Posted By Tim Crnko,
Tuesday, May 01, 2007
Updated: Sunday, February 10, 2013
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Part I, which appeared in the March/April issue, provided readers with information about basic operation and basic time-current characteristics of branch-circuit, low-voltage fuses and circuit breakers. This article covers three overcurrent protective device ratings, their application in design, and NEC compliance aspects of low-voltage branch-circuit fuses and circuit breakers. These overcurrent protective devices (OCPDs) are typically used in main service disconnects, feeders and branch circuits of residential, commercial, institutional, and industrial electrical systems. There are other OCPDs used in 600 V or less electrical distribution systems that this article does not directly address. However, many of these principles presented also apply to the other type devices. This article focuses on the basics and as you probably already know, the Code is comprehensive and complex. As a consequence, the information in this article cannot be assumed to be applicable for all types of applications and wiring situations.
Why Overcurrent Protection Is So Important
Table 1. Maximum Rating or Setting of Protective Devices*
Too often, installations are not safe due to improper selection, application, or maintenance of overcurrent protective devices. Improper application of a device’s voltage rating, current rating, or interrupting rating can result in equipment damage, injury, and/or death. For example, if a fuse or circuit breaker is chosen with the wrong ampere rating, the electrical equipment may not be protected under overload or short-circuit conditions, allowing destruction of the equipment, fire hazards and possible injury to personnel. If a fuse or circuit breaker does not have an adequate voltage rating, it can rupture while trying to interrupt an overcurrent. Finally, both fuses and circuit breakers can violently explode attempting to interrupt fault currents beyond their interrupting ratings. As an industry, we need to do better at specifying, installing, inspecting, and maintaining proper overcurrent protective device ratings for the application. It starts with understanding the OCPD ratings, how to apply them and Code requirements.
At a risk of oversimplification, the ampere rating of a fuse or circuit breaker is the maximum amount of current that it can safely carry without opening, under standard test conditions. Fuses and circuit breakers have a range of ampere ratings. NEC 240.6 lists the standard ampere ratings for fuses and inverse time circuit breakers. Standard amperage sizes of the Code are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000 and 6000. Additional standard ampere ratings for fuses are 1, 3, 6, 10 and 601. Manufacturers provide OCPDs in other ampere ratings and the use of these non-standard ampere ratings is permitted. Figure 1 illustrates a 200 A fuse and figure 2 illustrates a 225 A circuit breaker.
In selecting the proper OCPD’s ampere rating for an application, consideration must be given to the type of load and Code requirements. What is interesting is that the Code has so many different rules for determining the maximum fuse or circuit breaker ampere rating for different circuits. There are:
- Static loads such as heating where the normal current stays within an envelope of the full-load current or less, and does not have start-up currents greater than the circuit ampere rating.
- Devices that have momentary inrush currents, such as transformers, where the energizing current greatly exceeds the normal full current rating of the transformer.
- Loads that have high starting currents, such as across the line started ac motors which have starting currents four to six times the normal ampere rating that may persist for several seconds.
- Permitted tap conductor rules where conductors are tapped from larger ampacity conductors without an OCPD at that specific tap point.
The Code requirements have the intent to protect conductors and circuit components at their ampacities. What you find is that fuses and circuit breakers are either intended to provide:
- both overload and short-circuit protection, and are located at the line side of the circuit to be protected. (Examples would be heating and lighting branch circuits.), or
- only short-circuit protection, and are located at the line side of the circuit to be protected. In these cases, another device that is intended to provide overload protection is typically required and can be located further downstream. (An example would be a motor branch circuit.)
NEC 240.4 (2005) requires conductors, other than flexible cords and fixture wires, shall be protected against overcurrent in accordance with their ampacities as specified in 310.15, unless otherwise permitted or required in (A) through (G). There can be other components on a circuit, such as disconnects and contactors, and other Code sections require proper ratings so that overload protection is provided for these other components.
The general rule, for which there are numerous variances, is the ampere rating of a fuse or circuit breaker should not exceed the current-carrying capacity of the conductors. As a general rule, the ampere rating of a fuse or a circuit breaker is selected at 125 percent of the continuous load current. Since the conductors are also typically selected at 125 percent of the continuous load current, the ampacity of the conductors is typically not exceeded. For instance, for a 40 A continuous load, the conductor must be rated to carry 50 amperes (125% of 40A) and a 50-ampere fuse or circuit breaker is the largest that should be used (see figure 3).
As mentioned earlier, there are specific circumstances in which a fuse or circuit breaker ampere rating is permitted to be greater than the current-carrying capacity of the circuit. A typical example is motor circuits; dual-element, time-delay fuses generally are permitted to be sized up to 175 percent (or the next standard size if 175 percent does not correspond to a standard size fuse) of the motor full-load amps. For instance the motor circuit in figure 4 would allow fuses sized at 1.75 x 34 A = 59.5 amps. The next standard size is 60 A. Conductors would be sized (per 430.22) at 34 x 1.25 = 42.5 A minimum. An 8 AWG, 75°C conductor would be selected (50 A ampacity per 310.15 and Table 310.16), assuming the terminations are rated for 75°C conductors. This 60 A time-delay fuse is allowed because the required overload relay or "heater” will be sized 125 percent or less (assuming 1.15 SF motor) of the motor full-load amps and provides the overload protection for the circuit. Since the conductor is also sized at 125 percent of the motor full-load amps, the overload relay is intended to protect the conductor from overloads because it is sized at or less than the conductor capacity. Table 1 is a summary of the maximum ratings for common fuses and circuit breakers for single-phase and three-phase motors permitted per NEC 430.52 and Table 430.52. For this example, non-time delay fuses could be sized at 110 A and an inverse time circuit breaker could be sized at 90 A (for this same 8 AWG, 50 A conductor motor circuit application) [see figure 4].
There are additional exceptions, such as when the fuse-switch combination or circuit breaker is approved for continuous operation at 100 percent of its rating.
It is suggested readers review 240.4(A) though (G) for other permitted conductor protection compliance requirements. A few examples are following.
NEC 240.4(A) does not require conductor overload protection for fire pump circuits (see figure 5).
NEC 240.4(B) (2005 edition) allows the next higher standard OCPD rating (above the ampacity of the conductors being protected) to be used for OCPDs that are 800 A or less if the conductor ampacity does not already correspond to a standard OCPD size and if certain other conditions are met (see figure 6).
NEC 240.4(C) requires the ampacity of the conductor to be equal to or greater than the rating of the OCPD for overcurrent devices rated over 800 A (see figure 7). If there are no further changes in the 2008 Code process for this item, this requirement will be changed, allowing for the next standard sizes with certain limitations.
NEC 240.4(D) requires the OCPD shall not exceed 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG copper; or 15 A for 12 AWG and 25 A for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied. This is required unless specifically permitted in 240.4(E) through (G) [see figure 8].
One caution is in order. Selecting an OCPD ampere rating following the Code ampacity sizing rules does not ensure short-circuit protection for all circuit components. There are circumstances where further requirements come into play for short-circuit protection. However, this article is unable to adequately cover this topic.
Very simply, the voltage rating of a fuse or circuit breaker is the highest voltage that the fuse or circuit breaker is capable of safely interrupting, for all overload and short-circuit conditions at which it is rated to interrupt, under standard test conditions. The proper application of an overcurrent protective device according to its voltage rating requires that the voltage rating of the device be equal to or greater than the system voltage. For instance, a 600-volt fuse or circuit breaker can be used in a 575-V, 480-V, 208-V, or 120-V circuit. However, a 250-volt fuse or circuit breaker is not suitable for 480-V or 277-V applications.
There are two physical aspects to proper voltage rated OCPDs:
- Sufficient creepage and clearance distances to ensure there is not a conductive path or flashover between conductive parts of different phases, phase to neutral, or phase to ground. Figure 9 illustrates the creepage and clearance distances at the terminations of a disconnect. Circuit breakers and fuse holders/disconnects have minimum spacing requirements for specific voltage levels. Adequate creepage and clearance verification, ensuring a product is properly listed for an application, is evidenced by a NTRL mark that the product meets a specific product standard that is suitable for the application.
- The voltage rating of an OCPD is also a function of its capability to open a circuit under an overcurrent condition. Specifically, the voltage rating determines the ability of the OCPD to suppress and extinguish the internal arcing that occurs during the opening of an overcurrent condition. If an OCPD is used with a voltage rating lower than the circuit voltage, arc suppression and the ability to extinguish the arc will be impaired and, under some overcurrent conditions, the OCPD may not clear the overcurrent safely.
OCPDs can be rated for ac voltage, dc voltage, or both. Often an ac/dc voltage rated OCPD will have an ac voltage rating that is different from its dc voltage rating. For instance, some fuses are rated 600 Vac and 300 Vdc. When referencing manufacturers’ datasheets, if the rating is described as 600 V, this rating is typically assumed to be ac. However, product markings should be explicit such as 600 Vac or 600 Vdc.
There are two types of OCPD ac voltage ratings: straight voltage rated and slash voltage rated. The proper application is straightforward for overcurrent protective devices with a straight voltage rating (i.e., 600 V, 480 V, 250 V) which have been evaluated for proper performance with full phase-to-phase voltage used during the testing, listing and marking. For instance, all fuses are straight voltage rated and there is no need to be concerned about slash ratings. However, some circuit breakers and other mechanical overcurrent protective devices are slash voltage rated (i.e., 480/277 V, 240/120 V, 600/347 V). Slash voltage rated devices are limited in their applications and extra evaluation is required when they are being considered for use. This will be discussed under "Voltage Rating—Circuit Breakers.”
Voltage Rating — Fuses
Most low-voltage power distribution fuses have 250 V or 600 V ratings. Other fuse ratings are 125 V, 300 V, and 480 V. NEC 240.60(C) requires the voltage rating of cartridge fuses to be plainly marked on the fuse. NEC 240.61 allows fuses rated 600 V or less, to be used for voltages below their rating. NEC 240.60 (A)(2) allows 300 V rated cartridge fuses to be permitted on single-phase line-to-neutral circuits supplied from 3-phase, 4-wire, solidly grounded neutral source where the line-to-neutral voltage does not exceed 300 V. This allows 300-V cartridge fuses to be used on single-phase 277-V lighting circuits. Some Class T fuses are 300-V rated.
