Posted By David Dini,
Saturday, July 01, 2000
Updated: Monday, February 11, 2013
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According to the U.S. Consumer Product Safety Commission (CPSC), residential electrical equipment is involved in approximately 150,000 fires each year in the United States, which result in 850 deaths, 6,000 injuries, and more than $1.5 billion in property losses. A new technology called "arc-fault detection” has been developed to reduce this problem. Research conducted by UL has shown "arc-fault detection” to be a promising technology for further reducing the risk of fire beyond the scope of conventional fuses and circuit breakers that protect branch circuits. Using this new technology may save many lives and dramatically reduce the property damage and injuries caused each year by electrical fires.
In 1995, UL completed a research project for the CPSC and issued a report titled, "Technology for Detecting and Monitoring Conditions That Could Cause Electrical Wiring System Fires.” A number of technologies were identified and explored in the report, and although there was no single technology found that would protect against all electrical ignition scenarios likely to be encountered in residential wiring systems, arc-fault detection was identified as an effective means of reducing fires due to electrical arcing.
What is "arc heating?”
An electrically caused fire can occur if electrical energy is converted to thermal energy and the heat generated is transferred to a combustible material at a rate that will cause the material to reach its ignition temperature. One mechanism involved in converting electrical energy to thermal energy in an electrical distribution system is arc heating.
"Arcing” is defined as a luminous discharge of electricity across an insulating medium. The electrical discharge of an arc can involve temperatures of several thousand degrees Celsius. In determining the heating effects of an arc, the classical Joule heating equation involving current squared multiplied by resistance (I2R), however, does not fully explain the heating effects. Although the total power dissipated in the arc is equal to the total voltage drop in the arc multiplied by the arc current, power dissipation is not uniform throughout the arc.
In general, arcing can be divided into two categories: (1) non-contact arcing and (2) contact arcing.
"Non-contact arcing” is arcing that does not require direct physical contact between the conductors or "electrodes” where the arcing is taking place. Two types of non-contact arcing involving lower voltages are: a) arcing between conductors separated by insulation that occurs across the surface of the insulation, and b) arcing between conductors separated by pyrolyzed (carbonized) insulation.
With arcing between conductors separated by insulation, the mechanism of initiating an arc between stationary conductors separated by insulation will depend on the type and geometry of the conductors and insulation between them. In the case of typical air clearances found in an electrical residential distribution system, many kilovolts may be required to initiate arcing.
With arcing between conductors separated by carbonized insulation, arcing can occur at normal operational voltages. The resulting fault-current causes the carbon path to open and an arc is established similar to parting the conductors, as with contact arcing. Carbonized insulation between opposite polarity conductors or between a line-voltage conductor and ground can lead to an across-the-line arcing fault or a line-to-ground arcing fault.
Photo 1. This "guillotine" type tester simulates an unprotected metal chair or table leg penetrating through an extension cord for the Point Contact Arc Test
"Contact arcing” is arcing that involves direct or indirect physical contact between the conductors, known as electrodes, where the arcing is taking place, such as arcing between closing or parting conductors making or breaking a circuit. With this type of arcing, the arc initiation mechanism involves a hot point (essentially from I2R heating) at the last point of contact when a circuit is being interrupted (i.e., conductors initially in contact are parting), or at the first point of contact where a circuit is being established (i.e., conductors that are initially separated and subsequently come into contact).
Contact arcing is associated with normal operational arcing that occurs with any kind of air-gap type electrical switching device. Properly designed switching devices are capable of withstanding such arcing without excessive contact damage or generation of excessive heat. Contact arcing may also be associated with arcing faults due to the unintentional creation or interruption of current.
Technology for the 20th century—overcurrent protective devices
An overcurrent protective device (OCPD), such as a fuse or circuit breaker, is specifically designed to protect elec-trical circuits against the unwanted effects of over-currents. For example, when too many products are plugged into the same electrical outlet, and the total load current exceeds the rating of the branch circuit, the OCPD will open the circuit before damage or a fire occurs. An OCPD, however, is not designed to protect a circuit against arcing faults. Because of the time-current characteristics of the OCPD necessary to provide effective protection against overcurrents, some arcing faults, including damaging arcing faults, may have time and/or current characteristics below the threshold levels necessary to open the OCPD.
