Posted By IAEI,
Thursday, November 01, 2012
Updated: Wednesday, December 12, 2012
| Comments (0)
All three elements of the fire triangle — oxygen, ignition source, and fuel — must be present simultaneously and in specific quantities for a fire or explosion to occur. Often, in hazardous (classified) locations, the electrical system will be the ignition source of the triangle. This article looks at five methods of protection to prevent electrical systems from becoming ignition sources in these locations.
Location of electrical equipment
This method involves designing the electrical system so that much or all of it is located outside the classified areas. An accurate area classification of the space and careful placement of the electrical equipment are required. Depending on the type of process and the facility involved, this option could eliminate some or all special electrical equipment; it would be cost effective and would result in safer installations. [NEC 500.5(A) Information Note and 505.5(A) Information Note 2].
However, this method cannot always be used since in some cases the electrical components must be installed within the operating process; for example, pumps, compressors, instrumentation.
In other cases, the electrical equipment can be located to an area less likely to contain ignitible concentrations. Relocating equipment a relatively short distance away often has a major impact on safety and equipment requirements; the relocation can also result in decreased costs.
Figures 1, 2 and 3 demonstrate this protection method by relocating the luminaires. In figure 1, the luminaires are located within Division 1. In figure 2, the luminaires have been moved to Division 2. Then in figure 3, they have been installed outside Class 1, Division 1 or 2 locations. An accurate area classification and careful placement of electrical devices will always reduce both cost and risk of ignition in Class I locations.
Figure 1. Luminaires installed in Class I, Division 1 locations must be suitable for that location.
Figure 2. Luminaires installed in Class I, Division 2 location must be suitable for that location.
Figure 3. Luminaires installed outside a Class I, Division 1 or 2 location can be suitable for general use.
The principles are exactly the same for space classified using NEC 505, the Zone system. As in the Division system, a relatively minor relocation of many electrical devices could result in the use of ordinary location equipment. As the equipment location is moved from Zone 0 to Zone 2, the number of permitted protection techniques increases. This provides more flexibility, a decrease in cost, and a safer installation.
However, in Zone 0 spaces, Article 505 allows only electrical equipment that is protected by the intrinsic safety, or encapsulation techniques. This stipulation prohibits many electrical functions from taking place in that space.
Figure 4. Exhaust duct with the electric fan motor installed inside the duct (hazardous location)
Figure 5. Exhaust duct with the electric fan motor outside the duct (hazardous location)
With accurate area classification, on the other hand, much of the electrical equipment — lighting, alarm devices, security devices and communication devices — can be relocated to less classified or unclassified locations in many facilities. One example involves the location of the exhaust fan motor when ventilation is provided in hazardous (classified) locations. NEC 500.5(B)(1) Information Note 2, item 5 and 505.5(B)(1) Information Note 2 identify the "interior of an exhaust duct that is used to vent ignitible concentrations of gases or vapors” as a Division 1 or Zone 0 location. Figure 4 shows the exhaust fan motor located inside the duct, which is permitted in a Division system if the fan is explosionproof. In almost all applications, the motor could be located outside the duct in a Division 2 or unclassified location as shown in figure 5, and would achieve the same purpose using an ordinary location motor. With limited protection techniques permitted in Zone 0, installing the motor outside the duct may be the only practical option.
Containment of combustible materials
Controlling fugitive emissions
While the Environmental Protection Agency regulations address release of particles into the atmosphere in a different way than area classification, any control of release will have an impact on the area classification. This is not to say Division 1, Zone 0 or Zone 1 areas do not exist, but large-scale areas are not as common as they were in the past. It is no longer environmentally acceptable to vent waste products from a process, which, in some industries, could include flammable and combustible vapors into the atmosphere.
These EPA requirements have led to greater control of fugitive emissions, defined in American Petroleum Institute (API) Recommended Practice (RP) 500, Section 3.2.24 as "continuous flammable gas and vapor releases that are relatively small compared to releases due to equipment failures. These releases occur during normal operations of closed systems from components such as pump seals, valve packing and flange gaskets.”
Photo 1. Tight fill combustible liquid transfer
NFPA 30, Flammable and Combustible Liquids Code defines fugitive emissions as both continuous and intermittent releases and includes additional components that may be the source of release. Today vapor recovery is provided with many fill operations involving flammable or combustible liquids. Consequently, a much smaller volume of flammable or combustible vapor is released into the atmosphere. A common example of vapor recovery is used with gasoline delivery (see photo 1). As the gasoline tanks are filled, the vapors located above the liquid level in the tank are recovered into the tanker trucks, rather than being forced out the tank vents. These recovered vapors are then taken back to the bulk storage facilities, where vapor recovery is also provided (see photo 2).
Photo 2. Vapor recovery operation at the bulk storage facility
Adoption and enforcement of EPA regulations could have a different impact on area classifications from state to state, region to region and even from industry to industry since the goal of those regulations is environmental not prevention of ignitible concentrations as it relates to electrical installations. The National Electrical Code is used both in the U.S. and in other countries, some of which may have different regulations and may not have any environmental regulations.
Controlling the fuel side of the fire triangle
For Class II and Class III locations, dust collection systems and housekeeping have a major affect on area classification through the containment of combustible or ignitible material. These concepts provide health benefits to employees, greater mechanical life for moving equipment, and minimize overheating of electrical equipment that would otherwise be covered with layers of dust or fibers.
In order to realize the benefit of dust collection and housekeeping and also to use this method of protection by controlling the fuel side of the fire triangle, the installation must comply with the standards and recommended practices for the specific industry (see sidebar).
Section 3-3.2 of NFPA 499 indicates that areas designated as unclassified based on dust collection should be required to include adequate safeguards and warnings against failure. This might include some method to turn dust-producing equipment off when dust collection equipment is inoperable. Photo 3 shows a dust collection system that limits the dust cloud in this operation and, in turn, minimizes the dust layering. That in combination with housekeeping can greatly limit the amount of special electrical equipment needed for this operation.
Photo 3. Typical dust collection system (equipment shown inside of structure)
In many facilities, classified areas can be limited by providing closed storage vessels and transporting flammable and combustible materials through well-maintained closed piping systems. NFPA 497 and 499 both recognize these situations as unclassified areas. Special electrical equipment in the closed systems might be limited to instrumentation devices.
Containing with solid partitions
Separating hazardous locations with walls, enclosed vaults, or other solid partitions is another means to contain combustible materials and limit area classification. Containing the flammable or combustible materials is possible with Class I, Class II and Class III location materials. Since dusts and fibers/flyings are solid materials rather than gas, they may be easier to contain by separation. Where separation of smaller hazardous areas within a larger facility is used as a method of protection, extreme caution should be used to completely close off the classified spaces. All openings and penetrations through the separation must be considered for possible releases of the combustible materials. Figure 6 shows an example where an unpierced wall on one side of a structure that contains a Class I point of release ends the classified space. The opposite wall of the structure has openings that require the classified area to extend outside the structure.
Figure 7 shows a structure containing a Class II point of release. Walls on two sides of a structure are unpierced and an additional wall has an infrequently used self-closing door. The classified space does not extend beyond any of these walls. The other wall of the structure has a frequently opened door that requires the classified area to extend outside the structure.
Figure 6. Extent of classified locations can be extended due to openings in walls or structures
Figure 7. The classified space does not extend beyond the walls
NEC 500.5 recognizes the affect of ventilation in the area classification process. Adequate ventilation is defined in Section 1-3 of NFPA 497 as "a ventilation rate that affords either 6 air changes per hour, or 1 cubic feet per minute (cfm) per square foot of floor area, or other similar criteria that prevent the accumulation of significant quantities of vapor-air concentrations from exceeding 25 percent of the lower flammable limit.”
It should be recognized that the air change information included in this definition is not very specific. If the air changes noted do not limit the concentration of vapors to 25 percent of the lower flammable limit, the area classification should not be adjusted. In many cases where limited releases are occurring, those air change rates will likely provide adequate ventilation. In other cases where larger releases are expected, it may be determined that those air change rates are not sufficient to change the area classification. A review of the specific occupancy code, standard or recommended practice is advised prior to adjusting the area classification based on 6 air changes per hour. Some standards are very specific about the location of the exhaust and supply ducts, depending on the vapor density of the material involved. Some require product shutdown upon failure of ventilation. Others require gas detection systems to be interconnected with shutdown of a process. In every case, the code, standard or recommended practice for the facility should be considered prior to adjusting area classification based on ventilation.
Section 3-3.2 of NFPA 497 also indicates that outside installations and installations within open or partially open structures may be considered to have adequate ventilation and be classed "unclassified.” Nevertheless, a review of many of the occupancy documents will result in those spaces being classified. Section 1-2.3 of NFPA 497 indicates the occupancy document is a recommended practice and is intended as a guide that should be applied with sound engineering judgment. For example, when the information is used within its context and all of the parameters are considered, some outside installations will result in unclassified areas and others will result in classified areas. When it is determined that adequate ventilation has been provided, the specific standard for the industry or material involved will provide the guideline for the impact of ventilation on the area classification.
Great care should be used when the classification of a space and type of electrical equipment is changed based on the fact that ventilation is provided. Ventilation is required and may be provided in many facilities for reasons other than reducing the area classification. NFPA 500, Section 38.7 addresses attic ventilation; Section 54.4 requires ventilation of elevator rooms to maintain required temperatures during fire fighter service operations; Chapter 49 requires all rooms and occupied spaces in buildings to be ventilated; Section 126.96.36.199 requires various types of occupancies to conform to ASHRAE 62, Ventilation for Acceptable Air Quality. ASHRAE Standards provide ventilation requirements for spaces intended for human occupancy and specify minimum and recommended ventilation air quantities for preservation of the occupant’s health, safety and well-being.
NFPA 91, Standard for Exhaust Systems for Air Conveying of Vapors, Gases, Mists and Noncombustible Particulate Solids provides some general requirements for design and construction of exhaust systems which may be modified by other applicable standards. Systems that comply with this standard are limited to conveying flammables that are not more than 25 percent of the lower flammable limit unless they also meet the requirements of NFPA 69, Standard on Explosion Prevention Systems. Under normal conditions, fire detection and alarm systems are not permitted to shut down these air-moving devices.
Grounding and bonding requirements
Grounding is required for the protection of electrical installations, which, in turn, protect the buildings or structures in which the electrical systems are installed. Persons and animals that may come into contact with the electrical system, or are in these buildings or structures, are also protected if the grounding system is installed and maintained properly. The National Electrical Code does not imply that grounding is the only method that can be used for the protection of electrical installations, people or animals. Insulation, isolation and guarding are also suitable alternatives under certain conditions. Grounding of specific equipment is covered in several articles of the NEC. The scope and general requirements for grounding and bonding are contained in NEC 250.4. Included are the grounding and bonding performance requirements for grounded systems and ungrounded systems.
The Code places some special requirements for grounding and bonding in hazardous (classified) locations. These requirements can be found in NEC 501.30 for Class I locations; in 502.30 for Class II locations; and in 503.30 for Class III locations. For Class I, Zone 0, 1 and 2 hazardous (classified) locations, see NEC 505.25. Since this protection method is far more detailed than can be covered in this article, it is recommended that the reader refer to these Code sections, and to chapter 2 of Hazardous Locations, and to Soares Book on Grounding and Bonding, both books published by IAEI.
System shutdown and alarms
Methods of protection for hazardous locations often include engineered designs that incorporate specific ventilation systems, interlocks, and alarms. The basic objective of these systems is to minimize or remove one or more of the components of the fire triangle. Where movement of air is applied, the possibility of ignitible concentrations of hazardous atmospheres is reduced. A couple of examples are found in NEC 511.
Section 511.3(C)(1) indicates that the area 450 mm (18 in) above the floor in a commercial garage is unclassified if there is mechanical ventilation that provides a minimum of 4 air changes per hour or 1 cubic foot per minute of exchanged air across the entire floor area, and if the exhausted air is taken from a point within 0.3 m (12 in.) of the floor.
Section 511.3(C)(3) also indicates that lubrication service facilities — consisting of pit, belowgrade work area or subfloor work area — are classified according to whether ventilation is provided. If ventilation is not provided, any pit or subfloor work area is classified as Class I, Division 1 up to the floor level. If mechanical ventilation provides a minimum of 6 air changes per hour, the classification is Class 1, Division 2. There is no transfer of any Class 1 liquids, pits or work areas below grade level that are provided with not less than 0.3 m3/minute/m2 (1 cfm/ft2) of exhaust ventilation. The exhausted air must be taken from a point within 300 mm (12 in.) of the floor level of the pit or subfloor work area. Note that there are no requirements for interlocks or alarms in these Code rules; however, the Code does require that the exhaust ventilation for the pit of belowgrade work areas be operational at all times when the building is occupied or when vehicles are parked over the pit or belowgrade work area(s). A mechanical engineer can usually provide a design and documentation verifying to approving authorities that the require air movement or exhaust has been provided to allow the area to be considered unclassified. It is important that the occupant and operators of the facility understand the importance of these ventilation systems as they relate to building and personnel safety.