Voltage Rating — Circuit Breakers
Most circuit breakers used in low-voltage, power distribution applications have a voltage rating of either 125 V, 250 V, 480 V or 600 V. NEC 240.83 (E) requires the voltage rating of circuit breakers to be marked and not be less than the nominal system voltage.
NEC 240.85 details special requirements for the voltage rating of circuit breakers such as slash ratings. Some circuit breakers and other multiple-pole, mechanical overcurrent protective devices, such as self-protected starters and manual motor controllers, may have a slash voltage rating rather than a straight voltage rating. A slash voltage rated overcurrent protective device is one with two voltage ratings separated by a slash and is marked such as 480Y/277 V or 480/277 V (see figure 10). Contrast this to a straight voltage rated overcurrent protective device that does not have a slash voltage rating limitation, such as 480 V.
With a slash rated device, the lower of the two ratings is for overcurrents at line-to-ground voltages, intended to be cleared by one pole of the device. The higher of the two ratings is for overcurrents at line-to-line voltages, intended to be cleared by two or three poles of the circuit breaker or other mechanical overcurrent device. Slash voltage rated overcurrent protective devices are not intended to open phase-to-phase voltages across only one pole. Where it is possible for full phase-to-phase voltage to appear across only one pole, a full or straight rated overcurrent protective device must be utilized. An example of an application where a 480-V circuit breaker may have to open an overcurrent at 480 V with only one pole is when Phase A goes to ground on a 480-V, B-phase, corner-grounded delta system. Slash voltage ratings for circuit breakers are addressed in NEC 240.85 restricting their use to solidly grounded systems where the line-to-ground voltage does not exceed the lower of the two values and the line voltage does not exceed the higher value.
Overcurrent protective devices that may be slashed rated include, but are not limited to:
- Molded-case circuit breakers — UL 489
- Manual motor controllers — UL 508
- Self-protected Type E combination starters — UL 508
- Supplementary protectors — UL 1077
Two other special requirements are detailed for the voltage rating of circuit breakers in NEC 240.85:
- A circuit breaker with a straight voltage rating, such as 240 V or 480 V, shall be permitted to be applied in a circuit in which the nominal voltage between any two conductors does not exceed the circuit breaker’s voltage rating (see figure 12).
- A two-pole circuit breaker shall not be used for protecting a 3-phase, corner-grounded delta circuit unless the circuit breaker is marked 1Φ–3Φ (see figure 13).
NEC Article 100 defines interrupting rating as "The highest current at rated voltage that a device is intended to interrupt under standard test conditions.”
The rating that defines the capacity of an overcurrent protective device to maintain its integrity when clearing fault current is termed its interrupting rating. Interrupting rating for fault interruption is primarily related to the fuse or circuit breaker integrity to interrupt the fault current; it is not a rating that ensures protection for all downstream circuit components.
NEC110.9 requires devices that interrupt current to have a sufficient interrupting rating for the current that must be interrupted. Section 110.9 recognizes the difference between interrupting operating current and interrupting fault current. Circuit breakers and fuses are devices intended to break current at fault levels and 110.9 requires them to have an interrupting rating sufficient for the available short-circuit current at their line terminals. Equipment, such as disconnects and motor controllers, intended to interrupt operating current are required to be rated for the current that must be interrupted such as load current or motor locked-rotor current. This article pertains to fault current interruption by fuses and circuit breakers.
Figure 14 shows four sequenced photos taken from high speed filming of a test of a pair of 600-V, one-time fuses where the short-circuit current exceeded the fuses’ interrupting rating. These fuses have an interrupting rating of 10,000 A at 600 V. However, the test circuit was capable of delivering 50,000 A of short-circuit current at 480 V. This is a misapplication since the fuses do not have sufficient interrupting rating for the application. Notice in this test the large amount of destructive energy released by these devices as they rupture violently.
Minimum Interrupting Rating
NEC 240.60(C) states that the minimum interrupting rating of branch-circuit cartridge fuses is 10,000 A. NEC 240.83(C) states that the minimum interrupting rating of a branch-circuit circuit breaker is 5,000 A. A branch-circuit fuse or a branch-circuit circuit breaker must be properly marked if the interrupting rating exceeds these minimum ratings, respectively. These minimum interrupting ratings and markings do not apply to supplemental protective devices such as glass tube fuses or mini-breakers (supplementary protectors — UL 1077).
Figure 1 shows a fuse that has a UL Listing of 300 kA interrupting rating at 600 Vac and 100 kA interrupting rating at 300 Vdc. The interrupting rating for a given circuit breaker typically varies based on the system voltage. Figure 2 shows a circuit breaker with different interrupting ratings corresponding to various application voltage levels.
In figure 15 circuit, what interrupting rating must the fuse have?
Answer: at least 50,000 amperes. Classes R, J, T, L and CC fuses have an interrupting rating of at least 200,000 amperes.
Question:In figure 16, what interrupting rating must the circuit breaker have?
Answer:Some value greater than or equal to 50,000 amperes. It is important to realize that circuit breakers come in a wide variety of interrupting ratings. For example, a circuit breaker may be rated 10,000 A, 14,000 A, 18,000 A, 22,000 A, 25,000 A, 30,000 A, 35,000 A, 42,000 A, 50,000 A, 65,000 A, 100,000 A or 200,000 A. In addition, circuit breaker interrupting ratings are dependent upon voltage. Thus, a 480-V circuit breaker may have an interrupting rating of 65,000 A at 240 V, but 25,000 A at 480 V.
Is proper interrupting rating a problem in the industry? Many times the author has had people relate seeing a marked 42 kA panelboard with a 10 kA interrupting rated circuit breaker installed amongst the 42 kA circuit breakers. Or, 10-A IR Class H fuses installed where there is more than 10 kA short-circuit current available. These two examples are serious safety hazards. Many industrials are having arc-flash hazard studies performed for their facilities in order to provide a safer work place for their workers. The author’s firm does flash-hazard studies for industrial, commercial, and institutional facilities and the findings are a bit disconcerting. Numerous situations are being identified where the available fault current exceeds the interrupting rating for installed circuit breakers and fuses. Another industry situation: utilities routinely replace transformers due to greater kVA size required for facilities’ expansion or because the prior unit failed. Often the outcome is higher available short-circuit currents in the facilities which then may result in the installed OCPDs having inadequate interrupting ratings.
To ensure an electrical system is compliant with NEC 110.9 requires knowledge of the available short-circuit current at the line side of each overcurrent protective device. As depicted in figure 17, it becomes necessary to determine the available short-circuit currents at the location of each protective device. The fault currents in an electrical system can be easily calculated if sufficient information about the electrical system is known. However, this article does not address how to calculate the available short-circuit currents. There are easy to use tabular methods, hand calculation methods, as well as software applications that can be used to determine the available short-circuit currents in a system. Also, there are easy to use rules of thumb that may be used in certain situations.
As if this is not enough to know, there is more about interrupting rating. Generally, a circuit breaker should not be applied where the available short-circuit current at its line side terminals exceeds the circuit breaker’s interrupting rating. This is a requirement per 110.9. However, 240.86 has an allowance for fuses or circuit breakers to protect downstream circuit breakers where the available short-circuit current exceeds the downstream circuit breaker’s interrupting rating. The term given to this is a series rated combination, series rating, or series combination rating. The application of series ratings has many technical limitations and additional Code requirements that must be met for proper application. Series rated combinations allowed per 240.86 should be used sparingly. The most suitable and often the only proper application of series rated combinations is for branch circuit, lighting panels. Interested readers can obtain information from various industry sources on series ratings; the author’s company website has explanatory and application materials on series ratings, including a compliance checklist. Figure 18 illustrates the concept.
Single-Pole Interrupting Capability
The single-pole interrupting capability of a circuit breaker, self-protected starter and other similar mechanical overcurrent protective devices, is its ability to open an overcurrent at a specified voltage utilizing only one pole of the multi-pole device (see figure 19). Multi-pole mechanical overcurrent protective devices are typically marked with an interrupting rating. This marked interrupting rating applies to all three poles interrupting a three-phase fault for a three-pole device. The marked interrupting rating of a three-pole device does not apply to a single pole that must interrupt a fault current at rated voltage.
There are electrical systems with specific grounding methods that may require a three-pole circuit breaker to interrupt fault current at full voltage across only one pole. NEC 110.9 requires an overcurrent protective device to have an interrupting rating equal to or greater than the fault current available at its line terminals. This includes whether the device is interrupting the fault via a single pole or multi-poles. A fine print note was added to 240.85 of the 2002 NEC and 430.52(C)(6) of the 2005 NEC. These fine print notes alert users that mechanical devices, such as circuit breakers and self-protected combination controllers, have single-pole interrupting capabilities that must be considered for proper application. Although most electrical systems are designed with overcurrent devices having adequate three-phase interrupting ratings, the single-pole interrupting capabilities are easily overlooked. The electrical systems where this should be investigated are ungrounded systems, high-impedance grounded systems, and corner-grounded delta systems. These types of systems have long been common for continuous process applications and are becoming more commonly used for other applications in order to reduce the probability of flash hazards. The website of the author’s company has explanatory materials on single-pole interrupting capabilities.
The information in this article Overcurrent Protection Basics, Part II provided information on three important fuse and circuit breaker ratings: ampere rating, voltage rating and interrupting rating. These important criteria lay the foundation for a better understanding of overcurrent protection and code-compliance.
Read more by Tim Crnko
Posted By Jim Dollard,
Tuesday, May 01, 2007
Updated: Sunday, February 10, 2013
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What does the term electrical safety really mean? Stop reading for a moment and think about what electrical safety means to you. The hazards are electrical shock, arc-flash and arc-blast. Electrical safety is achieved by taking all of the necessary steps to provide our homes with safe electrical systems and by ensuring that everyone goes home from the job at the end of each day without suffering from shock, arc-flash or arc-blast.
When we discuss electrical safety, we must realize everyone uses and is exposed to electrical energy every day. We achieve the necessary level of electrical safety for all individuals both at home and on the job through two methods: (1) providing an electrical installation free from electrical hazards, and (2) through the implementation of electrical safe work practices.