The AFCI—technology for the 21st century
Photo 2. This "guillotine" type tester simulates an unprotected metal chair or table leg penetrating through an extension cord for the Point Contact Arc Test
To offer additional protection to electrical circuits against the unwanted effects of electrical arcing, manufacturers have begun incorporating arc-fault detection technology into a product known as an Arc-Fault Circuit Interrupter, or AFCI. An AFCI is a device intended to reduce the number of arcing-fault fires by opening the electrical circuit when an arc fault is detected. What differentiates an AFCI from an OCPD is the complex electronic circuitry in the AFCI that can identify specific characteristics or signatures of the current or voltage waveform that are unique to electrical arcing.
Setting the standard for AFCIs
In conjunction with the National Electrical Manufacturers Association (NEMA), UL began working with manufacturers and other interested parties in 1998 to form an Industry Advisory Group (IAG), to develop a Standard for Safety for AFCI products. In February 1999, UL published the First Edition of the Standard for Safety for Arc-Fault Circuit Interrupters (UL 1699). In this UL Standard, five different types of AFCI products are described as follows:
- Branch/Feeder AFCI– A device installed at the origin of a branch circuit or feeder, such as at a panelboard, to provide protection of the branch circuit wiring, feeder wiring, or both, against unwanted effects of arcing. This device also provides limited protection to branch-circuit extension wiring. It may be a circuit-breaker type device or a device in its own enclosure mounted at or near a panelboard.
- Outlet Circuit AFCI– A device installed at a branch circuit outlet, such as at an outlet box, to provide protection of cord sets and power-supply cords connected to it (when provided with receptacle outlets) against the unwanted effects of arcing. This device may provide feed-through protection of the cord sets and power-supply cords connected to downstream receptacles.
- Combination AFCI– An AFCI which complies with the requirements for both branch/feeder and outlet circuit AFCIs. It is intended to protect downstream branch-circuit wiring, cord sets and power-supply cords.
- Portable AFCI– A plug-in device intended to be connected to a receptacle outlet and provided with one or more outlets. It is intended to provide protection to connected cord sets and power-supply cords against the unwanted effects of arcing.
- Cord AFCI– A plug-in device connected to a receptacle outlet, to provide protection to the power-supply cord connected to it against the unwanted effects of arcing. The cord may be part of the device. The device has no additional outlets.
In addition to specific construction requirements, UL 1699 also incorporates extensive test requirements to determine that AFCI devices are: 1) effective in preventing fires due to arcing faults, 2) not subject to unwanted (nuisance) operation, and 3) not inhibited from operation by other loads or circuit characteristics that could mask or attenuate the unique properties of an arc signature. The AFCI must also pass rigorous tests that address environmental conditioning, overloads, endurance, short-circuits, voltage surge and other abnormal conditions.
Putting AFCIs to the test
Photo 3. A 12-amp vacuum cleaner with brush contacts on a universal motor is one of the loading conditions used to demonstrate resistance to unwanted operation of the AFCI
Tests that demonstrate the effectiveness of an AFCI in detecting and protecting against arcing are unique, and were developed specifically for UL 1699. All AFCIs must pass a point-contact arc test with either SPT-2 cord and/or NM-B cable. Point-contact arcing is a type of contact arcing that can occur when a sharp object cuts through parallel wires, first contacting one conductor, and then arcing when contact is made with the other conductor. For example, point-contact arcing can occur when an unprotected metal chair or table leg rests on an extension cord or appliance supply cord, and gradually or suddenly penetrates through the conductor insulation, contacting both conductors. In this case, a fire can result from ignition of the conductor insulation, or the ignition of other materials in the immediate area. An AFCI must detect point-contact arcing in circuits with available current as low as 75A, and operate to open the circuit within eight half-cycles of arcing current.
Additional arc-fault detection tests, depending upon the type of AFCI involved, include carbonized-path arc ignition, arc interruption and arc-clearing-time tests. These tests are designed to demonstrate the ability of the AFCI to detect non-contact arcing, such as arcing that can occur if electrical insulation breaks down.