Another method of protection can be provided by a combination of gas detection system and air changes, which are generally specific to the project or location. An example would be a design that might include an exhaust system that provides a minimum number of air changes in the appropriate location, depending on the properties (vapor density) of the gas involved. If the gas is heavier-than-air, exhausted air will generally be taken from locations close to the floor or grade level. If the gas is lighter-than-air, exhausted air is usually taken from locations near the highest point of the facility or location. Interlock systems are often used to provide warnings by audible and visual alarms, or both in combination, to warn qualified operators that the method or protection is not operational. In addition to interlocks for personnel warning systems, there could be a shutdown interlock system that removes power from the electrical system in the area where the exhaust or ventilation system has failed or is not operating. This can be accomplished by using shunt-trip breakers and contactors that are interlocked with the specific exhaust or ventilation system. Usually any necessary electrical circuits or systems, such as emergency lighting equipment of exit lighting are provided in these areas, but they are installed using the appropriate protection technique for the area so they remain operational.
Sometimes gas detection systems are used as the primary protection method and are part of a system design that includes not only detection of ignitible concentrations but also works cooperatively with an exhaust or ventilation system and a shunt-trip interlocking system for system shutdown. These systems are usually equipped with early warning audible and visual alarms to warn facility personnel or operators of ventilation or exhaust systems failure and ignitible mixture accumulations that are increasing to hazardous (explosive) levels. These types of engineered systems or designs are generally limited to applications in industrial facilities or to installations where there are qualified persons who are familiar with the operation and servicing of these safety systems. Even where these multilevel methods of protection are employed, safety depends on qualified persons that can respond appropriately if any necessary component of the designed system were to fail or become inoperative.
The Code requires documentation of areas designated as hazardous (classified) locations as provided in 500.4(A). Although this section only calls for the hazardous areas to be designated, often the documentation provided by engineering and design teams includes how the area was classified, together with information about associated exhaust or ventilation systems and interlocked systems that allow for definitive delimitation of the hazardous locations. The documentation should also include the hazardous area classification that can result from ventilation system failure. As indicated in NEC 500.4(A), the documentation is required to be available to those authorized to design, install, inspect, maintain, or operate electrical equipment at those locations. It is important that an adequately designed protection system be properly operated and monitored to maintain minimum levels of safety in hazardous locations.
Dust Collecting and Housekeeping
A few of the standards and recommended practices that include specific requirements related to dust collection and housekeeping are:
- NFPA 36. Standard for Solvent Extraction Plants
- NFPA 61. Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Products Facilities
- NFPA 120. Standard for Coal Preparation Plants
- NFPA 480. Standard for the Storage, Handling, and Processing of Magnesium Solids and Powders
- NFPA 481. Standard for the Production, Processing, Handling, and Storage of Titanium
- NFPA 482. Standard for the Production, Processing, Handling, and Storage of Zirconium
- NFPA 499. Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas
- NFPA 651. Standard for the Machining and Finishing of Aluminum and the Production and Handling of Aluminum Powder
- NFPA 654. Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids
- NFPA 655. Standard for the Prevention of Sulfur Fires and Explosions
- NFPA 664. Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities
Ventilation requirements for codes & standards
NFPA 30, Flammable & Combustible Liquids Code requires all atmospheric storage tanks that contain flammable or combustible liquids to be adequately vented to prevent the development of vacuum or pressure conditions that might distort the tank or exceed the design pressure of the tank. This adequate vent of the tank is not going to provide ventilation that is adequate to prevent the accumulation of vapor-air concentrations from exceeding 25 percent of the lower flammable limit. This code includes ventilation requirements in sections 188.8.131.52 for vaults that contain tanks storing Class I liquids; 184.108.40.206 for tank buildings; 220.127.116.11 for inside liquid storage areas; 18.104.22.168 for hazardous materials storage lockers; and 22.214.171.124 for enclosed processing areas handling or using Class I, Class II or Class III liquids above their flash point. With the ventilation requirements met, some of these spaces are Class I Division or Zone 1 and others are Class I, Division or Zone 2.
NFPA 57, Liquefied Natural Gas (LNG) Vehicular Fuel Systems Code requires buildings with indoor fueling to provide continuous mechanical ventilation or a mechanical ventilation system that is activated by a continuous gas monitoring system, which activates the ventilation system when a gas concentration of one-fifth of the lower flammable limit is present. Both of these ventilation options are required to shut down the fuel system if the ventilation system fails. The ventilation rate shall not be less than one cubic foot per minute per 12 cubic feet of room volume. When adequate ventilation is provided, some locations are classed Class I, Group D, Division or Zone 1 and others Class I, Group D, Division or Zone 2.
Additional requirements are provided for commercial marine vessels operating on LNG.
NFPA 120, Standard for Coal Preparation Plants allows adequate ventilation to reduce area classification from Class I, Division 2 where methane can reach ignitible concentrations to unclassified. Any equipment that is needed to restore the facility to a safe condition such as lighting, ventilation, and sump pumps must be installed based on Class I, Division 1 requirements. Additional gas monitoring and shutdown provisions are also required when area classification is reduced because of ventilation.
NFPA 409, Standard on Aircraft Hangars permits mechanical ventilation for vapor removal in accordance with NFPA 91, Standard for Exhaust Systems for Air Conveying of materials.
NFPA 651, Standard for the Machining and Finishing of Aluminum and the Production and Handling of Aluminum Powders includes a reference to the ventilation requirements in NFPA 30, where aluminum dusts or powders are present in the same area with flammable or combustible solvents.
NFPA 820, Standard for Fire Protection in Wastewater Treatment and Collection Facilities indicates that ventilation rates used in that standard are based on air changes per hour and are calculated by using 100 percent outside air for the supply air that is exhausted. Air changes are calculated using the maximum aggregate volume of the space to be ventilated under normal conditions. Ventilation is not required, but is permitted if designers and owners desire area classification reduction. Ventilation systems used to reduce area classification in this standard are required to have both supply and exhaust fans, a means to provide power from an alternate power source, a power loss alarm on the primary power source, and include a variety of ventilation rates depending on what location or function in the collection and treatment process is involved and area classification that is desired to be achieved. All continuous ventilation systems installed for these facilities are required to include flow detection devices which are connected to an alarm signaling system and include both visual and audible alarms located in specific locations.
NFPA 853, Standard for Installation of Stationary Fuel Cell Power Plants requires mechanical ventilation of rooms where fuel cell power plants are located. The exhaust rate must be at least 1 cfm per square foot of floor area for the room and not less than 150 cfm of total floor area. That standard also requires the ventilation to be interlocked so that the unit will be shut down upon loss of ventilation.
Excerpted from Hazardous Locations, Second Edition, International Association of Electrical Inspectors, Richardson, Texas. Updated to NEC-2011.
Posted By Jesse Abercrombie,
Thursday, November 01, 2012
Updated: Wednesday, December 12, 2012
| Comments (0)
All investments carry risk. But, as an investor, one of the biggest risks you face is that of not achieving your long-term goals, such as enjoying a comfortable retirement and remaining financially independent throughout your life. To help reach your objectives, you need to own a variety of investment vehicles — and each carries its own type of risk.
If you spread your investment dollars among vehicles that carry different types of risk, you may increase your chances of owning some investments that do well, even if, at the same time, you own others that aren’t. As a result, you may be able to reduce the overall level of volatility in your portfolio. (Keep in mind, though, that diversification can’t guarantee a profit or protect against all losses.)
To diversify your risk factors, you first need to recognize them. Here are some of the most common types of investment risk:
Market risk. This is the type of risk that everyone thinks about — the risk that you could lose principal if the value of your investment drops and does not recover before you sell it. All investments are subject to market risk. You can help lessen this risk by owning a wide variety of investments from different industries and even different countries.
Inflation (purchasing power) risk. If you own a fixed-rate investment, such as a Certificate of Deposit (CD), that pays an interest rate below the current rate of inflation, you are incurring purchasing power risk. Fixed-income investments can help provide reliable income streams, but you also need to consider investments with growth potential to help work toward your long-term goals.
Interest-rate risk. Bonds and other fixed-income investments are subject to interest-rate risk. If you own a bond that pays 4% interest, and newly issued bonds pay 5%, it would be difficult to sell your bond for full price. So if you wanted to sell it prior to maturity, you might have to offer it at a discount to the original price. However, if you hold your bonds to maturity, you can expect to receive return of your principal provided the bond does not default.
Default risk. Bonds, along with some more complex investments, such as options, are subject to default risk. If a company issues a bond that you’ve bought and that company runs into severe financial difficulties, or even goes bankrupt, it may default on its bonds, leaving you holding the bag. You can help protect against this risk by sticking with "investment-grade” bonds — those that receive high ratings from independent rating agencies such as Standard & Poor’s or Moody’s.
Liquidity risk. Some investments, like real estate, are harder to sell than others. Thus, real estate is considered more "illiquid” than many common investments.
Make sure you understand what type of risk is associated with every investment you own. And try to avoid "overloading” your portfolio with too many investments with the same type of risks. Doing so will not result in a totally smooth journey through the investment world, but it may help eliminate some of the "bumps” along the way.
Read more by Jesse Abercrombie
Posted By Steve Vidal,
Thursday, November 01, 2012
Updated: Wednesday, December 12, 2012
| Comments (0)
The programmable logic controller (PLC) is a microprocessor-based system that accepts input data from switches and sensors, processes that data by making decisions in accordance with a stored program, and then generates output signals to devices that perform a particular function based on the application.
An important review
It is important to review previous articles that dealt with magnetic motor starters, input and output devices, and ladder logic as they provide some necessary background for PLC operation. As you recall, the magnetic motor starter is the controller that operates the connected motor load. Two-wire and three-wire control circuits use various types of input devices to energize the coil of a magnetic motor starter. These input devices are pushbuttons, proximity sensors, liquid level sensors, photoelectric sensors, selector switches, and pressure transducers. Typical output devices are contactors, magnetic motor starters, solenoids, pilot lights, and intelligent display panels. These output devices will behave according to the connection of the input devices.
It is also important to review some background on logic functions and binary number states as they relate to PLCs. The binary number system has two numbers, namely 0 and 1. The 0 refers to a logic state low (off), and the 1 refers to a logic state high (on). Ladder logic is the symbolic language of motor control.
Two logic functions most common in motor control circuits are the "and” operation and the "or” operation. The "and” operation occurs when two contact devices are connected in series. For example, if two switches are connected in series, switch #1 "and” switch #2 must both be in the on position for the load to be energized. In terms of logic, this means switch #1 (input #1) and switch #2 (input #2) can either be in the on or off position. When both switches are on (logic state 1), the load is energized. When both switches are off (logic state 0), the load is de-energized.
The "or” operation occurs when two contact devices are connected in parallel. For example, if two switches are connected in parallel, either switch #1 "or” switch #2 can be in the on position for the load to be energized. In terms of logic, this means switch #1 (input #1) and switch #2 (input #2) can either be in the on or off position. When one of the switches is on (logic state 1), the load is energized. A truth table is a graphical way of showing how inputs and outputs behave according to logic function. The "ones” and "zeros” in the table are binary numbers that represent on and off states. Figure 3 illustrates PLC logic functions.
Figure 1. A 3-wire central circuit listing input and output fuctions
Figure 2. Ladder diagram as entered into PLC programing
Figure 3. PLC Logic functions
Necessity for a programming language
A traditional motor control circuit is normally a hardwired system; therefore, any required circuit design change is a rather involved process in terms of material and labor. The manufacturing and automotive industries were interested in automating the motor control process in a way that offered flexibility to make circuit design changes easier. The interesting challenge was to design a programming language that would allow the industrial electrician a familiar way to communicate with the electronics of the PLC. This programming language would utilize symbols encountered in conventional ladder diagrams of the hardwired variety.
The original purpose of the PLC was to allow electro-mechanical and electronic input devices to communicate with a computer that would perform logical operations on the input data and output a corresponding signal to some form of output device (see figure 2).
Understanding inputs and outputs
To properly understand PLCs, it is very important to break down functions into inputs and outputs. If we revisit the standard three-wire control circuit as shown in figure 1, you will notice there are a normally closed momentary stop pushbutton and a normally open momentary start pushbutton. These contact devices represent the input function. The coil of the magnetic motor starter represents the output function. The normally open start pushbutton energizes the coil of the magnetic motor starter and the normally closed stop pushbutton de-energizes the coil of the magnetic motor starter. In this example, the PLC would recognize two input functions; the stop and start pushbuttons, and one output function the coil of the magnetic motor starter.