Electrical Installation Requirements
Through the use of theNational Electrical Codewe provide installations that are essentially free from electrical hazards for the general public. This level of electrical safety achieved through implementation of an installation code, the NEC, also protects employees in their workplace. It is important to note that these safety-driven installation requirements are intended to protect persons who do not encounter exposed live parts or interact with electrical equipment. Electricians, inspectors, HVAC technicians, elevator technicians, maintenance personnel and others are exposed to electrical hazards when and where they access energized parts and/or interact with electrical equipment. As these employees become exposed to energized parts or they interact with electrical equipment, the safety-driven installation requirements of the NEC no longer protect them from electrical hazards.
Electrical Safe Work Practices
Protecting persons who are exposed to live parts or persons who interact with electrical equipment requires another level of safety, which is not provided by an installation code like the NEC. These individuals need to implement electrical safe work practices. Federal law requires that all employees be protected from all electrical hazards. OSHA requires that persons exposed to electrical hazards be protected. In both the 1926 Construction and in the 1910 General Industry Standards, OSHA mandates that employers shall protect employees. OSHA requirements for protecting employees who are exposed to live parts or interact with electrical equipment are for the most part "performance based” and do not give the employer adequate methods and means to protect employees from electrical hazards. NFPA 70E shows the employer when energized electrical work is justified and how to provide the necessary protection for the employee. NFPA 70E is a consensus standard and is not adopted locally as is the NEC. An employer who complies with NFPA 70E will meet the required levels of protection mandated by OSHA. To many electricians, inspectors and other workers who may be exposed to electrical hazards, it seems that the requirements of NFPA 70E are new. The talk around the gang-box on many jobsites and over lunch becomes a debate on whether or not we need to comply with these requirements. The reality is that Federal law requires that employers protect all employees from the hazards of electricity. OSHA is the shall and NFPA 70E shows the employer how.
The Requirements of the NEC
The NEC contains many safety-driven installation requirements that protect employees in the workplace. These requirements range from workspace clearances, GFCI protection, overcurrent protection, bonding/grounding and many other installation-based requirements designed to protect persons and property from hazards that may arise from the use of electricity. The NEC has evolved over the last few cycles by beginning to incorporate requirements during the installation of electrical systems that will provide safer working conditions for those who will repair, maintain, modify or renovate the electrical system.
The NEC has embraced multiple changes over the last few cycles that illustrate the trend in our industry to provide safer working conditions for all persons exposed to electrical hazards. These changes include but are not limited to the following sections in the 2005 NEC:
Article 100 definition of Qualified Person
110.16 marking requirements to warn persons of arc-flash hazards
110.26(C) and 110.33(A) requirements for doors to open in the direction of egress
210.4(B) simultaneous disconnect for multiwire branch circuit supplying more than one device or equipment on the same yoke
210.5(C) identification of ungrounded branch-circuit conductors
215.12 identification of all feeder conductors
240.86 engineered series rated systems in existing buildings
404.15(B) requires the term off to completely disconnect all ungrounded conductors
410.73(G) requires disconnecting means for ballast replacement
422.31(B) provisions for adding a lock to a disconnect
430.102(B) disconnect within sight of motors
490.46 ground bus for connection of safety grounds
These are just a few examples of how the NEC is recognizing the needs of those who will maintain, repair and renovate electrical systems. These revisions provide those who will maintain repair or renovate these systems with necessary information and modifications to facilitate safe working conditions.
The 2008 NEC has continued the trend towards providing installations that facilitate safe maintenance, repair and renovation. Dozens of locations in the NEC have been modified to require that a means to lock or add a lock to a required disconnect remain in place with or without the lock installed. This will allow anyone to effectively lock out a disconnect with just a lock. Electrical workers should not be expected to carry dozens of different types of lockout devices and hope that they have one to fit every switch or circuit breaker which needs to be locked out.
NFPA 70E, Standard for Electrical Safety in the Workplace
Implementing safe electrical work practices begins with an overall electrical safety program that directs activity appropriate for the voltage, energy level and circuit conditions to which employees may be exposed. The development of a written electrical safe work practice plan documents the overall program. All employees must be trained in electrical safety. The level of training in many cases may be different depending upon the tasks to be performed by the individual. NFPA 70E recognizes two categories of employees with respect to electrical safe work practices, qualified and unqualified. An electrical contractor or inspection agency may employ both qualified and unqualified employees. The difference is that an unqualified person is not recognized as being capable of working exposed to energized parts or interacting with energized electrical equipment.
At a minimum all employees must be trained (1) to understand and recognize all electrical hazards and their effects on the human body—shock, arc-flash, arc-blast, and (2) how to recognize an electrically safe work condition.
An electrically safe work condition is in essence a situation free from all electrical hazards. An electrically safe work condition exists when an individual is working on a piece of equipment or circuit which cannot be energized from any source. This situation is only possible: (1) when the equipment or circuit cannot be energized because the conductors to supply the equipment have not been installed, or (2) the equipment is effectively locked out and tagged out. See NFPA 70E, section 120.1, for detailed steps to achieve an electrically safe work condition.
In addition to understanding electrical hazards and an electrically safe work condition, qualified persons must receive additional training. Qualified persons must be trained to distinguish energized parts from other parts, to determine nominal voltages, to determine approach distances and in the decision-making process for determining the degree and extent of a hazard and the necessary planning and personal protective equipment needed to perform the task safely.
The Role of the Electrical Inspector
The electrical inspector plays a critical role in electrical safe work practices. Inspectors must take prudent steps to protect themselves from electrical hazards. In-house training of all inspectors in safe work practices is not an option; it is a requirement. The inspector gets to see firsthand how the electrical contractor operates. As an inspection is occurring, the inspector will certainly see if energized work has been or is being performed. The inspector should require that the electrical contractor schedule an inspection before the equipment is energized. Exposing yourself and others to electrical hazards to perform an inspection may not be justified by NFPA 70E and may create serious safety and liability concerns. NFPA 70E recognizes energized work as being justifiable in two situations: (1) infeasibility, the task cannot be performed deenergized, such as voltage testing, or (2) greater hazard, deenergization of the equipment creates a more significant hazard such as interruption of life support equipment.
Electrical contractors look to the inspector for guidance in the electrical installation; the inspector is the authority having jurisdiction. When the inspector embraces safe electrical work practices and demands that contractors properly plan their work and schedule inspections with all of the work completed in a deenergized state, everyone gets a safer workplace.
The real challenge of implementing an effective electrical safe work practice plan is getting everyone involved to change their working habits and the way each job or task is planned. We all resist change in our lives to some degree. How did we survive before computers and cell phones? We are all very comfortable in our environment and our working habits. It is natural that we resist change. A visit to a local hospital with a burn center may help one accept the change that an electrical safe work practice plan brings to the workplace. As safety coordinator for IBEW Local 98 in Philadelphia, I have seen my fellow members, friends of mine, in the burn center after an arc-flash incident. It is not something you want to experience for yourself. In fact, by embracing safe electrical work practices you can help prevent an accident.
Why would anyone work hot? Why would they expose themselves to those dangers when they could arrange a shutdown? In the electrical construction field, not involving line work or a service truck, 95 percent of energized work is done for one of two reasons, (1) poor planning, and (2) convenience. Poor planning occurs when a contractor energizes equipment before all work in that equipment is complete. For example, a project is to occur in two phases. The contractor must complete phase-1 before phase-2 begins. The switchboard in phase-1, however, will supply two feeders for phase-2. The contractor can take proactive steps by installing both feeders for phase-2 from the overcurrent devices into junction boxes which can be accessed for a splice, in a deenergized, locked and tagged out state as phase-2 of the project is completed. This eliminates exposure as well as a shutdown of the phase-1 project. Energized work also occurs regularly because of convenience. A typical mindset considering energized work justifies that work by thinking, "By the time I schedule a shutdown, I could have this done.”
This type of working culture that includes poor planning and convenience translates into a type of Russian roulette. There is not one live round, five empty chambers and one player. There is one live round, thousands of empty chambers and multiple players. Eventually something goes wrong. Burn centers and graveyards across the country are where the individuals who find the live round end up.
I would like to leave you with a thought about change. A quote from W. Edwards Demming sums up the reason that we must embrace safe electrical work practices: "It is not necessary to change. Survival is not mandatory.”
Electrical safe work practices are being embraced and implemented all over the country. It is difficult for some individuals and contractors to change. Survival makes the change easier to accept. Many electrical contractors and inspection agencies are forced to change. In order to survive, they develop written electrical safe work practice plans, lists of qualified persons and documentation of training because owners and developers demand it. We want everyone to survive. Electrical safety is changing and you are a major player in this arena. Take the next step. Implement an electrical safe work practice plan. When someone asks why you are making this change, tell him or her that you choose to survive.
Read more by Jim Dollard
Posted By Jacqueline Silvia,
Tuesday, May 01, 2007
Updated: Sunday, February 10, 2013
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When deciding what type of connector material works best in an electrical application, specifiers and inspectors alike may instinctively assume copper is the best choice. And indeed, it may be. But there are some applications for which aluminum poses a cost-effective, easier and longer lasting alternative. In order to ensure that electrical connections are proper, safe, and in accordance with electric code, it is important to first understand not only when it may be best for specifiers to select an aluminum connector, but also what specific traits to look for in each connector as well as a connector supplier. From there, it becomes much easier to test and approve the installed connection.
Photo 1. Compression connectors (Courtesy of FCI-Burndy Products)
Some material history
Copper has earned its solid reputation in the electrical connector industry, having proven particularly useful in residential installations. It should come as no surprise that performance levels with copper is high, as copper is, in fact, one of the oldest known metals. First used in about 8700 BC in what is now Iraq, and extremely popular in Ancient Egypt, copper owned 100 percent of the metals market for about 5000 years until the arrival of gold. Copper has had innumerable uses during this period, from early utensils, to ornaments and weapons, through the Bronze Age when it was alloyed with arsenic and tin.
All this being said, copper may be an omnipresent element, but it is far from the only choice for electrical connectors — a good thing considering its escalating cost in recent years. Aluminum is actually a better fit for some customers who have more industrial or commercial applications.
Aluminum is relatively young in age compared to copper. In 1787 Antoine Lavoisier identified bauxite as the oxide of a still undiscovered metal, and in 1825 Hans Christian Oersted has been credited with preparing the first metallic aluminum from bauxite, which is a claylike rock found in the earth. With its high electrical conductivity, ductility and low atomic mass, aluminum is frequently used in everything from electric transmission lines to the coating on telescope mirrors to aluminum foils used in food preparation and storage.