When AFCIs should—and should not— operate
AFCIs respond to a specific characteristic or signature of the current waveform associated with an arcing fault. These characteristics can be a combination of the presence or absence of certain frequencies, the duration of those frequencies, the rate of rise of electrical current, and similar phenomenon. To avoid unwanted or nuisance operation, AFCIs must distinguish between an arc-fault current waveform signature and current waveforms representing a variety of normal loading conditions. The loading conditions to demonstrate resistance to unwanted operation in UL 1699 include:
- inrush currents, such as tungsten lamps and motor starting;
- normal operation arcing, such as switch or thermostat operation, and brush contact on universal motors;
- non-sinusoidal waveforms, such as those associated with electronic dimmers and computer power supplies; and
- light bulb burnouts.
Operation inhibition tests in UL 1699 are intended to address a number of situations that could inhibit an AFCI from operating when actual arcing is occurring. These conditions range from the use of electromagnetic interference (EMI) filters, which could attenuate high frequency arc-fault signatures, to circuit impedance characteristics that may be inherent to different types of wiring systems used in older and newer homes.
AFCIs and the NEC
The 1999 National Electrical Code (NEC) includes a new Section 210-12 that requires all branch circuits that supply 125-volt, single-phase, 15- and 20-ampere receptacle outlets installed in dwelling unit bedrooms to be protected by an AFCI. This requirement is scheduled to take effect January 1, 2002. The introduction of AFCIs into the NEC for bedroom outlet circuits was based on permitting these devices to be introduced to the public on a gradual basis. However, this does not prohibit the use of AFCIs in other circuits or rooms throughout the home.
Toward a safer future
Manufacturers have already begun to seek UL Listing of their AFCI products to UL 1699, and interest in these products is expected to increase significantly as contractors, electricians and consumers become more aware of their availability and intended application. In the next century, the benefit from the use of AFCIs should be significant, not only in terms of property losses and injuries caused by electrical fires each year, but also in the saving of many lives.
Read more by David Dini
Posted By Michael Johnston,
Saturday, July 01, 2000
Updated: Monday, February 11, 2013
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Overcurrent protection for electrical equipment can be accomplished by several different methods. The general rules for overcurrent protection of conductors and equipment are found in Article 240 of the National Electric Code. Section 240-2 indicates Article 440 shall be used for protection of air-conditioning and refrigerating equipment. In Part C of Article 440, specifically Section 440-21, the Code states that the requirements of Part C of Article 440 are in addition to or amend the basic requirements in Article 240. This means that the rules in Article 440 are required to be used to obtain proper overcurrent protection for air-conditioning and refrigeration equipment. The process used to size protective devices and circuit components is similar to the process used with other types of motor-operated equipment. The proper application of overcurrent protection rules for air-conditioning and refrigeration equipment can be made relatively easy by following a few basic steps as outlined in this article. To better understand the Code rules, the characteristics of these types of motors must be understood.
Photo 1. The overcurrent protection requirements differ slightly than those for standard electrical motors
The Hermetic Refrigerant Motor-Compressor
Photo 2. The overcurrent protection requirements differ slightly than those for standard electrical motors
Hermetic refrigerant motor-compressors differ from standard electric motors in a couple of distinct areas. First, a hermetic refrigerant motor-compressor is different from a standard electric motor because it has no external shaft. The motor itself operates in the refrigerant in a sealed housing. Second, these hermetic refrigerant motor-compressors do not have a horsepower rating. The hermetic refrigerant motor-compressor is rated instead by the rated-load current, which is the average current the motor will draw under normally loaded conditions. Third, hermetic refrigerant motor-compressors employ a unique method of cooling. The motor windings and bearings are kept cool by the refrigerant. This characteristic shapes the overcurrent protection requirements. A hermetic refrigerant motor-compressor can be worked much harder than a standard motor. While the motor is operating, the compressor transforms the refrigerant into a liquid, which cools both the motor and the refrigerated product or space. The cooling characteristics depend on the type of refrigerant used, the flow rate of the liquid, and other factors such as flow rate and density. Therefore, the manufacturer of the equipment determines the characteristics of the overcurrent protection system. For this reason, the overcurrent protection requirements differ slightly than those for standard electrical motors (see Photos 1, 2, and 3).