In very simple terms, a PLC is designed to perform three tasks: (1) check the input status, (2) execute the program, and (3) update the output status. The PLC checks the input status by scanning each input to determine if the connected device is on or off and then records that information in memory. Next, the PLC has to execute the user program one line at a time to make decisions. For example, maybe the user program tells the PLC to turn on an output device if input #1 is on, and then turn off another output device if input #2 is off. The PLC will analyze these conditions and execute the appropriate action and then store that information in memory. Lastly, the PLC has to update the output status. This means it will send data to an output device such as the coil of a magnetic motor starter to enable some type of manufacturing process to begin. The time it takes the PLC to go through this cycle is called the scan time.
The PLC uses a programming language that is based upon readily identifiable symbols common to motor control. Handheld programmers or PCs are the most common methods for programming the PLC. Figure 2 is an example of programming code setup to perform an "and” operation. Switch #1 and switch #2 are connected in series to the coil of a relay. The first rung of the ladder diagram shows two inputs; namely, switch #1 and switch #2 and an output, namely the relay. Each rung of the ladder diagram should contain input(s) and output(s). The input(s) should be the first listed instruction and the output(s), the last listed instruction. Usually programming code requires the END command to be listed as the last instruction on the last rung of the ladder diagram.
The following specifications are for a GE Fanuc PLC. This list will give you a sense of the type of information that is important in the selection and application of the PLC.
80188 CPU 8MHZ Clock Speed
Input Points – 16
Output Points – 12
High Speed Counter – 10KHZ
Maximum User program – 1K
Registers – 256 Words
Internal Coils – 2560
Memory backup w/Lithium battery – 5 years
LED Status Indicators for I/O and CPU Status
Scan Rate – 18mS/1K of Logic
The programmable logic controller gives the end user a very flexible means to automate the control circuit process in a manufacturing environment.
Read more by Stephen J. Vidal
Posted By Steve Henry,
Thursday, November 01, 2012
Updated: Wednesday, December 12, 2012
| Comments (0)
Light-emitting diodes are not a particularly new technology. Affordable LEDs have been available since the late 1960s. From then until now, the technology has steadily progressed in terms of cost-effectiveness, color options, light output, and efficiency. Thanks to ongoing advancements in semiconductors, optics, and materials, LED applications have grown exponentially.
Because LEDs are rugged, reliable, and long-lived, they’re still the universal choice for indicator lamps on all types of electronic equipment more than 40 years after they first began to replace incandescent lamps. They soon found application in alphanumeric displays for calculators, clocks, watches, and appliances; and as brighter LEDs became available in more color options, they quickly became ubiquitous in traffic lights, animated signage, automotive brake and signal lights, decorative lighting, flashlights, and much more.
Photo 1. LED lighting is superior for safe lighting of variouss locations
All of these applications benefit from the relatively low cost of LEDs, their remarkable efficiency, their minimal environmental footprint, and their long and reliable operation even under extreme conditions. But the advantages of LED technology have never been available as a realistic choice for high-quality area and task lighting. Until now.
At the Gates to the City
For industrial settings, hazardous locations, and public areas such as parking garages, LED lighting has always been something of a golden city: rumored to be just over the horizon, but never actually within reach. But in the last two years, things have changed. Through advancements in technology and manufacturing, bright white LED luminaires for industrial lighting applications are now coming to market.
Recent legislation in the U.S. has led to the phase-out of mercury vapor ballasts and lamps as well as 150 to 500 watt metal halide luminaires. Lighting designers who used to choose these products for their broader spectrum compared to high-pressure sodium, and for their longer life compared to incandescent, must now look at other options. LED enters the market at the perfect time to fill these needs, while far exceeding government mandated efficiency standards.
While other lighting sources will continue to play their roles, LED clearly owns the future.
It’s no longer just a rumor; LED is here today and promises to become an increasingly dominant technology in the future for all kinds of industrial and general purpose lighting needs.
Figure 1. Comparison of lighting designs in a walkway installation
Whenever a new technology emerges, it takes time for standards to coalesce and for new concepts to become clear. That’s true not only for end-users, but also for manufacturers who are just getting started with technologies that differ radically from what they’re used to working with.
Consider the first few months after the introduction of compact fluorescent lamps. Manufacturers had to play a guessing game regarding which wattages and sizes would become standard, and what luminaires they would need to design to accommodate these standards. Through the passage of time, standards emerged, products based on incorrect guesses and bad ideas were weeded out, and customers gained access to proven products that would provide reliable performance and compatibility for years to come.
Although practical LED industrial lighting has emerged fairly recently, the field has stabilized to a point where this technology can be regarded with practically the same understanding and confidence that compact fluorescent lighting has earned. Poorly designed products are being purged from the market, misleading claims have been retracted, and the major manufacturers are providing products that perform as advertised.
The market is still experiencing somewhat of a holdover in which misconceptions, uncertainties, and fears continue to persist. By shedding light on the most widely misunderstood issues, we hope to help you make more informed decisions today and to have a better idea of what to expect for LED lighting in the years ahead. While other lighting sources will continue to play their roles, LED clearly owns the future.
Figure 2. LEDs are directional, enabling much higher efficiency compared to a conventional lamp mounted in a luminaire.
Bulb: HID lamps emit light in every direction; this light is controlled using a reflector or refractor. The result is poor utilization, with efficiencies as low as 40%.
LED directs the light to where it’s needed without the use of external optics. This results in efficiencies as high as 80%.
LED is Versatile
Let’s start with an overview of the features that make LED lighting suitable, and in many cases superior, for nearly any industrial lighting application. Well-designed LED luminaires are:
White and bright. Far from the dim, bluish-green flashlight you keep in your glove compartment, today’s industrial LED luminaires provide extremely high-quality light, comparable to any other lighting technology.
Long-lived. Correctly designed, LED luminaires offer up to 60,000 hours of illumination, with no droop and no penalty for frequent on/off cycles.
Highly directional. LED luminaires can be configured to produce virtually any horizontal and vertical distribution of light, from illuminating a tall, narrow fence line for security purposes to providing area lighting that allows production crews to work efficiently and safely.
Resistant to shock, vibration and corrosion. LEDs can be used in environments where other technologies fail — either prematurely or catastrophically.
Cold start capable. LEDs provide instant on and instant restrike capabilities to –40°C, with no warm-up time to full brightness.
Non-damaging. LEDs produce none of the harmful UV or IR radiation often associated with other lighting sources.
Safe for hazardous locations. Available LED luminaires are rated for use in areas where flammable gases and vapors are present under conditions defined by NEC Class I, Division 2 and IEC Zone 2.
Superior for difficult locations. LED lighting is often the best choice for areas with low clearance, severe weather conditions, excessive moisture or dust, corrosive atmospheres, and high ambient temperatures.
Low maintenance. Rugged and long running, LED lighting requires very little maintenance. If you choose a luminaire with an intelligent modular design, even end-of-life replacement of components becomes quick and simple.
With all these benefits, we still haven’t touched upon three of this technology’s most remarkable qualities: LEDs are efficient and environmentally friendly and cool. Because there are many misconceptions in the popular imagination and even in industry literature, we’ll devote the rest of this article to examining these three unique qualities of LED lighting.
Figure 3. While the surface and beam of an LED produce little heat, the T junction can become quite hot.
LED is Efficient
The main driver for LED adoption is efficiency. Achieving the lighting levels required for a particular application at the lowest possible energy input becomes critical as energy costs rise and as government regulations clamp down on waste of energy resources.
Comparing the efficiency of dissimilar systems can lead to confusion. Consider the following specifications (see figure 1):
A 175 watt pulse start metal halide lamp requires 208 watt input power to the luminaire (the excess power is lost in the luminaire’s ballast). The light output for each lamp is 17,500 lumens, and the average lighting when four luminaires are installed at a 13-foot height over a 100 x 15 foot walkway is 5.67 foot-candles. Total input power for the installation is 832 watt.
A 165 watt QL induction lamp requires 165 watt power to the luminaire. The light output for each lamp is 12,000 lumens, and the average lighting when six of these luminaires are installed over the walkway is 6.34 foot-candles. Total input power for the installation is 990 watt.
A 48 x 1.7 watt LED array requires 98 watt input to the luminaire (the excess power is lost in the driver). The light output for the luminaire is 5,400 lumens (this figure cannot be compared directly with lumens from a conventional lamp). The average lighting over the walkway when four of these luminaires are installed is 9.55 foot-candles. Total input power for the installation is 392 watt.
It may be tempting to look at the input power to the entire system versus the power actually consumed by the lamps, and to conclude that LED is the least efficient of the three. That would be a mistake. The important comparison is input power versus actual illumination at the point where it’s needed. In this scenario, LEDs are more than twice as efficient, providing brighter illumination at less than half the power consumption of either QL induction or pulse start metal halide.
Comparing lumen output
It may also be tempting to conclude that LEDs are less efficient than the alternatives based on the lumen figures given in the previous bullet points — 17,500 for the pulse start metal halide lamp, 12,000 for the induction lamp, and 5,400 for the LED luminaire. These figures cannot be directly and fairly compared. Figure 1 gives a lumen value of -1 for the LED luminaire. This value is used by our photometric software to distinguish LED from other types of lighting in its calculations to produce results that are valid across these very different technologies.
An LED luminaire incorporates an array of point sources that direct light precisely where it’s needed, with very little scattering or loss. Light distribution is controlled by the placement of LEDs, as well as by efficient use of optics that take advantage of the focal point presented by each individual LED. By contrast, conventional lamps cast light in every direction, and the luminaires incorporate hoods, reflectors, and lenses to direct light to where it’s needed and shade areas where it’s not. Due to scattering and absorption, only 40 percent of the available light reaches its intended destination, versus up to 80 percent for an LED luminaire (see figure 2).
A lumen rating calculated by totaling the light output of all LEDs in the luminaire is simply not comparable to a lumen rating for a lamp based on measurement of light output in all directions. Most light from the lamp never reaches its destination, while nearly all the light from an LED does, and this is why even our estimation of 5,400 lumens for a 48-LED array shouldn’t be used for purposes of comparison. LED is inherently incomparable with other lighting systems in terms of lumen output. What matters is how much light reaches the intended surface, and at what energy cost. In these terms, LED is the clear efficiency leader.
LED is "Green”
Properly designed, an LED luminaire produces a pleasant and eminently usable white light. But LED is also remarkably "green.” Several factors combine to give LED the smallest environmental footprint of any manufactured source.
LED is energy efficient
The most important "green” feature of LED is its energy efficiency. By choosing LED luminaires, you can significantly reduce your energy costs for lighting. Because 70 percent of electricity in the U.S. is produced by burning fossil fuels, your choice is also likely to cut emissions of greenhouse gases and toxic pollutants by half. Your total energy consumption is likely to be even lower when you consider that many buildings will require less cooling due to the lower operating temperature of LEDs. Consuming less energy is not only good for the environment; it’s also good for your company’s public image.
LED is Non-Toxic
LED lighting is also the only non-incandescent lighting source that contains no mercury. This eliminates any chance for mercury to escape into the environment either in operation or after disposal. Combined with LED’s inherent durability, the lack of mercury makes installation, maintenance, and disposal much easier. There are no special handling requirements. The extended lifetime of LED compared to most other lighting sources means less material being disposed over time — another ecological benefit. If you’re careful to choose a manufacturer that uses lead-free solder, you can be sure that no toxins are entering the environment when components do finally reach end-of-life.
Choose ENERGY STAR®- certified products for best performance
For manufacturers and customers alike, ENERGY STAR certification provides the benefit of knowing that products have been tested and approved by a reputable third-part organization to ensure they meet stringent environmental and operational requirements. Not all manufacturers seek or obtain this certification, which means they’re depending on customers to trust their claims and specifications without objective corroboration.
More than just meeting energy consumption requirements, ENERGY STAR certification means that a product has been tested to ensure the highest quality. Simply put, this government created certification body will not approve luminaires that don’t meet customer expectations, no matter how energy efficient they may be. Among many other requirements, ENERGY STAR certification ensures that LED lighting luminaires:
Use at least 75 percent less energy than equivalent incandescent lighting, and provide efficiency as good as or better than fluorescent lighting
Offer brightness equal to or greater than other technologies, with good distribution over the lighted area
Provide constant light output that decreases only near the end of the product’s rated lifetime
- Provide excellent color quality, with a shade of white light that remains clear and constant over time
- Turn on instantly, and use no power when turned off other than a maximum of 0.5 watts in the control gear
LED is Cool
If you own an LED flashlight, you know that LEDs put out very little heat. No matter how bright, you can touch the light source indefinitely with no discomfort. Moreover, LEDs produce no harmful ultraviolet or infrared radiation. These properties offer many benefits — lowering cooling costs, simplifying maintenance, prolonging product life, avoiding damage to eyes and sensitive equipment, and providing a margin of safety in hazardous environments.