Aluminum is also being specified more frequently in recent years as a great option for electrical connectors, including mechanical and compression terminals, splices and taps. In industrial and commercial installations, such as substations and utility distribution and transmission lines, aluminum connectors are particularly well-suited for these applications due to their lightweight composition, high conductivity, and ease of installation.
Mechanical vs. compression
If you find the specifier has selected aluminum for the installation, the next step is to ensure that not only have the proper aluminum connectors been selected, but that the most competent and knowledgeable connector supplier possible is involved in the process. Knowing that a renowned connector supplier is associated with an installation should give you the confidence that a job is being installed correctly and in accordance with codes. It is important to note though that aluminum terminals, splices and taps are offered in both mechanical and compression types, and there are advantages to each. Because there are no codes dictating which method to use, it is important to be aware of the merits of each in order to properly assess a quality connection.
Mechanical connectors are easy to install, requiring no special installation tooling. Though mechanical connectors may be individually more expensive than compression connectors, the capital investment incurred with the purchase of installation tooling for the compression process is substantial. Aluminum mechanical connectors are also reusable, have the flexibility to accommodate a wide range of cable, run cooler than conductors being joined, and have high mechanical strength.
Compression connectors do have their advantages however, and are typically the chosen method with larger organizations responsible for bigger installations. Compression installations are made to last — they are irreversible and offer an extremely high holding strength. Aluminum compression connectors deliver high quality connections at a low installed cost after the initial investment in special tooling has been made.
Determining whether to go with mechanical or compression connectors is usually an installation-driven decision for specifiers, with cost a fairly consistent underlying factor. After this decision has been made, there are still other variables to consider.
Choosing the right aluminum products
Whether you encounter aluminum mechanical or compression connectors, you should be careful to guarantee that the specifier selected a manufacturer that offers features such as:
- Dual-rated products for use on both aluminum and copper conductors.
- Connector sections that are heavy enough to carry full electrical loads of conductors and withstand the forces applied during installation.
- Contact surfaces that are finished and protected to prevent reformation of non-conducting oxides.
- Contact paths that are as short and direct as possible
- Connector designs that prevent moisture and corrosive media penetration into contact areas from causing potential corrosion.
- Ensure that pressure applied from bolts as well as from compression tools is well-distributed over the contact surface and does not weaken the conductor.
- Electro-tin plated contact surfaces that provide for durable, long-lasting, corrosion-resistant connections, if required.
Also, a sound electrical equipment manufacturer produces a wide enough range of aluminum products with the right materials and properties to meet exactly your application needs. For example, for bolted mechanical connectors, look for heat-treatable alloys that deliver the right combination of conductivity and strength. For compression connectors, you will want to make sure that a high-conductivity, malleable grade aluminum that supplies the right level of ductility is being implemented.
All aluminum connectors should have required hardware that is both high strength and provides resistance to corrosion and galling. Some companies go so far as to offer hardware that is coated with a lubricant that not only prevents galling, but also results in optimum performance for recommended installation torques.
Oxide film is an environmental byproduct of aluminum that, if not properly addressed, can be problematic. The effects of oxide film, which is present on all aluminum surfaces and can cause high contact resistance, can be best combated by employing a connector that incorporates a material designed to inhibit oxide and minimize galvanic corrosion. By ensuring that such a connector is implemented, you can help ward off detrimental corrosion during the service life of the connection.
Photo 2. Mechanical connectors (Courtesy of FCI Bundy)
Of course, all compression connectors should conform to applicable sections of the National Electric Code, and all products used are required to meet the UL 486A–486B Standard and potentially even have CSA 22.2 No. 65 certification. Again, there are manufacturers who offer products that meet all of these standards, enabling you to have good faith in the quality of the product and concentrate solely on the quality of the installation.
Proper installation is essential
Obviously, selecting the appropriate aluminum connector for the conductor and application is only the first step in ensuring a successful aluminum-based connection. After you have verified that an appropriate connector is being implemented, you need to make sure the specifier has:
- Measured and marked the recommended insulation strip length, then carefully cut and removed the insulation to avoid nicking strands
- Wire brushed the stripped length of wire and un-plated aluminum contact pad thoroughly to remove surface oxides
- Applied an oxide inhibiting compound to any exposed conductor surface before inserting the conductor into the connector
- For compression connectors, selected the appropriate installation tool and die — then completed the process with the required number of crimps
- For mechanical connectors, torque all hardware to recommended values according to hardware material and size
Aluminum connectors are by no means right for every application. There are certainly many installations, including those on the residential side, where copper is the material that makes the most sense. But in order to ensure that specifiers are selecting connections that work best for a given application and budget, you should consider all the feasible options, including the connector material and connection type, before making sure that the proper installation guidelines have been adhered to. By connecting with the right information up front, you’re much more likely to encounter successful connections on the job.
Read more by Jacqueline Silvia
Posted By Joseph Weigel ,
Tuesday, May 01, 2007
Updated: Sunday, February 10, 2013
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Electrical arc-flash and shock hazards have been recognized as particularly dangerous and fairly frequent occurrences that put the lives and health of electrical workers at significant risk. Statistics indicate that five to ten arc-flash accidents that involve a fatality or serious injury to an employee occur every day in the United States.
Those incidents most often occur when personnel are required (or choose) to work on electrical equipment while it is in an electrically energized condition. In an effort to reduce these incidents, the NFPA 70E® standard was published, and OSHA cites it as a definitive resource for employee electrical safety in the workplace.
Photo 1. With a flame resistant (FR) rating of 8 cal/cm2, NFPA 70E-2004 Hazard Risk Category 2* requires full face and neck protection which is provided in this case by a wraparound face shield rated for a minimum of 8 calories per square centimeter.
NFPA 70E 2004 Article 130.1, Justification for Work, is very clear that one of the fundamental requirements for safe electrical work practices in any facility is to place all equipment in an electrically safe work condition (i.e., lockout/tagout) before personnel works on or near the equipment, unless the circumstances meet certain conditions for exemption from that rule that are clearly outlined in NFPA 70E 2004 Article 130.1(A) (3). The arc-flash and shock hazards are only present when electrical equipment is energized, and that includes the steps of performing the process of lockout/tagout up to and including the point that the equipment has been verified to be in an electrically safe work condition, meaning it has been verified as not energized.
Arc-flash accidents result in devastating consequences to the workers involved and their families and are also very costly to the employer and its insurers. The average cost of a survived accident can easily reach $8 million to $10 million in direct and indirect costs. Legal settlements almost always involve citations of deficiencies in the employer’s safety program as a causative factor leading to the accident.
Inspecting energized equipment safely
Electrical inspectors face the same hazards as they perform the inspection process if the equipment is energized. In many cases when the inspector arrives on the jobsite, the equipment is closed up and energized. Many jurisdictions have regulations that require acceptance by the inspector before the equipment is energized, but many others do not. Often, the electrical contractor will energize equipment to troubleshoot or verify proper operation before the inspector arrives on the jobsite. Thus, it is likely in many cases that the equipment will be energized when the inspector arrives, and this presents safety hazards to the inspector whenever it is necessary to open the equipment in the course of the inspection.
Since a fundamental requirement of the NFPA 70E safety standards is to place all equipment in an electrically safe work condition as noted above, this is always the preferred condition prior to inspection. However, in a facility that is energized, the inspector should be aware that this option will often not be a popular approach, because it may disrupt facility operations and final troubleshooting and startup work by the contractor. If inspection must be done with the electrical equipment energized, there are clearly defined safety procedures that must be followed to ensure the safety of the electrical inspector and/or the electrical contractor’s personnel.
First, the NFPA 70E standards are clear that any time electrically energized parts are exposed, such as is the case when equipment covers are open or devices are withdrawn from their cells, only qualified persons are permitted to be in the work area (i.e., the flash protection boundary), and those qualified persons must be protected with the appropriate personal protective equipment (PPE). Unlike older existing facilities, most new facilities will have electrical single-line diagrams that are accurate and up-to-date, and this is critical for safe work, because they will accurately reflect all potential sources of supply to each piece of equipment in the facility. In addition, the short circuit and coordination studies are usually available and accurate in new facilities, and the equipment itself is typically in optimal working condition (i.e., new).
However, unless it was specified in the construction specification documents, it is unusual in a new installation to find that an arc-flash hazard analysis has been completed. This makes it more difficult for the electrical inspector or electrical contractor to know what the specific hazard levels are at each piece of equipment in the system, and the potential hazard levels define the hazard risk category of PPE that is required for energized work. Fortunately, NFPA 70E provides Table 130.7(C)(9)(a), which lists different equipment types, voltage classes and specific work tasks with requirements for appropriate PPE and tools to be used. When consulting this table, care must be taken to follow the footnotes that define the conditions under which the tables may be used.
Any time electrical inspectors need to be within the energized electrical equipment’s flash protection boundary with exposed energized parts to perform the inspection, they must be qualified persons as defined in NFPA 70E Article 100, and they must wear PPE that is appropriate for the hazards that are present. OSHA recognizes that in many cases workers are qualified to work only on electrical equipment at certain voltage levels. For example, many workers are considered by their employer to be qualified to work on energized electrical equipment at low voltage levels (less than 1,000 volts), but not on medium voltage equipment (1,000 volts or greater). Other more highly qualified employees are considered qualified to work at any voltage level.
Photo 2. The minimum PPE requirements of NFPA 70E-2004 Hazard Risk Category 4 include protection like an Oberon Arc40 coat, pants and hood with hard cap, along with his Hazard Risk Category 2 PPE. The worker is preparing to rack a Square D® Masterpact
Removal of covers on energized electrical equipment is in itself a fairly hazardous task. Many arc-flash accidents that are reported each year were caused by workers carelessly removing a cover from energized electrical equipment. Some electrical equipment may possess cover interlocks that are concealed and unknown to the inspector. Removing a cover with a concealed cover safety interlock may cause an upstream protective device to open. A good practice for inspectors to follow is to request that the electrical contractor’s qualified personnel remove all covers from equipment to provide access for inspection, and then reinstall the covers after the inspection is complete. The installing contractor should provide this access to the inspector, because the contractor will know the system and the safety requirements, and should be able to perform the task safely. Once the covers are safely removed for inspection, the inspector will still be required by the standards to be a qualified person wearing the appropriate PPE, even for a visual inspection.