Photo 3. The overcurrent protection requirements differ slightly than those for standard electrical motors
The Two Types of Motors
The first task is to understand the difference between a standard motor and a hermetic motor-compressor. Understanding the differences between the two types of motors enables us to apply the proper Code rules. Section 440-3 requires installations of air conditioning equipment that does not employ a hermetic refrigerant motor-compressor to follow the rules of Article 430, 422, or 424 as applicable.
It is easy to make mistakes. Take, for example, the fan coil units of a walk-in dairy case. Although the function of the fan-coil unit is to cool the walk-in cooler or freezer, the fan coil unit only employs standard motors blowing cool air across a set of refrigeration coils (see Photos 4 and 5). The fan coil unit is required to comply with the requirements for motors in Article 430. Unless the equipment employs a hermetic refrigerant motor-compressor, the Code requirements of Article 440 are not applicable.
Photo 4. Although the function of the fan-coil unit is to cool the walk-in cooler or freezer, the fan coil unit only employs standard motors blowing cool air across a set of refrigeration coils
The second task is to understand that the rules of Article 440 are in addition to, or amendatory of, Article 430 and other articles of the Code. The rules for motor circuits in Article 430 are the foundation of the special requirements for hermetic motor-compressors. Other applicable Code rules apply in any situation where Article 440 does not modify or amend those rules.
The Nameplate of Combination-Load Equipment
Air-conditioning and refrigeration equipment that employs only a single hermetic refrigerant motor-compressor shall comply with Parts C and D of Article 440. The equipment label or nameplate will state the rated-load current, locked-rotor current, nominal voltage, phase, frequency, and other data. The installer must provide overcurrent and overload protection per Parts C and D.
Photo 5. Although the function of the fan-coil unit is to cool the walk-in cooler or freezer, the fan coil unit only employs standard motors blowing cool air across a set of refrigeration coils
We will focus on the combination-load equipment. This type of equipment is more common than a single motor unit. An example of combination-load equipment would be a typical air-conditioning unit. One unit will contain several different loads in combination that comprise the total electrical load of the equipment. This type of equipment will contain at least one hermetic refrigerant motor-compressor. It may also contain a cooling fan or two and possibly a crankcase heater for the compressor. Thus this type of equipment is considered as combination-load equipment.
Figure 1. The two most useful numbers are the minimum circuit ampacity and the maximum overcurrent protective device
There are several alternative methods to provide proper overcurrent protection for the equipment covered by Article 440. The combination-load equipment manufactured today should have a nameplate. The nameplate details the data needed in order to provide proper overcurrent protection for the equipment. Section 440-4(b) requires that combination-load equipment be provided with a nameplate that gives the installer and inspector valuable information. The information on the nameplate includes the manufacturer’s name, voltage, phase, rated-load amperes, etc., and two very important items. The two most useful numbers are the minimum circuit ampacity and the maximum overcurrent protective device (see Figure 1).
Some equipment data plates indicate "Minimum Overcurrent Protective Device” ratings. The rating indicates the minimum size fuse or circuit breaker needed to allow the motor to start without nuisance tripping. If present, both minimum and maximum ratings must be followed in selecting the proper protective device.
Figure 2. For combination-load equipment having a nameplate as required by Section 440-4(b), the branch-circuit conductors are required to be "not less than the minimum circuit ampacity marked on the" nameplate of the equipment
The Branch Circuit Requirements
The branch-circuit conductor size requirements for hermetic refrigerant motor-compressors are located in Part D of Article 440. The sizing requirements for the branch-circuit conductors are very similar to those requirements for standard motors. Basically, the branch-circuit conductors are required to be sized at 125 percent of the rated-load current of the single hermetic motor-compressor or 125 percent of the branch-circuit selection current, whichever is less. However, for combination-load equipment having a nameplate as required by Section 440-4(b), the branch-circuit conductors are required to be "not less than the minimum circuit ampacity marked on the” nameplate of the equipment (see Figure 2). See NEC Section 440-35.
The manufacturer has already calculated the conductor size to be based on the total of all of the motor loads in the combination-load equipment times 125 percent. It is not necessary to do these calculations again. For this type of equipment, the installer and the inspector only have to install and verify that the branch-circuit conductors supplying the equipment have an ampacity equal to or greater than the minimum circuit ampacity marked on the nameplate of the equipment.