Industrial LED luminaires do produce some heat
Unlike a battery-powered flashlight, however, AC powered LED technology does produce a significant amount of heat outside the beam. It’s important to understand why this is so and how to manage the heat properly.
LEDs operate naturally on direct current. To light an LED on an AC circuit without destroying it, you need a driver that converts AC to DC and steps the voltage down from 120 Vac (or more) to 24 Vac. Unlike a flashlight battery, the output current is at very high amperage — much higher than the milliamps required to light the LED. This current is fed into the T junction at the rear of the LED (see figure 3).
The T junction can be compared to a tiny nozzle mounted on the end of a large fire hose. In stepping the input current down to meet the requirements of the LED, the T junction absorbs a substantial amount of energy — similar to the friction a large volume of water under high pressure creates when it meets with the constriction of a nozzle. This energy is released as heat. While the beam of an LED luminaire may be cool, the back side of the LED array can become quite hot. The T junction is the hottest spot on the luminaire.
Accurately determining its maximum temperature is crucial when rating products for use in the potentially flammable atmospheres of oil refineries, paper mills and other manufacturing environments.
The other main heat-producing component is the driver inside the luminaire unit, which is analogous to the ballast compartment in conventional lighting systems. The driver is a solid state device and as such it needs to operate within a specified case temperature rating.
Properly managing the heat generated within the LED luminaire is important for three primary reasons:
- Excess buildup of heat at the T junction can degrade the phosphor and reduce lamp life
- Excess heat at the driver unit can also reduce product life
- Inadequate heat management can limit the range of ambient temperatures for which the product can be specified.
Luminaires rated for maximum ambient temperatures below 55°C cannot be used in many areas of the world (for example, the Middle East) and in many specific applications (for example, smelting and casting).
A properly designed LED lighting luminaire will have a large external heat sink — often visible as a series of bare or powder-coated metallic fins surrounding the LED array itself. This heat sink is designed to pull heat away from the T junctions on each LED as well as from the driver housing. After several minutes of operation, the heat sink will become noticeably warm to the touch, even while the beam itself remains cool.
Prominent heat sinks may be unfamiliar to most people who are used to seeing LEDs in lower current applications such as signal lights, or outdoor luminaires that benefit from free flow of air and nighttime temperatures. For industrial lighting applications, when circuitry and lamps are housed in an enclosed and gasketed or explosion-proof luminaire, these heat sinks are critical. They ensure that LEDs achieve their full 60,000 hour lifespan with no degradation in the quality of light. The heat sink allows luminaires to operate reliably in temperatures ranging from –40°C to as high as 55°C.
Even with adequate heat sinking, good thermal management requires that luminaires be designed with the optimum number of LEDs to achieve the desired lighting levels. It’s possible to achieve a dramatically whiter, more intense light by adding more LEDs than the optimum number. This strategy will inevitably overdrive the system, reducing lamp life, damaging the phosphors, and causing a noticeable color shift or "droop.”
When too many LEDs are incorporated into the design, what began as an impressive display of white light may shift to an unacceptable color within weeks, and may die altogether within a few thousand hours of operation.
Locating and measuring hotspots
Luminaires designed for use in hazardous atmospheres must be rated according to stringent requirements to ensure that a spark or hotspot doesn’t ignite the atmosphere. If an internal ignition occurs it should not be allowed to escape from the luminaire into the surrounding atmosphere.
Construction and testing standards for these luminaires are controlled by the IEC, NEC, and other standards and testing bodies. For the most part the standards are well understood and consistent, but LEDs require a new approach to temperature rating.
Conventional wisdom based on more established technologies suggests that the hotspot is likely to occur on the surface of the lamp, but as we have seen this is not true with LEDs. The hotspot is at the T junction, which is sealed inside the LED assembly. The T junction is impossible to reach with a thermocouple in order to take a temperature reading directly.
Currently, different manufacturers and testing bodies use different methods to place the thermocouple as close as possible to the T junction, as well as different methods to analyze the results and estimate the true hotspot temperature.
We expect to see a single, accepted standard emerge for temperature rating of LED luminaires. In the meantime we suggest talking with the manufacturers whose products you are considering, and asking how they arrive at their temperature ratings and how much margin of error is built into the results. Because LED luminaires tend to have a lower temperature rating than most of the alternatives, you should be able to find a suitable product rated at a significantly lower temperature than the safety threshold for most applications.
LED is the Future
LED industrial lighting is here today, and it’s here to stay. The benefits it provides simply can’t be ignored, either by end users or manufacturers. Even local and national governments are taking notice of the benefits as they increasingly focus on the problems of energy consumption, greenhouse gases, pollutants, and toxic waste.
While LED technology is not the only choice, or in some cases even the best choice, it will rapidly become the leading choice to replace many of today’s energy hungry industrial lighting systems. In the years ahead — as standards become more firm, product lines more established, and customers more conversant with the technology — LED will also become a relatively easy choice. We’re not there yet. We’ve just entered the gates of the golden city, and there’s still a lot to explore.
The important thing is that we’ve seen enough to know where we’re going. The fundamentals are in place, and some very good LED products are already on the market. The knowledge of how LEDs work and the best ways to harness the technology are available. As with anything new, big, and potentially lucrative, a lot of misinformation has been made available as well.
Our goal in this article has been to present the facts you need to know about LED luminaires and to dispel the misinformation. Armed with this knowledge, you should be better prepared to enter the world of LED product evaluation for your own lighting design projects. With a little hands-on experience, you should soon be as comfortable with LED as with any other lighting system.
Read more by Steve Henry
Posted By Michael Furtak,
Thursday, November 01, 2012
Updated: Wednesday, December 12, 2012
| Comments (0)
Direct current (DC) arcing fault incident energy calculations are presented to assess the level of risk involved when working around high current DC apparatus. The proposed procedure allows evaluation of incident energy and arc flash boundaries, while taking into account as many circuit parameters as possible. These parameters include fixed or variable gap length, system voltage, available fault current, equipment configuration, circuit time constant and evaluated threshold energy for a second degree burn.
DC Arc Steady State Modeling
The resistance load line of the equivalent steady state circuit diagram representing a linear DC supply can be described by Equation (1) below:
where Vs stands for open source voltage and Rs is system resistance including source and feeders. The applications include battery packs, power converters and chargers, mining sites, public transportation, solar and wind farms, etc. Substituting into the Equation (1) voltage drop across an arc (Varc), arcing current can be resolved as:
where Isc is prospective short-circuit current at the arcing point. It is shown that arc voltage is mainly determined by the arc length, and the voltage is within the 10 to 20V/cm range for arc currents up to the order of 50kA.1 Incident energy exposure for an open-air arc where the heat transfer depends on the spherical energy density is then expressed as:
where tarc is the arc duration and D represents the distance from the arc. This formula assumes the radiant heat transfer. Not all of the arc energy will be transferred as radiant heat 2 especially within the short time interval after the arc was ignited. Therefore, the Equation (3) will produce a conservative but safe estimate of incident energy exposure. For the arc in a box, the enclosure has a focusing effect on the incident energy. For the selected enclosure type and test distance,3 the incident energies calculated for enclosures are 2.2 times larger than the incident energies calculated for open air.
Equation (3) written in terms of arc flash boundary, becomes:
where Et stands for threshold incident energy to second degree burn 4 evaluated as:
Figure 1. Sample arcing power and time to 2nd degree burn vs. arcing current at 0.5 meter distance away from arc in open air 600 VDC system.
DC Arc Transient Conditions
The problem of determining the arc flash boundary becomes less trivial when gap is not fixed and distance between anode and cathode is anticipated to increase by separation of the contacts. Also, the arc operates at the intersection of the arc volt-ampere characteristic curve and the resistance load line of the DC circuit. Therefore, the arcing current will stabilize itself at a fixed point on the curve and the arc will dissipate a relatively constant amount of power. However, it’s hard to predict how long it will take for the arcing current to stabilize before the arc burns out or is cleared by the upstream protective device. The load line may intercept the characteristic curve in two locations, but only one point is stable. The stable operating location is the point with the lowest arc voltage. 5
Stokes and Oppenlander 3 demonstrated that there is a minimum voltage needed to maintain an arc. That minimum depends on the current magnitude, gap width, and orientation of the electrodes. This transitional point can be expressed as:
where the length of the gap, Zg, is expressed in mm., It is measured in amperes. Above that minimum, the arc V-I characteristic can be expressed as: 3
To find the point where the arc V-I characteristic crosses the circuit load line, solve equations (2) and (7) using the iterative method. As the first approximation, assume Varc is equal to half of the system voltage Vs. Then, follow the steps below:
- determine Iarc from Equation (2)
- substitute Iarc into Equation (7) to determine new Varc
Cycle through the steps listed above until the answers for Varc converge. Additionally, circuit time constant affects current rise and protective device performance characteristics, thus impacting the arc duration. In this case, time current characteristic of the upstream protective device clearing the fault may have to be adjusted for the time constant. If this occurs, the process of determining the protective device operating time is cumbersome. First, the time-current characteristic of the protective device has to be analytically expressed as a function of the available fault current. A paper by Cynthia Cline6 provides an equation describing the relationship between the effective RMS current, the available fault current, and the number of time constants:
where the K factor is expressed in numbers of time constants n=tarc/tconst:
K=(1 + 2e-n /n - e-2n/2n - 1.5/n)0.5, (9)
This creates a dilemma due to the fact that one cannot determine the arcing time without the RMS value of the arcing current, and one cannot solve for the RMS current without the arcing time represented by the n term in Equation 9. This requires an iterative solution. As a first approximation, begin by assuming that Irms equals Iarc, determining tarc from the analytical expression for the fuse T-C characteristics tarc=f(Irms), determining the number of time constants n and calculating K from Equation 9, substituting its value into Equation 8 to calculate the new RMS current, and then solving for the arc duration again. Once the first approximation of the arc duration has been made, determine the new number of time-constants n, re-calculate the K term and substitute its value into Equation 8. This produces a new Irms. Re-calculate for a new tarc by using the new Irms and continue until the answers converge. Then, Equations (3) through (5) can be utilized to complete the DC arc analysis under the transient conditions.
Figure 2. Simplified block diagram for resolving arcing faults in DC power systems.
With numerous variable parameters on hand, which results in the difficulty to accurately model DC arc and to predict the arc V-I characteristic and thermal behavior, we decided to consider the worst-case scenario leading to an arbitrary burn hazard in the shortest possible time. Figure 1 shows arcing power as a function of variable arcing current for the DC equivalent circuit described by Equation 1. The red line on figure 1 represents time to 2nd degree burn as a function of heat flux.4 Note that the minimum time to 2nd degree burn, as well as any other burn hazard, coincides with the maximum power released by an arc, hence, the maximum heat flux.
For a fuse with an inverse time-current characteristic, the amount of arcing currents is inversely correlated with the fuse operating time, and consequently, with the arc duration. With the decrease of arcing current, power released by an arc will actually increase and reach its maximum value corresponding to the middle point on figure 1. Further decrease in arcing current will lead to a decrease in arcing power and an increase in arc duration time when it takes more time for the upstream protective device to clear the fault. Therefore, there is a minimum amount of time leading to a specified burn severity produced by any given DC arc, and, for the 2nd degree burn on bare skin, that time can be expressed as:
where Eb is equal to 1.2 cal/cm2/sec. The Equation (10) assumes rectangular flash pulse, thus producing the minimum time to 2nd degree burn under the specified circuit conditions. It can also be applied for hazards other than 2nd degree burn by selecting a different Eb factor on the right side of the Equation 10. When analytical expression for the protective device clearing the fault time-current characteristic is available, it is possible to examine power and energy released by an arc as a function of arcing current and arc resistance, and to determine maximum damage that can be caused by the arc during the selected time interval. A simplified block diagram on figure 2 describes the proposed approach for calculating incident energies and for determining arc flash boundaries in DC power systems.
With a better understanding of the DC circuit parameters and the DC capabilities of fuses, modeling DC arcs and selecting appropriate fuses for mitigating arc-flash hazard can be accomplished without much difficulty. The generalized solution presented in this paper considers the worst-case scenario, effectively eliminating the need for accurately predicting arcing gap and arc resistance.
1 "Electric Arcs and Arc Interruption.” C.E. Sölver. Chalmers University of Technology. February 2005.