It is a common industry practice for equipment manufacturers to ship new electrical equipment with all of the overcurrent protective devices’ trip unit adjustments set to their minimum settings. For example, circuit breakers with adjustable trip unit functions will always be set to minimum settings. During startup, the contractor will set each circuit breaker trip unit to the settings that are recommended in the system protective device time coordination study. Until the overcurrent protective devices have been properly adjusted, placing a load on the system will often result in nuisance tripping of some devices. This tripping is not in itself a safety hazard, but is something of which the electrical inspector should be aware. If a device nuisance trips when the inspector is inspecting the system with equipment doors open, it could startle the inspector or the contractor’s employee, and that could cause an accident to occur.
In Section 110.16, theNational Electrical Coderequires that electrical equipment must be labeled to indicate that there is a potential hazard to the worker of arc flash, arc blast, shock and electrocution. NEC-2005 does not require that specific hazard levels, such as the flash protection boundary distance, arc flash incident energy, PPE hazard risk category, shock protection boundaries and other details, be posted on labels. These "generic” labels are usually placed on new equipment by most manufacturers. Although these labels are useful as a hazard warning, they do not provide much information about the specific hazard levels in that particular installation. It is for that reason that the tables in NFPA 70E are useful until an arc-flash hazard analysis is completed. Future revisions of the NEC, including the 2008 edition, are likely to expand and clarify the labeling requirements, but they probably will not require specific details about the hazard level. Hazard warning labels that show these details are available from some suppliers, but they generally will not be found on equipment in a new installation.
Photo 3. The current requirement for application of hazard warning labels on electrical equipment (National Electrical Code 2005) only requires a generic hazard warning message to the worker. However, many Square D customers have decided to exceed the req
When inspectors work in existing facilities — for example, to inspect new equipment that is added to an existing system — there are a few differences of which they should be aware. Most existing facilities that are more than just a few years old will often not have very accurate documentation, such as the electrical single-line diagram, short circuit study and coordination studies. The reason is that as modifications were made to a facility’s electrical system over time, the existing documentation was not updated to reflect the system changes. Improper documentation, especially an out-of-date single-line diagram, makes the process of lockout/tagout more hazardous for the worker. In lockout/tagout, it is imperative that the worker be able to identify all supply sources to the equipment and all modes of operation in order to make certain that all potential sources are locked out and in an electrically safe work condition to prevent injury. Inaccurate electrical single-line diagrams make this process much more difficult.
In addition, the electrical equipment in older facilities has often not been properly maintained. This lack of maintenance can cause overcurrent protective devices to operate more slowly, or not at all, during an arcing fault, which will cause the arc flash incident energy to be difficult to calculate accurately, and protect against.
As an electrical inspector, your employer probably has a legal obligation to make certain that you are a qualified person according to the definition in NFPA 70E Article 100, that you have proper PPE (and use it), that you are furnished with any tools that are required to perform electrical inspections safely and that you have been properly trained on the hazards involved in performing your work. Your other "employer” — your family — has a vested interest in your continued well-being and safety as well.
But in the end, it is your responsibility to understand the hazards you face and understand how to perform your work safely. Thus, it is paramount to know how to perform your work safely, and be educated on how to protect yourself from injury.
Read more by Joseph Weigel
Posted By Todd Wimmer,
Tuesday, May 01, 2007
Updated: Sunday, February 10, 2013
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Behind the scenes, a lot of mechanical engineering goes into creating and maintaining a tropical rainforest near Lake Erie, just 40 miles from the Canadian border. It is a feat accomplished by Cleveland (Ohio) Metroparks, a recreational authority that provides an "emerald necklace” of woodlands, golf courses, hiking trails and other attractions surrounding this Midwestern city—known as much for its sports teams as its world-class orchestra and Rock ‘n Roll Hall of Fame. Without state-of-the-art HVAC technology, it is doubtful this unique tropical habitat—located within the Cleveland Metroparks Zoo complex—could exist; and it continues to benefit from equipment improvements, such as advanced motor and drive design.
Photo 1. First opened in 1992, The RainForest at Cleveland Metroparks Zoo is a domed, simulated biosphere that contains some two acres of plants and wildlife, comparable to that found in tropical rainforests and jungles. Its air handling system, which inc
The RainForest at Cleveland Metroparks Zoo contains two acres of plants and wildlife similar to that found in rainforests around the world. Each year, over one million visitors come to this two-story, domed, simulated biosphere to experience what it is like walking through tropical regions of Central America, Africa or Asia—and see some 600 animals in a natural setting, including birds, monkeys, reptiles, and colorful fish that ply lagoons, swamps and warm rivers.
A Fragile Environment in a Rugged Climate
Photo 2. Shown are two heat wheels, 10 feet in diameter, which are the means of moving moisture and heat from the RainForest facility and returning fresh air. They are turned at a rate of 7 - 18 times a minute, using ABB motor/drive Direct Torque Contro
Despite wide swings in temperature and humidity on the Great Lakes, where "Alberta Clipper” storms can swiftly deliver below-freezing temperatures in winter, and sun-drenched summer days top 90 degrees F, visitors and inhabitants of The RainForest enjoy a nearly constant 76 degrees F and 76-percent humidity. This is due to a robust HVAC system that has evolved over the years to incorporate components that have improved the system’s reliability by 100 percent.
Not only does The RainForest, a $30 million investment, envelope visitors in exotic surroundings, but it also serves as a reminder of what is being lost—unless more rainforest and jungle acreage can be protected from overdevelopment.
Heat Wheel Application Critical to Constant Temperature and Humidity Control
Direct Air Systems, Inc., with locations in Cleveland and Columbus, Ohio, working in conjunction with Zesco Inc., specialists in electrical-mechanical motion control and based in Cleveland, provide HVAC service to The RainForest, as well as other notable local sites, including the Rock ‘n Roll Hall of Fame.
The RainForest has two air handler units that are 100 percent outside air. To provide and maintain optimum environmental conditions for the facility, Direct Air Systems installed SEMCO energy wheel systems for the units, one of which has a throughout of 60,000 cfm used primarily for cooling; while the other unit, rated at 40,000 cfm, is equipped with a pre-heater and humidifier rack. Both units have side-by-side,10-foot diameter, 1,000-pound dry desiccant heat wheels, which are necessary to conserve 18,000 pounds of water every day, transferring moisture from The RainForest’s stale exhaust air and giving it to the dry outside air stream once every 2.5 hours.
Photo 3. Compact ABB ACS 800 drives, part of the ABB Direct Torque Control solution, require a much smaller footprint than earlier drive models. This is especially helpful in areas where space is limited.
The term desiccant refers to material bonded to the surface of the heat wheels that collects moisture, as well as odors, which are then exhausted out of the building via the upper portion of the wheels.
The wheels rotate anywhere from seven to eighteen times a minute, depending on the humidity level. Fresh air, referred to as process air, is drawn in on the bottom portion of the air handlers and filters through the wheels.
The fresh air’s temperature and humidity are moderated by the wheels’ slow revolution and the fact that the wheels’ mass and desiccant surface transfers a portion of the heat and moisture collected from the interior. Heaters, when necessary, warm the air before it passes to The RainForest’s spacious interior, which has over 60 temperature zones, including those for offices, cafeteria, and gift shops.
Air Handling Units Built into Facility
Rather than being roof-mounted and exposed to the elements, as is commonly done with air handling units, the ones serving The RainForest are built into the facility to maintain unit efficiency that would otherwise be lost in Cleveland’s warm summers and cold winters.
Efficiency and Simplicity Distinguish the System
The desiccant process was selected for both efficiency and simplicity. It was concluded that boilers, z-ducts, heat pipes or other methods did not compare to the 85 percent efficiency the heat wheels provide. Additionally, heat wheels are extremely simple to operate. The thinking was that the simpler the fundamental mechanical equipment, the greater the reliability and ease of maintenance. That proved to be the case—up to a point. While the technology should have worked flawlessly, a nagging problem developed.
Each wheel rotates with a custom fabricated 31-foot long belt and, when first installed, was equipped with a 1-HP ac electric motor rated for 1750 rpm, and a mechanical gearbox to provide a 5:1 gear reduction.
At the time of installation, this was a fairly common equipment configuration. However, it was discovered that mechanical gearboxes used for The RainForest were failing at an alarming rate. Once a year, one of the gearboxes had to be replaced. There was no discernable pattern pointing to a particular wheel-and-gearbox arrangement. It was random. The only constant element in the problem was the routine failure of a gearbox.
The difficulty was finally identified. It dealt with the revolutions per minute. The pace was too slow for the gearboxes’ splash lubricating systems to engage properly. As a result, parts were not being properly oiled and were wearing out prematurely.
Path to a Solution
Photo 4. More than one million people a year visit The RainForest, enjoying a nearly constant 76ºF, 76 percent humidity environment maintained by the air handling system.
Since the use of a gearbox was the common approach when the heat wheels were installed, the issue was not initially seen as one of equipment selection, but viewed as a problem of application. Each heat wheel installation is custom-made and has to contend with its own set of dynamics.
Solutions to the problem included continuing the practice of simply replacing gearboxes as they failed; however, it soon became obvious that this was expensive and somewhat unpredictable.
Also considered was using gearboxes that lubricate differently, as well as making customized gearboxes specific to The RainForest application. As more thought went into devising ways to handle the lubrication problem, it was concluded that these possibilities were bordering on experimentation. The RainForest, with its 600 animal inhabitants, was not a good candidate to take such risks, especially when a more up-to-date solution was available.
With Drives, No Gearboxes Required
Direct Air Systems thought about their experiences with other HVAC applications and mentioned to the Cleveland Metroparks Zoo maintenance team an ac drive/ac motor solution that did not require gearboxes. This was becoming an increasingly common arrangement and had a good track record. It was also state-of-the-art technology, moving away from the problems and complexities that moving-parts mechanisms presented.
"When we saw we weren’t getting too far with the gearbox-lubrication issue, we turned to equipment that was available to us—now,” Steve Snyder, president of Direct Air Systems, explained. "The direct torque approach we recommended was something that would be cost-effective, easy to maintain, and simple in its operation. Gear lubrication would not be an issue. Plus, direct torque is proven engineering. The Cleveland Metroparks sought bids for the project, as it is a public agency, and our bid was selected.”
The retrofit involved ABB’s Direct Torque Control solution, which uses the ac motor’s torque as the primary control element.