The Branch-Circuit Short-Circuit Ground-Fault Protection
The nameplate data is also used to select the proper size or rating of the branch-circuit short-circuit and ground-fault protective device. The manufacturer may limit the choice of devices. Fuses and/or HACR rated circuit breakers are normally used for this type of protection.
The branch-circuit short-circuit ground-fault protective device for hermetic refrigerant motor-compressors is required to not exceed 175 percent of the motor-compressor rated-load current. The rating or setting of the protective device may be increased if the initial setting is insufficient for the starting current. The maximum rating or setting is limited to 225 percent of the rated-load current of the motor-compressor or the branch-circuit selection current, whichever is greater. See Section 440-22(a) of the Code.
A hermetic motor-compressor draws locked-rotor current at startup. The branch-circuit short-circuit ground-fault protective device is permitted to be increased by these percentages to allow the motor-compressor to start without tripping the overcurrent device. However, for combination-load equipment, the Code requires that the nameplate be marked to indicate the maximum rating of the overcurrent protective device. The manufacturer has again already done the calculation for the installer or inspector. No additional calculations are necessary in the field to size the overcurrent protective device.
Figure 3. Be sure to use a HACR rated circuit breaker if so indicated on the nameplate
Sometimes the manufacturer of the combination-load equipment will specify fuses as the overcurrent protective device. This is important information and must be followed. If the nameplate says fuse only, the equipment has been evaluated and tested only with a fuse. The manufacturer has determined that only a fuse provides the proper overcurrent protection for the hermetic refrigerant motor-compressor and the other internal components. Use of a circuit breaker would be in violation of the Code Section 440-4(b) and 440-22(c). This would also be in violation of Section 110-3(b). This is the equivalent of not following the manufacturer’s instructions provided with the equipment. Not following the instructions is the same as not following the Code.
Most manufacturers permit either fuses or HACR circuit breakers as the protective device. If the equipment is marked "maximum fuse size*” and the * at the bottom of the nameplate indicates "or HACR circuit breaker” then the equipment has been evaluated and tested for use with either form of overcurrent protection. A HACR breaker is a type of circuit breaker that is listed for group applications. In other words, the breaker is able to supply proper protection for both the larger compressor motor circuit as well as the components of the smaller fan motor circuit. Be sure to use a HACR rated circuit breaker if so indicated on the nameplate (see Figure 3).
The "maximum overcurrent protective device” rating is the other very important number on the data plate. The overcurrent protection device marked on combination-load equipment is marked "maximum” such as "maximum fuse size.” This means that specified size cannot be exceeded. The device could be less than that maximum size.
Figure 4. Values on a typical air-conditioning unit nameplate
It may seem that the conductors are improperly protected. However it is the combination of the maximum size short-circuit and ground-fault protective device together with the overload protection system of the equipment that is providing the overcurrent protection for all circuit components. If the overload protection is field-installed for a hermetic refrigerant motor-compressor, the overload sizing must be in accordance with Section 440-52 and must not exceed the manufacturer’s values.
In Figure 2, the nameplate indicates the minimum circuit ampacity and the maximum overcurrent device. Based on the nameplate data the conductors are required to be capable of carrying 27.8 amperes. Remember, with combination-load equipment the 125 percent factor is already used by the manufacturer to determine the 27.8-ampere total. A No. 10 THWN copper conductor is an acceptable size for the circuit conductors. The maximum overcurrent protective device marked on the equipment is 40 amperes. It appears as though the No. 10 THWN conductors are improperly protected. This is not true. The 40-ampere fuse or HACR circuit breaker provides the short-circuit and ground-fault protection. The overload protective device limits the normal running current to the proscribed values.
The overcurrent protective device could be a device rated smaller than 40 amperes as long as it can handle the starting and running current of the equipment. These maximum values are often misunderstood to be the only size allowable by the Code, when in fact it is the value that must not be exceeded.