2 "Electric Power Transmission Systems.” 2nd Edition. J. Robert Eaton, Edwin Cohen. Prentice-Hall, Inc.
3 "DC Arc Models and Incident Energy Calculations.” R. Ammerman, T. Gammon, P.K. Sen, J. Nelson. IEEE Transactions on Industry Applications, Vol. 46, No. 5, September/October 2010.
4 "Evaluation of Onset to Second Degree Burn Energy in Arc Flash.” M. Furtak, L. Silecky. Electrical Safety Measures, March/April 2012.
5 "Arcing Faults on Direct Current Trolley Systems.” P. Hall, K Myers and W. Vilchek. Mine Safety and Health Administration.
6 "Fuse Protection of DC Systems.” Cynthia Cline. Annual Meeting of the American Power Conference. April 1995.
Read more by Michael Furtak
Posted By Tim Crnko and Mark Hilbert,
Monday, September 03, 2012
Updated: Thursday, September 20, 2012
| Comments (0)
What matters to a person when they are in a facility that has capacity for many people and an emergency situation such as fire, flooding, storms, earthquake, explosion, or merely loss of normal utility electrical power occurs? Naturally, this depends on the situation, but the basic human instinct is to want to escape the event without physical or extreme emotional trauma. We desire safety for our family, ourselves, and others.
In buildings where large numbers of people can assemble, such as office buildings, schools, high rises, hotels, theatres, arenas, and hospitals, there are electrical loads that are intended to provide for human safety in emergency situations. The 2012 NFPA 101 Life Safety Code provides minimum life safety requirements for the design, operation, and maintenance of buildings and structures. Examples include emergency loads such as elevators, emergency lighting, emergency egress lighting, alarm systems, ventilation, smoke control, fire pumps, and more. These life safety loads need to operate when there is an emergency and need to operate as long as possible, even if the building and electrical system supplying these loads are under physical distress. And if a portion of the electrical system is damaged due to fire or other causes, it is imperative to restrict any resultant electrical outage to the minimum portion of the electrical system thereby keeping as many vital loads powered as possible. Lives may depend on these loads.
A key requirement for the electrical systems that power these life safety loads is reliability. It is superior to that typically provided for ordinary loads and thereby increases the probability people can survive or escape during emergency events. The reliability of electrical systems supplying life safety loads and functions is just as important and possibly more so for first responders. Merely relying on the normal system does not provide sufficient continuity of electrical power for life safety. Additional alternate electrical source(s) (in case the normal source is lost) and an electrical distribution system designed, installed, and maintained to be more reliable are necessary. Components such as generators, automatic transfer switches, and circuit wiring with higher reliability features are needed. This increases the cost, space requirements, and design, installation, and maintenance complexity of the facility electrical system. But not having these could mean loss of life or severe injuries if an emergency event occurs.
Figure 1. This overly simplified electrical distribution system one-line depicts the various circuits and panels. Each emergency panel may have one or more life safety type loads or functions with one or many branch circuits for each life safety function.
There is an ever continuing trend to utilize more and increasingly sophisticated life safety and emergency electrical functions. In other words, for facilities where large numbers of people assemble, there are more emergency loads with more complexity and interdependencies. Elevators have long been used for transporting first responders and their equipment. However, now in some cases, elevators are being used for emergency egress of building occupants; the 2012 NFPA 101 has added new requirements for occupant evacuation elevators. Now there are sophisticated public safety communication systems, fire detection and alarm systems, and more. As you walk in the middle of a large unfamiliar building at night ask yourself what you would do if a major fire or explosion ensued and all the lights go out, and none of the life safety loads functioned.
Figure 2. Cascading overcurrent protective devices will result in unnecessary loss of power to other life safety loads.
The 2011 NEC has minimum installation requirements for the electrical systems supplying these vital life safety loads. These minimum requirements provide the baseline for the electrical system reliability. Systems can be designed and installed for more reliability than the NEC requirements, but the minimum NEC requirements must be met and not compromised.
The bottom line is that if it is your family at the top of a high rise building during a major fire, their path of egress to safety relies heavily on the electrical system. Whether the normal source or the emergency source is supplying the power to the life safety loads, come "hell or high water,” your family’s fate may very well be in the hands of the electrical system. If they are on the 20th floor and have to exit via a stairway, will the egress lighting be operational to illuminate their pathway? Will the smoke control system keep the stairway free of smoke so they can survive? Will the public safety communication system be powered to deliver vital messages to assist in their journey to exit the building or to a safe location? Will the elevators be powered so the first responders and their equipment can quickly get to where they need to be and they can move quickly to other locations? The answer to these questions lies within the integrity of the "emergency system.”
If fire, smoke, or physical damage to the building causes a fault in the emergency electrical system, will the fault be localized to only the faulted circuit, or will multiple levels of overcurrent protective devices cascade open and unnecessarily cause loss of power to life safety loads? Remember, it could be your family relying on the integrity of the electrical system to ensure their pathway to safety.
The authority having jurisdiction (AHJ) has an important role. They very well could be the last line of defense against an electrically compromised pathway to safety. From an enforcement standpoint, those responsible for approving the electrical installation have the responsibility to ensure the minimum requirements are met for these vital life safety systems. If AHJs are able to review the plans and installations and ensure code-compliance, your family has an increased probability of coming home if ever they are caught in a building when an emergency event occurs. In some cases, the AHJ has to establish the "interpretation of the rules” per NEC 90.4 which is an especially significant responsibility when it concerns the emergency electrical system.
It is important to mention that besides the NEC and NFPA 101, there are other applicable codes and standards that are not mentioned in this article but that have relevant requirements for specific life safety functions, systems, or loads.
Systems Vital for Life Safety
The societal demands upon modern building life safety are necessitating more life safety systems, more interdependency of various electrical functions which are supplied by emergency systems, and greater reliability.
Elevators are an example. In normal conditions elevators are a convenience to move people vertically in a building. Over the decades, we have been educated not to use the elevators during fires. However, elevators can have the provision for first responders to take control of elevators for improving their speed of response. In some cases, elevators might only be supplied by the normal electrical system and have no provision for emergency power. In other cases, elevators may be on the emergency system and therefore supplied by either the normal or emergency source, depending on the circumstances.
However, in emergency conditions elevators are now permitted to serve as occupant evacuation elevators (2012 NFPA 101 7.14) if approved by the AHJ and if all requirements are met (includes NFPA 101, NEC, and others). The NFPA 101 requirements include the emergency command center continuously monitoring and displaying the elevators, elevator lobbies, elevator machine rooms, and status of many related functions serving the elevator system. The following are a few such interdependent functions powered by the emergency electrical system. The lobbies of occupant evacuation elevators must be equipped with a status indicator display system to communicate visually to occupants the condition of use: (1) normal use, (2) available for occupant evacuation, or (3) out of service – use the stairs.
If elevators are used for occupant evacuation, then the building must be protected throughout with a fire alarm system and a voice/alarm communication system with capability to give voice directions on a selective basis to any floor. In addition, occupant evacuation elevator lobbies must be equipped with a two-way communication system to permit communications between persons in the elevator lobby and the emergency command center or another designated location. This illustrates the level of sophistication modern building life safety systems may have; there are multiple interdependent systems vital for human safety that are powered by the heart of the electrical system, the "emergency system.” Losing one or more of these systems reduces the probability the occupants will survive.
Photo 1. This photo was taken by Mark Hilbert after an earthquake in Hawaii. The entire island was without power because the utility generators sensed the movement and shut down. The only thing illuminated in the entire hotel was the stair tower.
There are many other life safety electrical loads or systems that may be deployed in a building. These can include fire pumps, emergency lighting, egress lighting, smoke control systems, fire detection and alarm systems, communications systems, video systems, ventilation, HVAC for specific applications, monitoring various electrical functions and other functions.
Reliable Electrical Power
Reliable power supply and reliable electrical circuits for life safety loads are vital. The NEC has specific requirements for fire pumps in Article 695 and elevators in Article 620. In addition, Article 700 Emergency Systems, Article 701 Legally Required Systems, and Article 708 Critical Operations Power Systems provide requirements for electrical distribution systems that demand more reliability than normal systems.
Per NEC Article 700, the life safety loads and systems are powered by the normal electrical source and, in addition, with one of the type emergency electrical power sources complying with 700.12. So in simple terms, life safety loads or functions are powered from an emergency electrical distribution system which is supplied by a normal source and an emergency source. There can be a wide variance in the electrical distribution system layouts based on the building type and life safety loads or functions required by the AHJ or by chosen design. Figure 1 is an overly simplified electrical distribution system one-line depicting the various circuits and panels. Each emergency panel may have one or more life safety type loads or functions with one or many branch circuits for each life safety function. So what makes NEC Article 700 emergency systems more reliable than normal NEC electrical systems? In addition to complying with Chapters 1 – 4 of the NEC, emergency systems must comply with NEC Article 700 which can supplement or modify Chapters 1 – 4. In addition, there may be requirements applicable for an emergency system from Chapters 5 for Special Occupancies, 6 for Special Equipment, and other Chapter 7 articles for Special Conditions. The following discussion will highlight some of the key Article 700 requirements that provide the increased reliability to power life safety loads.
700.3 Requires AHJ witnessed testing of the emergency system upon installation and periodically.
700.4 Capacity and rating for simultaneous operation of all loads. All equipment must have short-circuit ratings equal to or greater than the available fault current and all overcurrent protective devices must have interrupting ratings greater than the available fault current.
700.5 Automatic transfer switches and all transfer equipment shall be identified for emergency use.
700.10 All boxes and enclosures must be permanently marked as part of an emergency circuit or system. Emergency circuit wiring must be separated from other wiring. Wiring must be located to minimize failures due to adverse conditions such as flooding, fire, etc. There are special fire protection requirements for some occupancies which may require a two-hour fire rating.
700.12 If normal power is lost, emergency power shall be available to loads within 10 seconds. This section provides the type of alternate power sources that may be used. The specific type of alternate source utilized is typically based on the performance required by the NEC and other codes and standards predicated on the type occupancy and type of service required.
700.15 Emergency lighting branch circuits can only supply lighting designated as required for emergency use.
700.16 The failure of one lighting element must not result in total darkness to any space requiring emergency illumination. This is an extremely important element (no pun intended) that ensures that while under emergency conditions, the objective of avoiding spaces without illumination is met.
700.20 Only authorized personnel shall have control of emergency lighting.
700.25 Only authorized personnel may have access to emergency branch circuit overcurrent protective devices.
700.26 Ground fault protection with automatic disconnecting means, as required by 215.10, is not required for the alternate emergency source. This is an important permission since this relegates the delivery of power to life safety loads at a higher priority than the damage resulting due to a ground fault. However, if this permission is granted then ground fault indication is required in accordance with 700.6(D).
700.27 "Emergency overcurrent protective devices shall be selectively coordinated with all supply side overcurrent protective devices.” This requirement is especially important when the emergency is a fire, explosion, earthquake, or similar event which may increase the risk of a fault on a circuit. Fires may cause the ionized gases to initiate a fault in equipment or the resulting high temperature may result in a short circuit. Earthquakes and explosions can cause faults in equipment and circuits as well. If the installed overcurrent protective devices are not selectively coordinated for all levels of available fault current, a fault on one part of the system may unnecessarily cascade two or more levels of overcurrent protective devices upstream. Cascading overcurrent protective devices will result in unnecessary loss of power to other life safety loads.
See figure 2. A branch circuit fault such as X1 or X2 should only be cleared by the branch-circuit overcurrent protective device (1 or 1A, respectively). No feeder overcurrent protective devices upstream should open for a branch-circuit fault (X1 or X2). If a branch-circuit fault X1 unnecessarily opens the feeder 2 overcurrent protective device, an entire emergency panel will be without power versus only the life safety load(s) on the faulted branch circuit 1. Or if branch-circuit fault X2 unnecessarily opens the feeder 2A overcurrent protective device, a whole bank of elevators will unnecessarily be without power versus only the one elevator on the faulted branch circuit. If the cascading goes even higher in the system, such as when feeder 3 overcurrent protective device unnecessarily cascades open for a branch-circuit fault (X1 or X2), all the emergency loads will unnecessarily be without power. The unnecessary opening of overcurrent protective devices can jeopardize the life safety of the building occupants and first responders who may need those specific life safety loads.
People’s lives depend upon the emergency power sources and emergency distribution systems delivering power to life safety loads during emergency situations. In essence, emergency systems are "insurance” that people are more likely to survive an emergency event, but there is a price to pay for this reliability. All the extra electrical equipment it takes to comply with NEC Article 700, Emergency Systems, has costs plus the additional cost for the associated engineering, installation, and maintenance. However, the bottom line is that it’s going to be someone’s family member on the 20th floor of that building during a fire, earthquake, explosion, or other emergency situation that has to find a pathway to safety. If it was your family member, wouldn’t you want these systems designed, installed, inspected and maintained as required?