The original 1-HP ac motor and gearbox equipment in each of the energy wheel systems was removed and replaced with an ABB 5-HP induction motor/ac low-voltage drive combination. This arrangement allows the motor to be connected directly to the motor/load without the need for a gearbox or pulse encoder. The ABB solution allows full motor torque down to zero speed.
Through the use of an algorithm, the ABB drives, in this case variable speed ACS models, can run without an encoder to provide speed feedback. The algorithm enables the drive to calculate the state of the motor’s torque and flux 40,000 times per second. Elimination of the encoder further reduces maintenance and decreases downtime.
Although each energy wheel system is controlled by individual Johnson Controls systems, the status of the motors and drives is monitored by The RainForest’s comprehensive Johnson Controls building management system.
In the event of a control failure, the ABB ACS drives are designed to go, automatically, to a pre-set rpm rate, to ensure heat transfer is maintained. Spare motors are inventoried at The RainForest and drives are kept at Direct Air Systems’ office location, minutes away from the facility.
Since the installation of the ABB motor/drives combination over four years ago, there has been no interruption in service. Direct Air Systems is seeing increasing use of direct torque control. "It is definitely one of the approaches we recommend,” Snyder explains. "Often there is more than one way to solve a problem. Based on the circumstances in this instance, the direct torque control method proved to be a good solution. We have applied it on other projects, as well. It has three characteristics we like. It’s cost-effective, simple and reliable.”
A Model for Saving the Rainforests of the World
Through contributions by visitors to The RainForest, and the efforts of other organizations and governments, limited amounts of scarce acreage of rainforests and jungles now are protected from overdevelopment. Continuing support will set aside even more of this valuable land that not only serves as a wildlife refuge for native plants and animals, but also improves the Earth’s environment by taking carbon dioxide out of the atmosphere. Additionally, these protected areas are proving to be important sources for tomorrow’s miracle medicines.
Featured on Animal Planet, The RainForest is one of several attractions at the Cleveland Metroparks Zoo, which covers 168 acres and features the largest collection of primate species in North America among its 3,000 animals.
Read more by Todd Wimmer
Posted By Michael Johnston,
Tuesday, May 01, 2007
Updated: Sunday, February 10, 2013
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May is electrical safety month, during which the electrical industry draws attention to all aspects of electrical safety for persons and property. Section 90.1(A) of the NEC provides the essence and basis for this concept that is integral to the rules in the Code. These rules are all built on a foundation and premise related to practical safeguarding of persons and property from electrical hazards. While the NEC primarily governs electrical installations and systems on the premises, there are various rules that correlate with safe electrical work practices that are provided in safety standards beyond the coverage of the NEC. This article provides a review of some essential electrical installation rules that directly relate to worker safety. The requirements for locating disconnecting means at electrical equipment are one such set of rules that affect electrical safety for workers. The disconnecting means location and its use provides the basis for this article.
Photo 1. Disconnecting means installed in sight from an equipment location.
When a disconnecting means is in an electrical circuit, it provides a means to shut off the power in emergencies or for repairs, maintenance, or routine service operations. The basic requirements from NFPA 70E and OSHA standards generally are to place electrical circuits and equipment in an electrically safe work condition when one is performing work on these systems. The NEC covers the required locations for equipment disconnecting means. How workers must use them to meet the requirements for safe work practices is not covered by the NEC. It is up to the workers to obtain electrical workplace safety training to understand how to recognize and avoid hazards associated with electricity.
The definition ofqualified personsin Article 100 has been strengthened in recent editions to correlate clearly with safety training requirements for workers. This definition was again revised for the 2008 NEC to indicate just that. The words "to recognize and avoid the” have been added to the definition to clarify what the safety training is expected to accomplish and to correlate with Section 110.6(D)(1) NFPA 70 E.
Figure 1. Qualified persons must be trained to recognize and avoid electrical hazards.
"Qualified Person. One who has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training to recognize and avoid the hazards involved.” [See Proposal 1-45 (Log No. 2589) on page 27 of the NFPA 70 2007 Report on Proposals for more details about this revision].
Qualified persons should be trained to identify and understand the relationship between electrical hazards and possible injury or death. Employees that are qualified by receiving such safety training are better equipped to make educated decisions involving their safety and often the safety others (see figure 1). The changes to this definition provide consistent correlation between the requirements for qualified persons provided in NFPA 70E-2004 Section 110.6(D). The revised definition in the NEC clarifies that an essential element of being trained includes understanding not only what the hazards are, but how to avoid them.
General Rules for Equipment Disconnecting Means
Photo 2. Disconnecting means installed within sight from the motor or driven machinery location
The requirements for equipment are provided in chapter 4 of theNEC. This is where the rules for required means of disconnect for equipment first start to appear in the Code. There are requirements for equipment disconnecting means scattered throughout chapters 4, 5, and 6 of the NEC, and each of these requirements focuses on electrical safety for workers. Three specifically defined terms are typically used within the general rules dealing with the location of equipment disconnecting means. The terms are in sight from, within sight, and within sight from. These concepts are defined in Article 100 as follows:
In Sight From (Within Sight From, Within Sight). Where this Code specifies that one equipment shall be "in sight from,” "within sight from,” or "within sight,” and so forth, of another equipment, the specified equipment is to be visible and not more than 15 m (50 ft) distant from the other.
There are two critical elements of this defined term. Each entity, such as equipment and disconnect, must be visible from the other and the distance between must not exceed 15 m (50 ft) [see photo 1].
Photo 3. Disconnecting means installed for air-conditioning equipment
The required disconnecting means provides a ready means for disengaging power in the event that quick shutdown operations are necessary, but it also serves as a safety disconnect for routine operations such as equipment maintenance. The concept of the disconnect being within sight from the equipment it supplies affords an inherent safety feature for workers. Basically with a disconnect visible and not more than 15 m (50 ft) from the equipment it supplies, workers have the means to monitor the safety switch while it is in the open position. Equipment disconnecting means meeting the in sight from requirements of the Code do not have to be capable of being locked in the open position because of this monitoring capability related to its proximity to the connected equipment. Many circuit breakers, general duty switch, snap switches, and other types of electrical switching mechanisms can meet these general requirements where Code rules call for a disconnecting means at equipment. Be sure to select and apply a disconnecting means within its ratings as required by other Code rules. Some
Photo 4. Disconnecting means installed for an electric motor
examples where disconnecting means are required within sight from electrical equipment are found in 422.32, 430.102(B) and 440.14 [see photos 2, 3, and 4]. In many cases, multiple motors are supplied from a single motor control center (MCC) that is within sight from all the motors and equipment it supplies. In these cases additional disconnecting means installed at the motor location is not necessary unless the within sight from requirement is not met. In these situations, an additional disconnecting means is required.
Disconnecting Means Lockable in the Open Position
Photo 5. The provision for adding a lock is inherent to the disconnecting means
The Code provides the minimum rules for electrical safety for persons and property. We have just reviewed the longstanding general requirements for equipment disconnecting means locations. It is important to review another important Code concept related to electrical worker safety. This concept involves disconnecting means that are not located within sight from the equipment they supply. Now the minimum Code rules are less restrictive for the required disconnecting means locations, yet more restrictive for their operational characteristics at the same time. In other words, if certain restrictive provisions in the Code are satisfied, an equipment disconnecting means is permitted to be located out of sight from the equipment it supplies, but it is required to be capable of being locked in the open (off) position (see photo 5).
Photo 6. Circuit breaker with locking provisions installed
For some types of equipment, such as motors and motor-driven machinery covered by 430.102(B), the disconnecting means is required to be located in sight from the motor and driven machinery. Only where the disconnecting means location introduces increased hazards and is impracticable, or for specific industrial applications, is this disconnecting means permitted to be located out of sight from the motor or equipment it supplies [430.102(B) Exception (a) and (b)]. If the specific conditions of this exception can be satisfied, the disconnecting means is permitted to be located out of sight from the equipment, but it must provide specific locking characteristics. First, the disconnecting means must be individually capable of being locked in the open or off position and second, the provision for adding a lock to the switch or circuit breaker is required to remain in place with or without the lock installed (see photos 6 and 7).
Photo 7. Motor control disconnecting means equipped with locking provisions.
This requirement in theNECworks hand-in-hand with the locking-out and tagging-out requirements of the electrical workplace safety Standard 70E. Electrical workers are afforded a ready means to insert their own lock and tag when presented with this work condition in the field. It is important for electrical workers to carry a lock and key for this reason. Workers that do not utilize this provision are putting themselves at risk. There are also other requirements that go along with placing the disconnecting means in the open position and locking it. Workers should verify that the switching means did in fact physically open all ungrounded conductors of the circuit by verifying open blades or contact mechanisms. The next important step is to verify the absence of voltage at the equipment after it is disconnected from the electrical circuit or system. This is accomplished by testing for voltage. There are also requirements for personal protective equipment during the process of testing for voltage. More specific procedures and requirements for locking-out, tagging-out procedures and voltage testing are provided in other safety standards and are beyond the scope of this article. The rules in the NEC ensure that workers have the locking means to put the system in an electrically safe work condition and keep it that way until the equipment is ready to be re-energized. The key to electrical safety is the training and implementing of good practices by qualified electrical workers. What good is the locking means if one does not use it? Who is at risk? The worker, his or her family, the organization he works for, the list is extensive.
New Requirements in the Code
The 2008NECdevelopment process is almost complete. Many existing rules have been revised, and new articles have been incorporated into this edition of theCode. A series of revisions that relate to the concepts of disconnecting means being capable of being locked in the open position were acted upon favorably by various code-making panels. The phrase capable of being locked in the open position appears over 25 times in the Code. In each case, the objective of the requirement is consistent and relates to worker safety. It is logical that the rules should also contain the same requirements relating to the disconnecting means locking provisions. Many of these have been revised to require that the provision for adding a lock remain with the switch or circuit breaker with or without the lock installed. The effect of this change is that where the disconnects are not provided within sight from the equipment they supply, the switch or circuit breaker must include provisions for adding a lock. These locking provisions have to be part of the equipment, either inherent to the equipment design or by an accessory feature that can be installed on the equipment.
The Code does not recognize portable locking devices that must be carried to the equipment and then attached to the equipment so a portable lock could be installed. The NEC covers installations and does not cover such portable locking provisions that could satisfy requirements in NFPA 70E, more specifically, the provisions in Article 120. This was one of the primary reasons for the changes to these NEC sections referencing disconnecting means that are capable of being locked in the off position. It all relates to worker safety, and establishing and maintaining consistency between NEC rules that share the same purpose.