Equipment Requiring Two Supply Voltages
Figure 5. Consequently, since the equivalent full-load current of this A/C unit is 19.3 amperes, the next higher rating must be used, and the disconnect switch must have a minimum of a 5-horsepower, 230-volt, single-phase rating
Air-conditioning and refrigeration equipment that requires two supply circuits at different voltages is required to be marked with a nameplate indicating the minimum circuit ampacity and maximum overcurrent protective devices required for each of the circuits supplying the equipment. This can appear on the same nameplate but it is more common to see individual nameplates. It is not uncommon to see a refrigeration compressor rack in a grocery store that requires the two circuits [see Section 440-4(b)].
Disconnecting Means Rating
The rules in 440-12 determine the minimum rating and interrupting capacity of the disconnecting means. Where the air-conditioning or heat pump compressor unit consists of a hermetic refrigerant motor-compressor(s) in combination with other loads, such as the fan motor, the horsepower rating of the disconnecting means is based on the summation of all currents at both rated-load condition and also at locked-rotor condition. For example, using the values on a typical air-conditioning unit nameplate, (see Figure 4) the 18-ampere rated-load current (RLA) of the compressor motor is added to the 1.3 ampere full-load current (FLA) of the fan motor.
The total of 19.3 amperes is then considered to be the equivalent full-load current for the combined load. According to NEC Table 430-148, the full-load current rating of a 230-volt, single-phase, 3-horsepower motor is 17 amperes, while the full-load current rating of a 230-volt, single-phase, 5-horsepower motor is 28 amperes. Consequently, since the equivalent full-load current of this A/C unit is 19.3 amperes, the next higher rating must be used, and the disconnect switch must have a minimum of a 5-horsepower, 230-volt, single-phase rating (see Figure 5).
Figure 6. Section 440-14 requires a disconnecting means for air-conditioning and refrigeration equipment to be located within sight from the equipment it supplies
The ampere rating of the disconnecting means must also be at least 115 percent of the sum of all currents at rated-load condition. This minimum rating would then be 115 percent x 19.3 amperes = 22.19 amperes. If the disconnecting means includes or serves as the branch-circuit overcurrent protection for the unit, the rating required for the overcurrent device, rather than this minimum rating, would generally be the determining factor in sizing the disconnecting means. A fused disconnect switch containing either the maximum or minimum sizes of fuses listed on the nameplate would exceed this 115 percent minimum requirement. If an unfused disconnect switch is used as the disconnecting means, however, then this 115 percent rating and the horsepower rating would establish the minimum switch rating.
There is one other consideration in establishing the correct size of the disconnecting means serving the air-conditioning unit. The disconnecting means rating must also be based on currents at locked-rotor condition. Refer to NEC Table 430-151(A) for the conversion of locked-rotor current (LRA) to horsepower. In our example, the nameplate indicates that the motor-compressor LRA is 96 amperes. Since the nameplate does not give a LRA for the fan motor, we assume it to be six times the FLA or 6 x 1.3 amperes = 7.8 amperes. Adding this to the motor-compressor LRA of 96 amperes gives us an equivalent LRA for the combined load of 103.8 amperes. Again referring to NEC Table 430-151, we find that for a single-phase, 230-volt motor with a 103.8 amperes motor locked-rotor current, the disconnect switch should be based on a 5-horsepower rating. See NEC Section 440-12.
Photo 6. The disconnecting means may be located on or within the air-conditioning or refrigerating equipment.
Trying to use the nameplate ratings to size the disconnecting means can be confusing. For example, consider the nameplate information "minimum circuit amperes = 26″ and "maximum overcurrent protective device = 35.” Is a 30-ampere disconnect suitable for use with this particular unit? This is why the locked-rotor ampere marking is important. Since there is no horsepower rating on the hermetic refrigerant motor-compressors, the locked-rotor equivalent must be obtained by using the values in Table 430-151(A) or (B), as appropriate. Using the rated total load amperes of the equipment, we can determine if the disconnecting means has a sufficiently large horsepower rating. Disconnect switches having the same ampere rating may have different horsepower ratings. Installers and inspectors should carefully observe the markings on both the equipment and the disconnecting means. The rating of the disconnecting means is especially critical for larger equipment. Disconnecting means for equipment with an equivalent horsepower rating in excess of 100 horsepower are required to comply with Section 430-109. If general duty switches are used as disconnecting means for equipment exceeding 100 horsepower, the disconnecting means is required to be marked "Do Not Operate Under Load.” The installer normally applies this additional marking.