Read more by Tim Crnko
Read more by Mark Hilbert
Posted By Stephen J. Vidal ,
Monday, September 03, 2012
Updated: Thursday, September 20, 2012
| Comments (0)
NEC Article 100 defines a controller as "a device or group of devices that serves to govern, in some predetermined manner, the electric power delivered to the apparatus to which it is connected.” Section 430.2 gives a more motor-specific definition: "For the purpose of this article [Article 430], a controller is any switch or device that is normally used to start and stop a motor by making and breaking the motor circuit current.”
The magnetic motor starter is such a controller and utilizes electromagnetically operated contacts that start and stop the connected motor load. A control circuit with momentary contact devices connected to the coil of the magnetic motor starter performs this start and stop function. A three-pole full-voltage magnetic motor starter is made up of the following components: set of stationary contacts, set of movable contacts, pressure springs, operating coil, stationary electromagnet, set of magnetic shading coils, and the moving armature.
It is also important to remember that a magnetic motor starter is a contactor that has the addition of an overload relay assembly that provides running overload protection to the motor. Selection of the thermal overload relay is done using the manufacturer’s table included with the magnetic motor starter. It is always important to know the full load current (FLC) of the motor, the service factor (SF) of the motor, and the ambient temperature in which the equipment is being operated. Thermal units are based on an ambient temperature of 40° C (104° F).
Types of starters
Magnetic motor starters are commonly available as full-voltage (across-the-line), reduced-voltage, and reversing. A full-voltage or across-the-line magnetic motor starter applies full voltage to the motor, which means it is designed to properly handle the levels of inrush current that will develop as the motor is started (see figure 1).
Figure 1. Full-voltage (across-the-line) magnetic motor starter
Reduced-voltage starters are designed to limit the effects of inrush current during motor startup. These are available in electro-mechanical and electronic formats.
Figure 2. Full voltage reversing starter
Reversing starters are designed to reverse shaft rotation of a three-phase motor. This is accomplished by interchanging any two-line conductors that supply the motor load. The reversing magnetic motor starter features a forward and a reverse starter as part of the assembly (see figure 2). Electrical and mechanical interlocks are provided to ensure only the forward or the reverse starter can be engaged at any given time, but not at the same time.
Comparison of NEMA and IEC starters
In this article we will focus on how NEMA (National Electrical Manufacturers Association) and IEC (International Electro-Technical Commission) relate to the selection and application of magnetic motor starters.
NEMA magnetic motor starters are available in various voltage and horsepower ratings with the following designations: sizes 00 through size 9, consecutively. These NEMA sizes classify a magnetic motor starter by voltage and maximum horsepower. Examples of AC voltages include 24V, 120V, 208V, 240V, 277V, 480V and 600V varieties. The magnetic motor starter is also offered in different types of enclosure depending on the environment in which the equipment will operate, not to mention DC coils. Typical protective enclosures are NEMA 1 (general purpose), NEMA 4 (watertight), NEMA 12 (dust-tight) and NEMA 7 (hazardous location).
IEC-style magnetic motor starters are usually available in a modular format with a contactor and an overload relay. Three-phase contactors are available in 208V, 230V, 460V and 575V variety with corresponding maximum horsepower ratings. IEC magnetic motor starters are often supplied as part of original equipment manufacturer (OEM) equipment, as are NEMA starters.
If we compare the NEMA magnetic motor starter to the IEC magnetic motor starter, we would notice the following differences:
IEC device typically is physically smaller than the comparable NEMA device but not in all cases, especially in larger sizes.
The life cycle can be different between NEMA and IEC devices. Performance evaluation between NEMA and IEC, as well as variances in how manufacturers develop the data (not validated by 3rd parties so test methods could vary greatly). The general safety performance of either IEC or NEMA devices is evaluated by a 3rd party testing agency in North America. The EU does permit self-certification, but manufacturers of NEMA devices also use self-certification for NEMA-specific performance characteristics. A NEMA controller is typically from an OSHA accredited lab, while an IEC device may be self-certified, with a CE mark, or certified by a lab that may not be OSHA accredited. NEMA starters can now be used in conjunction with electronic/solid state overload relays that are adjustable.
IEC device has an adjustable overload relay assembly, while the comparable NEMA device has a fixed and removable overload relay assembly. In addition, NEMA devises can be used in conjunction with electronic/solid state overload relays that are adjustable.
IEC device should normally be protected with fast-acting current-limiting fuses, while the NEMA device can be protected with conventional time-delay fuses, but this varies from product to product and from manufacturer to manufacturer.
Many IEC and NEMA devices are designed for use with conventional (non–current-limiting) fuses and circuit breakers, at least for standard fault SCCRs. Fact-acting, current-limiting may be used for high fault SCCRs and/or Type 2 coordination.
The end user should carefully consider all these requirements before making the decision to install a NEMA magnetic motor starter or an IEC magnetic motor starter in their specific application.
Read more by Stephen J. Vidal
Posted By Chad Kennedy,
Monday, September 03, 2012
Updated: Thursday, September 20, 2012
| Comments (0)
Selective coordination of the emergency system is intended to provide a high level of reliability and continuance of supply needed for systems providing emergency functions. As with all electrical installations, an effective plan review and installation inspection of the overcurrent protective devices (OCPDs) coordination for emergency systems is fundamental in approving and inspecting such systems. In addition, understanding which circuits require emergency systems also must be considered. Chapter 27 in the International Building Code (IBC) establishes a number of occupancies and systems that require emergency power such as egress illumination, exit signs, and hazardous materials occupancies. Let’s start by looking at the foundational elements for selectivity found in the NEC.
The 2011 NEC defines selective coordination in Article 100 as…
Coordination (Selective). Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.
In layman’s terms, selective coordination means that only the overcurrent protective device nearest to a fault should clear the fault. This applies to all fault types: overload, ground fault, and short circuit.
For emergency systems, the requirements for selective coordination are provided in NEC 700.27.
700.27 Coordination. Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.
The "mystery” of selective coordination compliance can be easily overcome with training. Where desired, current staff capabilities can be expanded with education and experience and several OCPD manufacturers have excellent programs to assist in this area. Although it is not necessary to understand the myriad of devices and equipment considered in a selective coordination analysis, enforcers intending to review coordination designs should understand the types of documents which should be supplied and how to interpret them.
How do I get started? From an enforcement perspective, there are two basic paths which can be taken. A community that has a plan review department often has the expertise to understand and review the coordination documents while smaller communities without plan review may have a policy which relies solely on the designer or professional engineer to provide the appropriate documentation for field inspection of settings.
It is important to remember that the NEC requirements apply to the installation; however, the performance requirements come from other documents. In addition to specifying which systems must have emergency power, Chapter 27 of the 2012 International Building Code requires compliance with NFPA 110 in Section 2702.1.
IBC 2702.1 Installation. Emergency and standby power systems required by this code or the International Fire Code shall be installed in accordance with this code, NFPA 110 and 111.
Health Care facilities establishes the minimum performance requirements for Healthcare Systems. Photo courtesy of John Watson
NFPA 110-2010, Standard for Emergency and Standby Power Systems, sets the minimum performance requirements for Emergency Systems and NEC Article 700 specifies the material and process associated with putting equipment in place and making it ready for use in accordance with the specified performance requirements. NFPA 110 Section 6.5.1 requires OCPDs be coordinated to optimize selective tripping and A.6.5.1 recognizes there are certain systems which require practical limits or levels.
NFPA 110, 6.5.1 General. The overcurrent protective devices in the EPSS shall be coordinated to optimize selective tripping of the circuit overcurrent protective devices when a short circuit occurs.
NFPA 110, Annex A, A.6.5.1. It is important that the various overcurrent devices be coordinated, as far as practicable, to isolate faulted circuits and to protect against cascading operation on short circuit faults. In many systems, however, full coordination is not practicable without using equipment that could be prohibitively costly or undesirable for other reasons. Primary consideration also should be given to prevent overloading of equipment by limiting the possibilities of large current inrushes due to instantaneous reestablishment of connections to heavy loads.
NFPA 99-2012, Health Care Facilities Code, establishes the minimum performance requirements for Healthcare Systems and NEC Article 517 specifies the material and process associated with putting equipment in place and making it ready for use in accordance with the specified performance requirements. NFPA 99 clauses 126.96.36.199.2, 188.8.131.52.1 and 184.108.40.206.1 set the performance requirements for Types 1, 2, and 3 essential electrical systems with additional explanation provided in Annex A clauses A.220.127.116.11.2, A.18.104.22.168.1 and A.22.214.171.124.1.
NFPA 99, 126.96.36.199.2Selective Coordination. Overcurrent protective devices serving the essential electrical system shall selectively coordinate for the period of time that a fault’s duration extends beyond 0.1 second.
NFPA 99, Annex A, A.6.4.212. It is important that the various overcurrent devices be coordinated, as far as practicable, to isolate faulted circuits and to protect against cascading operation on short-circuit faults. In many systems, however, full coordination could compromise safety and system reliability. Primary consideration also should be given to prevent overloading of equipment by limiting the possibilities of large current inrushes due to instantaneous reestablishment of connections to heavy loads.
While these performance requirements provide the minimum acceptable level, many system designers and engineers will design to obtain higher levels. It is also important to note that some jurisdictions across the country have set specific requirements for the qualifications of selective coordination design personnel and set performance levels which must be followed.
Reviewing the coordination documents is just the beginning, ensuring that the installation matches the coordinated design can be an equally challenging hurdle once the equipment has been installed. Manufacturers typically ship equipment with the circuit breaker settings at minimum levels and fusible disconnects without fuses installed. Depending upon the system design, there may be other adjustable devices such as ground-fault relays, circuit protection relays, zone-selective interlocking and energy-reducing maintenance systems with required settings and adjustments. Proper device verification for each circuit breaker or fuse and the settings for any adjustable devices are essential to the performance of the system. A systematic approach using the emergency system one line diagram and the selective coordination documentation is required to perform the system inspection including device type, and settings.
Since the selective coordination is device and settings dependent, how long will the system be coordinated after inspection? Replacement of fuses or circuit breakers with different types or from a different manufacturer has an impact on the selective coordination of the system. It is important that the maintenance staff understand these constraints and communicate this to their staff. In some cases, marking should be added to provide this communication at the device to ensure that quick response actions don’t impact the coordination by installation of the wrong fuse or breaker.
Key installation items to verify:
Compliance begins with understanding where emergency power is required. The IBC, Chapter 27, specifies systems and occupancies which must have emergency power.
Determination of the required performance level utilizing NFPA 110, NFPA 99 and any local jurisdictional rules.
A selective coordination study and resulting documents providing device details. Simply walking through and looking at the system devices will not verify the coordination unless a study has been completed.
Ensure adjustable trip settings for the system devices are appropriately set.
When using ratio tables for fuses or selectivity table for circuit breakers, confirm all of the fuses and breakers in the system are the appropriate size, type and from the same manufacturer.
Check the ground fault and other protection relay adjustments and settings.
Consider recommending markings to communicate proper adjustment of breaker settings, and replacement of fuses with specific manufacturer brand and type.
Selective coordination of emergency systems may seem like a difficult task but like many worthwhile challenges the most difficult step is getting started. The life safety importance of emergency systems warrant the performance and installation requirements provided in NFPA 99, NFPA 110 and NEC Article 700. Understanding the codes and standards requirements along with any jurisdictional regulations form the foundation for plan review and installation inspection of these systems. The key implementation items provide areas to verify and potential maintenance concerns to consider as the system is inspected.
Read more by Chad Kennedy
Posted By Jim Hayes,
Monday, September 03, 2012
Updated: Thursday, September 20, 2012
| Comments (0)
We seem to live in a world where technology changes weekly; we’re inundated with so much new hardware like smartphones and tablets and broad concepts like "Smart Grid” that it’s difficult to keep up-to-date. Everybody seems to think "fiber optics” is a new technology, too, although some of us have been involved with the technology for over thirty years!
Without these last three decades of fiber optics, we’d be missing a lot of communications we depend on — like the Internet, mobile smartphones or cheap worldwide phone calls. Computer networks would be slower and more expensive. Cities would not be able to ensure security with video surveillance or to manage traffic with smart traffic lights. If it involves data and/or communications, it probably involves fiber.
Within the same thirty years, think how many other technologies have been introduced, peaked and become obsolete: VCRs, CDs, desktop PCs, and more. Yet fiber optics has slowly matured, becoming the preferred medium for most communications.