The NEC is primarily an installation Code that includes some rules that have a direct or indirect impact on safety for electrical workers. The rules for equipment disconnect locations and provisions for locking means are good examples of such rules. These rules are only as good as the workers that use them to their advantage. Electrical inspectors have a responsibility to ensure that the required disconnecting means for equipment are provided where Code rules indicate they are necessary. Where the disconnecting means are not located within sight from the equipment, inspectors must ensure that the disconnecting means provided out of sight are capable of being locked in the open (off) position (see photos 6 and 7). It is up to trained qualified persons to use the locking means in compliance with other applicable electrical safety workplace and industry standards. In short, the NEC requires the locking provision for equipment, and NFPA 70E requires safe work practices. It is up to individuals to understand the hazards and to use locking and tagging methods to ensure their own safety and often the safety of others. Revisions in the 2008 NEC provide consistent requirements regarding locking means provisions for equipment disconnecting means that are not located within sight from the equipment. Numerous electrical safety training programs are available that can better equip employees to work safely and reduce risks in a trade where many exist.
We are all extremely busy these days. Many choices we make and practices we carry out are those that we take for granted, such as talking on a cell phone while driving, climbing a ladder to paint a room in the home, or verifying no fingers are in the way of a slamming vehicle door. Society tends to get complacent when it comes to electrical safety. Worker safety is taken for granted far too often. Electrical safety and worker safety require both individual and organizational efforts to be effective and beneficial. Having the approach that "it won’t happen to me” is not a good practice and not in the best interest of everyone affected. In the electrical business, there are installation codes that result in installations that are essentially free from electrical hazards. Some NEC rules provide means for workers to exercise safe work practices. The NEC location requirements for disconnecting means, and specific provisions for disconnects that are capable of being locked out (off) when not located in sight from equipment provide an installed means for workers to ensure safety by adding their lock to equipment and circuit disconnects. It is up to qualified workers to recognize and avoid hazards, not only for themselves but for their families. Use the lock and tag when the disconnecting means is not enough, and do what it takes to understand and apply electrical workplace safety rules. It is the best practice and the one that your family would expect you to follow.
Read more by Michael Johnston
Posted By Len Frier,
Tuesday, May 01, 2007
Updated: Sunday, February 10, 2013
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Deregulation of electric utilities is sweeping the country and is now available almost everywhere. The theory is that competition in the purchase of electric power would result in cheaper electricity and make utilities more responsive to consumers. This may be good in some areas and bad in others but it does put certain new elements of an electrical system under the authority of the local jurisdiction.
Utilities have historically been exempted from requirements in the National Electrical Code. They controlled all of their system up to and including the watthour meter in a facility. Electrical inspectors inspected from the meter into the facility. That is no longer the case. It is now possible for an owner to own the electric meters and all of the wiring from the main service drop. It is attractive for landlords of large apartment projects or other multiple occupancy facilities to buy their electricity wholesale and sell it to each tenant retail. Additionally, a tenant is less likely to leave the air-conditioning or heat on unnecessarily when they are paying for its usage.
Often, there is no control or requirements on these meters. Local public utility commissions may have requirements on the accuracy of a meter but usually not on safety. What’s more is that these meters can be lethal since they are usually connected to unfused wiring. Circuit protection is usually on the load side of a meter with no protection of the meter itself. A short circuit in the meter may only be isolated by the primary protection of a transformer or a circuit protective device feeding multiple meters. When inspecting a system an electrical inspector is usually not accustomed to looking at the meter. Additionally, there may not be a clear indication as to whether it is a utility meter or a customer-owned meter. Considering that there are watthour meters on the market that cost less that $10.00 each, the potential for a very dangerous condition exists.
Wiring and devices within a facility are usually properly protected yet in order to assure safety are still required to be labeled by a nationally recognized testing laboratory (NRTL). However, a watthour meter located prior to any overcurrent protective device in the building presents a major hazard in an installation. These meters have to withstand short-circuit currents from shorts in a building and surge voltages from outside the building. In addition, sever stresses should not cause the meter elements to burn open, preventing the meter from accurately registering the power consumption. The standards to which these meters are tested take all of these possibilities into account and result in a meter that is both safe and accurate.
To be safe, watthour meters not owned by the utility should be listed and labeled by a nationally recognized testing laboratory (NRTL) just as any other electrical device in a facility. To provide further assurance of quality, accuracy and dependability, the meter should also be certified to comply with the ANSI C12 compilation of standards. These standards assure accuracy and functionality in outdoor (wet, cold, hot and humid) environments. Safety determined by UL Standard 61010-1 does not necessarily include all of the environmental conditions a meter can be subjected to. Therefore the safety testing must include many of the ANSI C12 requirements. Although, performance and accuracy are not necessarily under the jurisdiction of the authority having jurisdiction, it would be serving the public well to know that the metering is accurate, dependable in addition to safe.
Read more by Len Frier
Posted By Jay D. Crutcher, Esq.,
Tuesday, May 01, 2007
Updated: Sunday, February 10, 2013
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For two and half years in IAEI News, Jesse Abercrombie has been addressing financial issues of concern to those in the electrical industry. For this issue, he has submitted a guest article by Attorney Jay Crutcher, who will discuss a new tax in Texas. It is not yet known whether other states are implementing similar taxes. —The Editor
There’s a new tariff in town called the Texas Margin Tax. The margin tax replaces the old franchise tax and affects virtually every business entity in Texas (including out-of-state companies "doing business” in Texas). Vast numbers of limited partnerships, previously exempt from the old franchise tax, will now pay margin tax and it could be a sizeable amount. Corporations and limited liability companies that paid the old franchise tax will calculate margin tax using methods significantly different from the old franchise tax. Businesses now face new reporting rules for combined groups and tiered-partnership arrangements. In short, the margin tax affects business owners in fundamental ways — how they structure (or restructure) their businesses, how they account for their business operations, how they file their margin tax returns, and bottom-line, how much tax they pay. As a business owner, you need to know exactly how the margin tax affects your business so that you can take proactive measures to minimize your tax liability. Otherwise, you could face a surprising tax bill or overlook tax planning opportunities. This article describes who pays the margin tax and how much they pay.
The margin tax applies to a taxable entity. Most business entities, including limited partnerships, are considered to be a taxable entity. Limited partnerships were exempt from the old franchise tax. Prior to the margin tax, many businesses operated as a limited partnership to avoid franchise tax liability. One of the primary legislative purposes behind the margin tax was to eliminate the old franchise tax exemption for the vast majority of limited partnerships.
The margin tax does not apply to a passive entity. To qualify as a passive entity, the business must be organized as a general partnership, limited partnership or non-business trust. A limited liability company does not qualify as a passive entity. Also, the business must satisfy specific passive income tests prescribed by the margin tax statute. Oddly enough, the margin tax statute expressly provides that rent is not passive income.
Margin Math: Taxable Margin
The margin tax rate is 1% for most businesses. Some businesses qualify for a lower rate of 0.5%. A business pays margin tax based upon its taxable margin. The business calculates taxable margin using one of three methods: (1) the cost of goods sold method, (2) the compensation method, or (3) the 70% percent method. The business uses the method that results in the lowest taxable margin. The business can elect to use a different method from year to year and is not required to use the same method each year. The old franchise tax was based on a net income concept. By changing the tax base to a gross margin concept, the margin tax potentially disregards many deductions.
Taxable Margin: The Texas Six-Step
A business calculates its taxable margin in basically six steps:
- First, the business determines its total revenue. Revenue exclusions apply for certain types of revenue. Special rules apply to specific types of businesses, including construction companies and contractors;
- Second, the business determines its cost of goods sold. The margin tax statute enumerates eligible costs and excluded costs. Special rules apply to construction companies, projects involving real property, and companies that lease heavy construction equipment;
- Third, the business determines its compensation. Compensation includes W-2 wages and, in certain cases, distributive shares of income from partnerships, limited liability companies and "S” corporations. However, the compensation deduction is limited to $300,000 per person. Compensation does not include amounts paid to undocumented workers or 1099 payments. Special rules apply to specific types of businesses, including management companies and managed entities;
- Fourth, the business determines the lower of three separate calculations; (1) total revenue minus cost of goods sold (i.e., the cost of goods sold method), (2) total revenue minus compensation (i.e., the compensation method), or (3) total revenue multiplied times 70% (i.e., the 70% method). The business elects each year to use the method that results in the lowest margin;
- Fifth, the business apportions its margin as between Texas and other states in which it conducts business; and
- Sixth, the business subtracts other allowable deductions specifically granted by the margin tax statute.
Combined Groups and Tiered-Partnership Arrangements
The margin tax statute requires combined groups to file on a combined group basis. This reporting rule represents a significant change from the old franchise tax law that prohibited consolidated reporting. Combined groups are affiliated entities, whether corporate or non-corporate, that have 80% common control and are engaged in a so-called unitary business. A unitary business generally includes separate parts of a single entity or a controlled group of entities that, in either case, constitute a single economic enterprise by reason of interdependent and integrated business activities.
The margin tax statute permits a form of combined reporting for tiered partnership arrangements. A tiered partnership arrangement includes partnerships that are 100% owned by one or more other taxable entities and may consist of one or more tiers. In this case, the other taxable entities may pay and report the margin tax attributable to their respective ownership interests in the partnership.
Tangling with the Texas Margin Tax
Businesses must assess the business, legal and tax implications resulting from the Texas Margin Tax. Business structures need to be viewed (and perhaps revisited) in light of the fact that most limited partnerships are now taxable. Tax planning needs to be revamped to carefully consider the different methods used to calculate the margin tax, how to maximize the benefits of each method and how to minimize margin tax liability. Tax reporting will need to be revised to include combined groups and, as appropriate, tiered partnerships.