Photo 7. The disconnecting means may be located on or within the air-conditioning or refrigerating equipment.
Location of the Disconnecting Means
Section 440-14 requires a disconnecting means for air-conditioning and refrigeration equipment to be located within sight from the equipment it supplies (see Figure 6). The disconnecting means may be located on or within the air-conditioning or refrigerating equipment. See Photos 6 and 7.
There are two exceptions to this general requirement. One exception allows a cord and plug to be utilized as the disconnecting means for portable or window-type air-conditioning equipment, and the other exception allows air-conditioning equipment in a large industrial process line to have a means out of sight but capable of being locked in the open position (see Photo 8).
UL 1995 – Heating and Cooling Equipment. (This standard covers central heating, central air-conditioning, and heat pumps.) UL 484 – Room Air Conditioners. These product safety standards detail the necessary safety tests and determine the required nameplate markings and instructions that are included by the manufacturer of the equipment. For example, paragraph 36.3(i) of UL 1995 specifies the equipment shall be marked with a "maximum overcurrent protective device size.” A typical nameplate will show the "MAXIMUM FUSE” and/or "MAXIMUM CIRCUIT BREAKER” size. If the nameplate specifies only fuses, then the unit is intended to be protected by fuses only. If the nameplate requires HACR (Heating, Air-Conditioning and Refrigeration) circuit breakers, then the circuit breaker protecting the unit must be marked "HACR.”
If the nameplate includes both fuses and HACR circuit breakers, as is the case of our nameplate example, then either is acceptable.
The selection process for hermetic motor-compressor circuit components is somewhat different from that of other motors. Using the markings on the end-use equipment helps ensure proper protection.
Read more by Michael Johnston
Posted By George Gregory,
Saturday, July 01, 2000
Updated: Monday, February 11, 2013
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The arc-fault circuit interrupter (AFCI) is emerging as a new device in the National Electrical Code and in residential installations to enhance electrical safety. New technology generally fosters questions and concerns about the workings and application of the technology and this article will address some of the most frequently asked questions surrounding AFCI:
- How does the AFCI work? and,
- What is the state of the National Electrical Code regarding AFCIs?
This article addressees both questions.
What is the Purpose of an AFCI?
Figure 1. Typical time-current characteristics of a circuit breaker
We need to think about why the AFCI was developed in order to assist our understanding of how they work. Its purpose is to eliminate certain unwanted arcs as potential, electrical fire causes. Electric arcs operate at several thousand degrees Celsius at their center. They also generate a pressure wave that will blow molten metal or burning material from their center onto ignitable materials. Either the high temperature or the materials discharged from the center of the arc can cause a fire. The intent of the AFCI is to detect hazardous arcing and turn off the circuit in order to greatly reduce the potential of fire from an arc.
Since an overcurrent protective device (OCPD), a circuit breaker or fuse, will detect and interrupt an arc above the OCPD characteristic curve, circuits are already protected against these higher current arcs. The AFCI addresses arcs below and to the left of the characteristic curve of an overcurrent protective device. The blue colored area in figure 1 represents the unprotected area in which the AFCI detects potentially harmful arcs.
Arcing Fault Hazards
We are all aware that arcs occur in useful circuits today. For example, arcs are found in universal motors, light switches, and thermostatically controlled appliances. Also, we find circuit signals that may look much like some arc characteristics in other equipment and appliances. Computers, audio equipment, light dimmers and many other devices with electronic controls generate "noisy” or unusual signals that could appear to be an arc to some form of detection devices. We wish for the circuit to continue to supply energy to these useful items, but we want the circuit to be interrupted when a potentially hazardous arc emerges. The AFCI must distinguish between useful conditions and hazardous arcing.
Figure 2. Connections to an AFCI
Hazardous arcing faults may occur in any of three configurations:
- Series (such as in a broken or frayed wire or at a loose connection),
Line-to-neutral faults will generally be at higher current levels, close to the system available short-circuit current at the point of the fault. In a study done by Underwriters Laboratories for the Electronic Industries Association (EIA), data shows that available short-circuit current at receptacles in residences ranges from approximately 75 A to 1650 A with an average of 300 A for 15 A branches and 467 A for 20 A branches.1 This data gives a good idea of the current levels available in branch circuits and it is for this reason that the point contact tests for AFCIs in UL 1699, Underwriters Laboratories Inc. Standard for Safety for Arc-Fault Circuit Interrupters, are done at 75 amperes and greater.