Along the way, all those who install cabling have embraced fiber technology. I started training NECA electrical contractors how to install fiber optics over twenty years ago. Today the majority of these contractors install fiber optics for the same customers they have always served. Installers of data cabling, security systems, building management systems or just about any cabling inside buildings or out have been trained in fiber optic installation.
Thankfully, we rarely hear "fiber optics is expensive, hard to install and fragile because it’s made of glass” any more. On the other hand, fiber has become so common, we sometimes don’t hear about some of the more important uses of the technology and how it facilitates many other technologies.
Photo 1. Telecommunications. Photo courtesy of John Watson
Let’s start with data centers. All that Internet data that creates billions of web pages or enables "cloud” computing has to be stored somewhere and made easily accessible — over fiber optics, of course. What we call a "data center” is really a "data warehouse” with shelves of servers that find and distribute the data and disk storage from many terabytes of data. A typical data center will have thousands of links connecting servers to storage and consume megawatts of power.
Speed and power are the keys to data center networks (sometimes called SANs or storage area networks). 1 gigabyte Ethernet is being phased out for the 10 gigabyte version, and 40 and 100 gigabytes are becoming available. Copper cabling was OK for gigabyte links, but its development has been five years behind fiber at 10G and it’s not seriously being considered for 40 to 100G. Besides the bandwidth advantage, fiber consumes about ⅕ as much power per link as copper at 10G, and power consumption is a big issue for data centers.
Cellular Phone Networks
You are probably already addicted to smartphones or tablets and depend on cellular phone networks for these mobile devices. Cellular phones have always connected into the worldwide phone network that is based on fiber for long distance and metropolitan links. The growth in mobile data use is staggering. In its first 3½ years on the market, the iPhone alone caused an 8000% growth (that’s 80 times) in AT&T’s mobile network traffic. To meet traffic demand, cellular towers have been converting copper or digital radio to fiber connections to allow more radio bandwidth for user connections.
I’m sure you’ve noticed the proliferation of antennas on cellular towers to meet this demand for cellular bandwidth. Each of those antennas requires a connection to the base, typically with large coax cables running up the tower. These are being replaced with fiber, too, using composite cables that carry copper conductors as well as fiber, providing power and connections to all the antennas with a much smaller, less costly cable.
Photo 2. Traffic signal. Photo courtesy of John Watson
Besides widespread use of those smart mobile devices, we’ve increased our consumption of data in the home too, with Internet video being the fastest growing source of traffic. Streaming video, led by Netflix, now accounts for the bulk of Internet traffic. Video consumes vast amounts of bandwidth; just think about streaming a movie over your Internet connection that would fill a 4–5 gagabyte DVD!
To keep up with consumer demand, fiber is being brought closer and closer or even all the way to the home. CATV was an early adopter of fiber since it increased system reliability as well as allowing the first broadband Internet access using cable modems. Using the latest cable modem technology, DOCSIS 3, CATV can deliver Internet access up to 100 megabits/s or more.
Telecommunications companies (Telcos) are still trying to catch up with CATV for Internet access. Most telcos continue to use their old copper wires to connect the home but are installing fiber closer and closer to the home. Copper is limited in bandwidth by physics; longer links of copper telephone wire have considerably less bandwidth than shorter ones. While technology called DSL (digital subscriber line) can provide 10–40 megabits per second connections, it requires "clean” copper wires. Many homes in the U.S. have older copper that has deteriorated over the years, making DSL problematic. One telco guy I know says we’ve already mined all the copper we can for DSL — it’s time to convert to fiber.
And it’s fiber to the home (FTTH) that is the long-term solution for home connections. With fiber, you have virtually infinite bandwidth. Fiber costs more but has two potential paybacks: (1) the potential to sell consumers more services and (2) lower maintenance costs. In addition, new technology like passive optical networks (PON) that split one fiber link to serve up to 32 users can save lots of money, making fiber only slightly more expensive than DSL.
In 2005, a Telcordia survey said that FTTH could pay for itself in lower maintenance costs compared to the high maintenance required by older copper wires. Verizon took this report to heart, creating FiOS, the biggest FTTH system in the U.S., and saving billions of dollars in maintenance costs by replacing their aging copper cable plant with fiber. Converting customers from just POTS (plain old telephone service) to HDTV and Internet users added billions more in revenue too.
Photo 3. Fiber has been used by utilities for years. Photo courtesy of John Watson
Today, there are about 800 FTTH projects being built in the U.S. Google made waves in the industry by announcing they would build a gigabyte FTTH network in a U.S. city as a demonstration project. Kansas City (KS+MO) was chosen for this project that is now being installed. While Google was talking about gigabyte FTTH, Verizon actually demonstrated it in their FiOS network in Massachusetts. But the city of Chattanooga, Tennessee, outdid everybody, installing a citywide gigabyte FTTH network using the local electrical utility network.
Chattanooga’s gigabyte FTTH network truly illustrates the reasons for cities to install FTTH. The local telco and CATV companies did not do the work, but the city electrical utility had been looking at reading meters online. It was an easy — I said easy, not cheap — expansion to offer broadband Internet. But the city’s commitment to providing broadband helped convince Volkswagen to bring their U.S. manufacturing plant to Chattanooga, creating 3000 jobs initially and 2000 more in the future.
Other regions and cities are also promoting FTTH for its ability to attract high-tech jobs. A consortium of college towns has founded "Gig-U,” an organization dedicated to bringing gigabyte FTTH to their cities to attract tech companies. A startup has even received $200 million to provide those networks.
FTTH networks must have fiber equipment at every home (or subscriber, if in an apartment or condo building) to interface to the customer’s phone, TV and computer. These optical-to-electrical conversion boxes require power and connection to the customer’s devices, either through current cabling or cables installed by the system operator. Power generally is by AC power cubes although more complex uninterruptible power supplies are sometimes used to allow customer connections during power outages.
Cities are installing municipal networks for many other uses than FTTH. Municipal networks are expanding rapidly because they encompass many types of services, not just broadband Internet.
Cities and suburbs are installing communications systems to connect administrative, public safety and other city offices. They are installing surveillance cameras for security and traffic monitoring. Traffic signals are being monitored and controlled to improve traffic flow to save energy. Schools are being connected to provide high speed Internet to students and school administrations.
Wireless (WiFi) systems are being installed for both private municipal use and public access. Utilities are expanding their communications systems to increase efficiency and to read meters remotely. Many cities are also realizing that they can lease "dark fibers” to phone or CATV companies as well as other companies that desire high-speed connectivity.
Photo 4. Utilities with long distance companies to install fiber along high voltage lines. Photo courtesy of John Watson
While consumer demand for broadband drives much of the growth in fiber optics, many other applications are growing as well. The buzz about "Smart Grid” refers to the work being done to increase the efficiency and security of the power grid. Just imagine the scope of the problem. In the U.S., there are over 2000 utilities generating power and hundreds of thousands of photovoltaic solar systems pumping power into the U.S. power grid.
Keeping this network under control is a massive job. Last year here in Southern California, we had a power outage because a tech at one small generating station flipped the wrong switch and put 5 million users in the dark for the evening. Making a power grid more efficient requires communications and that’s done on fiber.
Utilities and Optical Power Ground Wires
Fiber has been used by utilities for decades. Utilities were among the first to recognize the advantages of fiber as a non-conductive communications cable that could not only connect generating or distribution facilities but even monitor high-voltage lines using fiber optic sensors. Utilities partnered with long distance companies to install fiber along high-voltage distribution lines, with the communications companies selling telecom services and the utilities getting free links for their use.
Together they pioneered an extremely useful cable design, a high-voltage transmission line with fiber in the center for communications. It’s called optical power ground wire (OPGW) and forms the communications backbone in many areas of the country. Since fiber is totally immune to electromagnetic interference, power lines can carry massive amounts of communications signals at much lower costs than when fiber cables must be installed independently.
Fiber is a major component of smart grid technology. One college we know is developing a smart grid program with a major utility, and fiber represents about one-third of the course. And as utilities add more fiber to manage their systems, they get closer to the home, eventually reaching what one person called FTTM — fiber to the meter — and they can offer gigabyte connections to every home.
Fiber has also made big inroads into industrial environments. Over the last few decades, most machines have become automated with computerized controls that send data back to computers that analyze and manage the factory floor. Auto plants use fiber for welding robots, critical assembly and inspection tools, control of other machinery and the assembly lines. Aluminum smelters are another place where fiber is popular because they use extremely high electrical currents in processing the aluminum that interferes with other communications media. Video cameras monitor the activity of workers, machines and materials. HVAC has become computerized in part to improve the factory environment but also to reduce energy costs.
Photo 5. In the desert in the southwest, solar generation stations are being built
Most factories have in common large physical plants and a harsh environment.
Industrial fiber optic cable plants usually require more protection than in commercial buildings. Cables are often run in metal conduit and terminations are made in sealed enclosures. Cables are sometimes run along the roof and dropped to floor locations, so extra support for the vertical runs can be an issue.
Business and Government Uses
Perhaps the most complex fiber optic cable plants are in the biggest buildings like airports, convention centers and other government facilities. These buildings are likely to include networks for phones and computers, indoor antennas for mobile WiFi and cellular devices, security cameras and entry systems, building management systems and entrance facilities with connections to many outside communications systems.
These large buildings have been used for some interesting experiments in cabling and networks. Here in the U.S., some contractors have been using FTTH PONs [fiber to the home, passive optical networks] scaled for use in buildings. A PON network has many advantages including easier installation with only one fiber per user, higher bandwidth capacity and lower cost. The cost is even lower than a copper network because of the network architecture and market volume; with millions of units having been built and installed in FTTH networks. Perhaps the two biggest advantages of a PON network in a building are easier network management and security. PON networks are scaled to handle millions of users so network management in a smaller building network is easy. And since it splits fibers to connect users using the same signal, it’s necessary to encrypt every user, increasing security.
A network architecture we saw recently in a foreign airport is making inroads into computer networks too. Called fiber to the office (FTTO), it uses a standard Ethernet architecture over fiber with small, inexpensive switches placed near the user’s desk and shared by four computers or VoIP phones. Besides replacing four big copper cables with one small fiber cable, it also eliminates the need for expensive telecom rooms on every floor of a building with their requirements for electrical power, AC and data-quality grounding.
Alternative Energy Systems and Fiber
Fiber has also become an enabler for energy. One of the earliest uses for fiber was in mines where it was used for its long distance capability and inherent safety since it does not involve electrical current in its transmission. Fiber was also used for energy exploration, allowing the connection of many seismic sensors over a large area on land and underwater remote vehicles to reach the deepest parts of the ocean (and discover the Titanic too). Pipelines rely on optical fiber running along their lengths for communications and monitoring for problems.
Perhaps the most interesting use of fiber in energy is in two growing fields, wind and solar power. Wind power requires careful monitoring of each tower to make it efficient as the wind changes and to connect it to the grid. Each tower is connected to fiber as part of the generation network.
Solar power is even more interesting. In the desert in the Southwest, several gigawatts of solar generation stations are being built. Unlike the typical small system that uses photovoltaic panels, these large systems combine hundreds or thousands of mirrors to focus extremely high power light to create steam that runs a generator. It’s really just like a nuclear power plant but the reactor is 93 million miles away — and much safer!
Some systems heat water or antifreeze to generate steam, but a new system uses plain old salt (NaCl) in the heat exchanger. The higher heat capacity of salt allows storing heat underground so steam can be generated during night hours, a big advantage over photovoltaic systems.
These solar systems require moving the mirrors continuously to keep the light focused properly. Every one of the mirrors spread out over hundreds or thousands of acres requires control and feedback, and that is provided by optical fiber. And of course the generators and heat exchangers require equally good monitoring and control, all on fiber.
How Fiber Impacts You
For electrical inspectors, most of what you need to know about fiber optics is in the NEC or on the Fiber Optic Association website (www.thefoa.org). We even have created with NECA an ANSI standard on fiber optic installation that describes what installing fiber optics "in a neat and workmanlike fashion” means. FOA provides the ANSI/NECA/FOA-301 Standard free to the industry. Contact FOA for your digital copy. But the proliferation of fiber uses in every aspect of today’s technical world means it’s harder to keep track of where the fiber is and what the fiber is doing.
Here at the Fiber Optic Association, the professional society of fiber optics, we try to track all the uses of fiber and help the industry keep up-to-date with our newsletters and websites. Most of our focus is on the designers and installers of fiber optic cable systems, but recently we’ve had requests from electrical inspectors for more information and even requests from IAEI chapters for seminars on fiber optics.
Generally, we send callers to our website or our online training website (fiberu.org) for free technical information online. For one chapter, the FOA sent one of our Master Instructors, Arnie Harris in Philadelphia, to speak. Arnie’s talk was well-received and we’re certainly willing to entertain more requests for seminars.