This article is for general information purposes only and is not intended to constitute legal or tax advice and should not be viewed as such. Readers are encouraged to consult with their attorney or tax advisor to consider the matters discussed in this article in light of their unique tax circumstances. ©2007
Read more by Jay D. Crutcher
Posted By David Young,
Tuesday, May 01, 2007
Updated: Sunday, February 10, 2013
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To protect your and your company’s wallets, it is very important to understand the rates for which you are being charged for electricity. To get your feet wet, I am going to share with you and discuss in detail the electric rates of a typical utility. The example I am using is a utility that publishes their rates on the Internet. The electric rates for which you are being billed may vary greatly from my example. I recommend that you contact your utility to get a copy of your rate and find out what other rates are available to you. In most states, if a residential, commercial or industrial establishment wants electric service, they do not have a choice as to the service utility. In some states, customers have a choice as to the utility from which they purchase their energy. This utility is sometimes called the supply utility. The supply utility may be the same or a different one from the service utility. To accommodate these two functions, some electric utility rates have two components, one called delivery charges and the other called supply charges. Customers pay the delivery charges of their service utility and the supply charges of the supply utility.
Photo 1. Residential Meter
I understand that most customers have chosen to have the same utility for both functions. If utilities other than your service utility offer supply in your state, compare the supply charges of the two utilities. You might be able to save some money by changing to a different supply utility.
To keep things simple in my discussion of rates, I am not going to show the delivery and supply charges separately. Imbedded in most delivery charges is a customer charge. The customer charge is usually a minimum bill associated with having electric service.
Basic Residential Rate
For our example utility, the basic residential rate has a customer charge of $7.36 per month and the total energy charge is 13.1456 cents per kWh for the summer months June through September and 14.3249 cents per kWh for the first 500kWh and 12.6017 cents per kWh for the excess over 500kWh for the winter months October through May. For this rate there is not much one can do to save money except to turn off lights and appliances when not in use. For a residence that uses 1000 kWh of energy each month, the bill would be $141.99 per month in the winter months and $138.82 per month in the summer months. The annual cost would be $1691.20 on this rate. For this rate, the supply charges are about 76% of the total bill.
Residential Electric Heat Rate
Some utilities offer a special rate for residences where the primary source of heat is electric resistance heat or electric heat pump. For our example utility, the customer charge for this rate is $7.36 per month and total energy charge is 14.1284 cents per KWh for the summer months June through September, and 15.5948 cents per kWh for the first 500kWh and 8.6716 cents per kWh for the excess over 500kWh for the winter months October through May. Obviously, the big savings in this rate is the 8.6716 cents per kWh for the winter months. For a residence that uses 1000 kWh each month in the summer months and 2000 kWh per month in the winter
months, the annual cost would be $2317.86 on this rate. If the customer originally had oil or gas heat and then switched to electric heat but did not notify the electric utility to change them to the electric heat rate, the annual cost would be $2699.34 on the basic residential rate. I have heard of customers paying the higher rate for years before they realized their error. I am sure there are customers who still pay the higher rate.
Table 1. Basic Residential Rate
Residential Time-of-Use Rate
Some utilities offer a rate where the cost of energy changes with the time of day. For our example utility, the customer charge for this rate is $11.32 per month and the energy charge during the summer months is 22.8253 cents per kWh during on-peak hours of the day and 6.58390 cents per kWh during off-peak hours of the day. On-peak hours are 9:00 a. m. to 8:00 p. m. Monday through Friday. During Daylight Savings Time, on-peak hours are 10:00 a. m. to 9:00 p. m. Monday through Friday. During the winter months, the energy charge is 22.7142 cents per kWh during on-peak hours of the day and 7.6336 cents per kWh during off-peak hours of the day. To accomplish the metering function for this rate, the utility installs a sophisticated electronic meter that has a built-in computer. The computer has a very accurate clock and keeps track of how much energy is used during the on-peak and off-peak time periods of each day.
Table 2. Residential Electric Heat Rate
Note the huge price difference between on-peak and off-peak charges. If your work schedule is such that you are only home from 8:00 p. m. to 9:00 a. m. and you turn off your heat (winter) and air-conditioning (summer) when you are not home, you could save a bundle. If you get home at 6 p. m., the savings may not be as much, particularly if you use an electric stove. On the weekend, you do not have to change your lifestyle since you are on the cheap rate all weekend.
Residential Time-of-Use With Demand Rate
Some utilities offer a time-of-use rate where the customer is charged for energy and demand. You will recall from part one, peak demand is the peak power and is based upon the maximum instantaneous current.
Table 3. Residential Time-of-Use Rate
A family comes home from the beach on a hot summer day. Mom turns on the air-conditioner because the house is hot. Dad goes to the refrigerator and chest freezer and stands there with the door open for five minutes trying to decide what to cook for supper. Both units turn on. Dad turns on the oven and two burners of the electric stove to cook supper. Oldest daughter jumps into the shower. The electric hot water heater turns on. Mom throws most of the beach clothes into the washing machine. The son decides to dry his beach towels without washing them. Now the electric clothes dryer is on. Oldest daughter gets out of the shower and finds cold air coming out of the air-conditioner vent. She turns on the electric wall heater in the bathroom while she dries her hair with a hair dryer the size of a chain saw.
Would you believe 100 amps at 240 volts? That is 24,000 Watts (24 kW) peak demand. Since a 60-minute demand is the average power used for a 60 minutes period and it is not likely that all the appliances would be on for a full hour, the 60-minute demand will probably be less. That may not be true if you have six teenagers.
Table 4. Residential Time-of-Use with Demand
For our example utility, the customer charge for this rate is $11.32 per month and the energy charge during the summer months is 7.3405 cents per kWh during on-peak hours of the day and 5.4510 cents per kWh during off-peak hours of the day. For this rate, on-peak hours are 8:00 a. m. to 9:00 p.m Monday through Friday. During Daylight Savings Time, onpeak hours are 9:00 a. m. to 10:00 p. m. Monday through Friday. During the winter months, the energy charge is 8.5299 cents per kWh during on-peak hours of the day and 6.2911 cents per kWh during off-peak hours of the day. The demand charge is $10.493976 per kW during the summer months and $9.982711 per kW during winter months.
The billing demand during the summer months is the greatest demand established during any 60-minute clock hour of the month, during on-peak hours, taken to the nearest whole kW. The billing demand for each of the winter months is the greater of the maximum demand established during any 60-minute clock hour of the month, during on-peak hours, taken to the nearest whole kW or 75% of the greatest billing demand as created during the most recent summer billing months.
It is easy to see that the energy charges are low for this rate but the demand charge can kill you. At $10.49 per kW, this rate might not be a good choice for the family who goes to the beach each month. I think that the complexity of this rate is one reason why very few people switch to this rate. As I suggested with the nondemand time-of-use rate, if you are only home between 8 p. m. and 9 a. m. each day this rate might save you a lot of money.
Next time, in Part 3, I will get into the details of the rates that apply to commercial and industrial facilities.
Read more by David Young
Posted By Leslie Stoch,
Tuesday, May 01, 2007
Updated: Sunday, February 10, 2013
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In my experience, a discussion of conductor numbers and ampacities in cable trays is frequently met with a snicker or knowing smile. Could it be that the rule for wiring in cable trays is sometimes taken less than seriously? We have all seen trays overloaded with cables, if not at the time of installation, then in the fullness of time. Once the trays are in place as originally designed, it’s far too easy to add cables, especially when the trays follow a convenient route to the end destinations of the added cables.
Occasionally, people are surprised that there is a rule for cable trays and that it can greatly increase the minimum conductor sizes given in Tables 1 to 4. So in this article, let’s review the much maligned Canadian Electrical Code Rule 12-2210, Ampacities of Conductors in Cable Trays.
We will take a detailed look at the maximum permissible ampacities for conductors installed in ladder, ventilated and non-ventilated cable trays. For those of us less familiar with cable trays, Section 0 includes some very precise definitions for all three types. But for our purposes, it’s enough to know that all have side rails and a bottom. The bottom of a ladder tray looks something like a ladder where the spacing between the rungs exceeds 50 mm. A ventilated tray has a solid bottom that has ventilation openings not exceeding 50 mm in longitudinal length. A non-ventilated tray is totally enclosed on the top, bottom and both sides with no ventilation openings.
CEC Rule 12-2210 prescribes that when installed in a cable tray, allowable conductor and cable ampacities are based on Tables 1 to 4 for copper and aluminum conductors with adjustments based on the spacings between cables and the type of tray selected.
When spacings between cables in a ladder or ventilated tray are maintained at greater than 100 percent of the largest cable diameter in the tray, the minimum conductor ampacities may be determined from:
- Tables 1 or 3 for copper or aluminum single-conductor cables; or
- Tables 2 or 4 for copper or aluminum multiple-conductor cables, ampacities corrected in accordance with Table 5C for the number of conductors when they exceed three per cable.
For example, with 100 percent of the largest cable diameter spacing maintained, a cable that contains six current-carrying conductors would require a correction factor of 80 percent based on Table 5C, applied to the ampacities derived from Tables 2 or 4.
When spacings between cables in a ladder or ventilated tray are maintained at any distance between 25 percent and 100 percent of the largest cable diameter in the tray, the allowable ampacities obtained as above would need to be further corrected in accordance with Table 5D unless a deviation from the rule is permitted. Table 5D is arranged in up to six conductors or cables arranged horizontally and two rows vertically.
If for example we decide to install a single layer of five cables horizontally in a ladder or ventilated tray, spaced at 50 percent of the largest cable diameter apart, the allowable ampacity as determined from above example would need to be reduced to 83 percent in accordance with Table 5D.
But hold on—the most exciting part is yet to come! When cable spacing in a ladder or ventilated tray is less than 25 percent of the largest cable diameter in the tray or for any spacings in a non-ventilated tray, the allowable cable ampacities are based on Tables 2 or 4 for copper or aluminum conductors corrected in accordance with Table 5C for the total number of conductors in the tray.
Let’s say for example we have four 3-conductor cables in a ladder tray, spaced less than the 25 percent of the largest cable diameter apart. And let’s suppose that all of the conductors are considered as current-carrying as defined in the CEC. This would give us twelve conductors in the tray. Table 5C shows that the allowable ampacities of the cables in this tray would need to be corrected to 70 percent of their Table 2 or 4 ratings. No doubt everyone faithfully applies Table 5C as shown in this example at every given opportunity.
At the end of Rule 12-2210, there is a reminder that the cable ampacities must be further reduced when the trays are installed in a location where ambient temperatures may exceed 30ºC. For example, when installing 90ºC rated conductors in a location where the ambient temperature may reach 40ºC, a further correction to 90 percent of the above calculated values would be necessary.
As with earlier articles, you should always consult the electrical inspection authority in each province or territory as applicable for a more specific interpretation of any of the above.
Read more by Leslie Stoch