Read more by Jim Hayes
Posted By Randy Hunter,
Sunday, September 02, 2012
Updated: Wednesday, September 19, 2012
| Comments (0)
Outside branch circuits and feeders — what is so special about these circuits that we require a completely separate article when they are located outside? What makes them different from inside branch circuits and feeders? The scope of this article covers the requirements for branch circuits and feeders running on or between buildings, structure, or poles on the premises, which would also include the wiring for the supply of utilization equipment that is located on or attached to the outside of buildings, structures or poles.
Photo 1. Feeders — any conductors downstream of the main service disconnect is considered a feeder (to the last set of overcurrent devices).
Before we start this article, we need to review just a little. Some of these outside systems may look very similar to and can easily be confused with utility-owned wiring methods. In order to understand the difference between the two, we need to establish where the service point is located. Once this point is found, then anything that is both outside and downstream of the service disconnect would come under the scope of this article.
A new definition has been included to describe a substation. While substations are normally under the control of utilities, in some instances customers are supplied with power at higher than normal voltages from the utility and therefore own their own substations. This has created the need for theNECto include rules for these substations, as they are part of the system past the point of service and therefore under the scope of theNEC. Please review this new definition in 225.2. As with several other articles, in 225.3 we are referred to a table that lists other articles within the code that have additional requirements for specific situations or equipment.
Moving on to the general requirements, the first item is the requirement for conductor covering, which is required within 10′ of any building or structure other than supporting poles. There is an exception for the grounded or grounding conductor which may still be bare within 10′ of buildings, but only where specifically permitted elsewhere in the code. The ampacity of the conductors for under 600-volt systems will be according to the load as calculated in 220.10 and Part III of Article 220 and then meeting the requirements of Section 310.15. As usual in the code, not all the answers can be found in one location.
Photo 2. Here is the service point for the overhead conductors in photo 1.
By now we should be used to bouncing from article to article to complete an installation. It’s very important to be aware of how the code works and the interaction between various parts of the code. When teaching in a classroom style location, it is actually quite fun to play the game of asking where in the code do we go for this and that as we work through installation scenarios. Even those who are very good at the code will often find themselves searching the code for an exact reference, which they recall but can’t remember exactly where it is located. The more I deal with the code the more I find it important to locate the exact language to ensure I am using that portion of the code exactly as written and not just how I "remembered” it. This became especially critical in the position of AHJ. So the message here is not to feel like you are not good at the code if you can’t quote verse and chapter. We are really challenged because the code is a living document which changes every three years.
Conductor size and support requirements
We now step into 225.6 where we have the conductor size and support requirements. This is broken down into two categories, first Overhead Spans and then Festoon Lighting. Let’s identify where we commonly see these installations.
First, the overhead spans would be wiring going from building to building or pole to pole, usually contained within the confines of one’s property. These may be providing power to detached buildings, wells, signs, area lighting, or other exterior equipment. Depending on the voltage, over 600 volts or below, we are limited to how small the conductor is that may be used for these spans. Over 600 volts requires a minimum of 6 AWG copper, or 4 AWG aluminum, individual conductors; if using a cable assembly, the limits go down to 8 AWG copper or 6 AWG aluminum. For 600 volts or less, we are allowed to use 10 AWG copper or 8 AWG aluminum for spans up to 50 feet, and for spans over 50 feet you are required to step up one size to 8 AWG copper, or 6 AWG aluminum. There is one exception which is the use of a messenger wire, which normally utilizes some type of a steel cable or hybrid wire which has a center strand of steel; in this case, you do not have to increase conductor size when going over 50 feet.
Festoon lightingis not a commonly used term. If we check back in Article 100 we can find the definition, which is: "a string of outdoor lights that is suspended between two points.” This is the lighting method used for outdoor carnivals, flea markets, or holiday sales lots. Here we have the requirement that the conductors not be smaller than 12 AWG unless they are supported by a messenger wire, and if any spans exceed 40 feet then they shall be supported by a messenger wire. It goes further and even tells us how the messenger is to be supported, that being strain insulator devices. We are also told what the conductors or messenger wires may not be attached to; it is not permissible to attach to a fire escape, downspout or plumbing equipment. This makes sense when you consider the issues that may arise if we have a conductor get damaged and short out to the messenger wire, which may then energize a fire escape, downspout or plumbing system and present a dangerous condition. Often, these systems run over long distances and the current needed to have the overcurrent device operate in a timely function may not be optimal, so we can’t take the risk of energizing anything accidentally.
Photo 3. This shows a medical office building where the main service is in the building on the right and the emergency power system and generator are in the parking garage to the left (see inset photo).
In our part of the country (Southern Nevada), festoon lighting was something that we seldom had the opportunity to inspect during most of the year. However, during the fall it seems to come around for Halloween and the holiday season, where we have quite a few pumpkin sales locations and holiday tree lots. These often present several conditions which challenge a good inspector, as they seem to find unique ways to create power and lighting systems, often without the benefit of qualified electrical professionals. As these seasons would come around, I would remind the inspectors to review 225.6(B), to freshen their memory, and hopefully cut down the number of phone calls I would get.
Calculating loads for outdoor installations
Sections 225.7 and 225.8 deal with Lighting Equipment Installed Outdoors and Calculations of Loads 600 Volts, Nominal, or Less. Here we find some limitations depending on the voltages used and references to other code articles for load calculations. Most of this is not out of the ordinary, so we will move on to 225.10 where we have a list of wiring methods which shall be permitted for circuits not over 600 volts on the outside surfaces of buildings. My first question when reviewing this list is, what’s not there? The one wiring method that is definitely not there is nonmetallic-sheathed cable, which when we look it up in Section 334.12(B), we find it’s not allowed to be installed in a wet location. If we review the definition in Article 100, we find that anything on the exterior of a building is considered a wet location due to the fact it is subject to exposure to various types of weather. Another thing to consider is that due to condensation, various wiring methods run outside of buildings are subject to moisture inside conduits, as well as underground installations.
Moving on to 225.11 through 16 we find requirements for conductor entrance and exit from buildings, open conductor supports, spacing, supports over buildings and attachment to buildings. Most of these are simply references to other code articles where we have the language detailing these installations. Most of them refer us to Article 230, Services, which we will cover in the next article in this series.
Outdoor installations often have many similarities whether it is a branch circuit, feeder or service conductors. Some of the installation methods overlap, and the main article used is the one for the most commonly found outside circuit, which is Services, Article 230.
Where violations frequently occur
Next we have to cover a couple of areas where violations occur frequently, Masts as Supports, 225.17 and Clearance from Overhead Conductors and Cables, 225.18. Most of the violations happen in the clearances area. In 225.17 we cover masts as supports and here we find an interpretive term. The language states that where a mast is used as the final span support it shall be of "adequate” strength. First, we have to understand that the final spans are the ones at the end of a run. Those supports in the center of a run have equal tension pulling in two directions, so they basically just need vertical strength; however, the end supports have the pull in just one direction, and therefore have to be strong enough for the lateral pull in one direction. Now what is adequate when you are an inspector? We have run across this issue several times, and locally we just defaulted to the local utility standard, and what they found is that intermediate metallic conduit (IMC) or rigid galvanized steel (RGS) are of sufficient strength for their overhead service drops, so we adopted this standard for outside feeders and branch circuits, as well.
The most critical issue, in my eyes, is the clearance for overhead conductors and cables, which is dealt with in 225.18. We have four basic rules that consider two factors, (1) the voltage of the conductors and (2) the anticipated traffic under the overhead spans. The required distances for clearances are measured from finished grade, sidewalks, or any platform or projection from which the conductors might be reached. Starting out with the lowest clearance, which is 10 feet, this is the minimum clearance for conductors operating at 150 volts or less to ground and subject only to pedestrian traffic. Moving on to condition 2, which is over residential property and driveways and commercial areas not subject to truck traffic, where the conductors operate at 300 volts or less to ground, here we find the minimum clearance at 12 feet. If we have the same conditions but we have a voltage over 300 volts, we make the clearance move up an extra 3 feet to a total of 15 feet. So what have we left out? Only the most important condition, in my thoughts, and that is the area over public streets, alleys, and roads subject to truck traffic; driveways other than residential; and other land traversed by vehicles such as cultivated, grazing, forest and orchard. In these locations, we must maintain an 18-foot clearance no matter what the voltage. A new requirement for the 2011 Code is the clearance requirement over railroad tracks, which is 24.5 feet.
Photo 4. Example of festoon lighting
Continuing with clearances, we next review 225.19, which is titled Clearances from Buildings for Conductors of Not over 600 Volts, Nominal. In (A) Above Roofs, we find the general rule which states that the conductors and cables will be 8 feet above the roof surface and that height shall be maintained for a distance of 3 feet in all directions from the edge of the roof. The main thought here is that the conductors will be high enough that you won’t come into contact when working on a roof. As with many sections of the code, the general rule is followed by several exceptions; and in this case, we have four. I’ll go into detail on one of them and then challenge you to review the rest on your own. In Exception 2, where the roof has a pitch of 4 inches of elevation change in 12 inches of horizontal distance, which makes it quite steep and therefore un-walkable, the clearance may be reduced to 3 feet.
Continuing with clearances in (B) the clearance from signs, chimneys, radio and TV antennas, tanks and other non-building or non-bridge structures, the clearance shall be not less than 3 feet in any direction. This feeds right into (C) which covers horizontal clearances, which also is 3 feet.
The next clearances covered are those for the final span when it is near any windows, doors, porches, balconies, ladders, stairs, fire escapes, platforms, projections or similar surfaces from which the conductors may be reached, bunching all of these together as they require, again, a 3-foot clearance to conductors. With all of these distances and clearance requirements it would just be a natural assumption that conductors should not pass below any openings through which items are moved, and that very requirement is explicitly stated in (D)(3). The most obvious location mentioned is a barn loft through which things are loaded.
Vegetation as Support
Skipping down to 225.26 Vegetation as Support, vegetation such as trees shall not be used for support of overhead conductor spans. This is very commonly violated. For some reason people just have a difficult time telling the difference between a wooden pole and a living tree, I guess.
Raceway Seal is a new item in 225.27 added in the 2011NEC. A similar requirement was covered in a previous article, but I think it is important to mention that any underground raceway entering a building shall be sealed.
Supplied by Feeders or Branch Circuits
This brings us to Part II of Article 225, which is Buildings or Other Structures Supplied by a Feeder(s) or Branch Circuits(s). If you are familiar with Article 230 (or have been cheating and reading ahead), you will notice that just about every item in this section is also covered in 230. This further supports the idea that services and outside feeders and branch circuits have many conditions which are the same. With this in mind and to reduce repetition, I will only touch on the unique issues here and we will cover the rest of the basics in our next article.
In 225.30 Number of Supplies, we are informed that only one feeder or branch circuit may supply a building, and of course there are exceptions. Note that for the purpose of this section, a multiwire branch circuit shall be considered as a single circuit. Also, a new item was added for the 2011 Code which states that if a feeder or branch circuit originates at one of these buildings, and is to feed something back in the original building or structure, only one circuit may feed back. This may sound totally confusing, but here is an example: the main building has the normal power service, and therefore has a feeder out to the parking garage, and this feeder goes to a panelboard which feeds all the normal lighting circuits. In this garage is located the emergency generator and the emergency distribution system. It feeds all the emergency lights for the stairs and other areas of the garage, but it also feeds the emergency power for the main building. Well, the code doesn’t allow twenty branch circuits to be fed back to the main building for this application. One circuit or feeder would feed to the main building from the emergency equipment in the garage; and once back at the main building an emergency panelboard would have to be set to feed the individual circuits within that building.
Access to Overcurrent Protective Devices
The last item to cover for 225 is 225.40 Access to Overcurrent Protective Devices, which covers the situation where one may not have ready access to the feeder overcurrent device. This could be for a variety reasons; for instance, you may be a tenant in a separate building on a property under one ownership. When that tenant does not have access to the feeder overcurrent device, the code requires that branch-circuit overcurrent devices shall be installed on the load side of the feeder, that they shall be readily accessible, and — a key point here — they shall be sized smaller than the feeder overcurrent device. The idea here is if you have an issue and a device opens, generally it will open where the tenant has access to it and can restore the power when the problem is solved. This will work most of the time, unless a high-fault current issue happens, which may also take out the feeder device on the line side.
This concludes our discussion of Article 225. Again, I challenge you to open the code and review the actual code language. Time and space certainly limit the amount of coverage possible when writing these articles, so please follow along and fill in the voids. Also, now would be a good time to compare 225 and 230 to see the similarities and be prepared for the next installment when we will dive into Article 230, Services.
Read more by Randy Hunter