Figure 3. A typical circuit breaker AFCI unit.
A research program conducted during the development of UL 1699 indicated fire ignition of test materials with arcs of 5 amperes and higher. For this reason, tests identified as carbonized path tests in the Standard are done with arcing currents of 5 amperes and greater.
The purpose of this section is not to examine the testing standard or arc phenomena in depth, but rather to get an idea of how an AFCI works. So far, we know that the AFCI must detect a hazardous arc with current levels up to those of an overcurrent protective device characteristic. It must be capable of opening the circuit when one is detected. It must distinguish between hazardous arcs and normal circuit conditions involving arcs or signals that look like an arc. (See Figure 2)
How Does an AFCI Work?
Any statement about how AFCIs function today may be changed by the introduction of a new product tomorrow that employs a different technology. However, we can gain a general idea of how they work by understanding that the current and voltage waveforms of an arc have some distinctive characteristics.
Today’s AFCIs employ two complementary methods of detection: arc recognition and ground fault detection. We will look at each of them.
Figure A. The simple case of arcing current in a resistive circuit
The information in the sidebar tells us that voltage and current waveforms will exhibit some unique characteristics including the following:
- Flat "shoulders” in the current around current zero
- Arcing current lower than ideal current
- Voltage across the arc approaching a square wave
- Voltage spikes each half cycle as the arc ignites and extinguishes
- High frequency "noise”
Figure B. The simple case of arcing current in a resistive circuit
Voltage and current signals can be taken by an AFCI unit connected as indicated in figure 2. The connections and current transformer coils indicated in figure 2 constitute one method of obtaining the circuit information for the unit. There may be many other means of obtaining the same information. The AFCI unit will use the circuit information to perform an analysis to determine whether an arc is present and whether it is an arc that should be interrupted. It will very likely be looking for several simultaneous indications of arc presence and persistence in order to verify that the signal is from a hazardous arc. On determining that a hazardous arc is in the circuit, the unit signals interruption of the circuit.
The various manufacturers of today’s AFCIs use different methods of obtaining circuit information and analyzing it. The descriptions in this article provide a generic overview and are not intended to represent exact methods employed for a specific product. The signals may be captured by variations of the configuration of figure 2. Also, some of the units use a microprocessor to perform the analysis, while others use electronic circuitry to perform a direct analysis. The characteristics presented in this article are indications of available characteristics and are not necessarily those used by the various manufacturers. However, all of the units perform an analysis of circuit signals to identify the arc.
Ground Fault Detection
In evidence from fires caused by electrical arcing, line-to-ground leakage is involved, if not the direct path of the fault in many of the instances. Since an arcing fault is probably in the process of burning away insulation, there is a high likelihood that stray current will reach a path to ground. The path might be through metal appliance housings, fluids, equipment grounding conductors or conduit. Where arcing is involved, a frequent path is arc tracking across insulation that has pyrolyzed (carbonized) from the heat of the arc.
Since line-to-ground faults are a major factor in hazardous arcing faults, present manufacturers have included equipment level ground fault detection as an integral part of the AFCI detection system. Unless labeled otherwise, this ground-fault protection is not the 6-milliampere protection intended for personnel. That is, it is not GFCI protection unless so labeled. For present products, it is a 30-milliampere protection of the kind employed for protection of heating tape and other equipment.
The UL 1699 Standard does not require the 30-milliampere detection level. In fact, it requires detection at 5 amperes and above for faults to ground because it is at the 5-ampere level that fire causes are evident from research experiments. However, since the lower level ground-fault technology is readily available, some manufacturers have chosen to incorporate it.
Forms of AFCI
The UL 1699 Standard covers a number of types of AFCI, each to protect against unwanted effects of arcing.
The Branch/Feeder AFCI is a device installed at the origin of a branch circuit or feeder to provide protection of branch circuit and feeder wiring. This device also provides limited protection to appliance and extension cords. The B/F AFCI is available in circuit breaker form today but is not limited to that form.
The Outlet Circuit AFCI is a device installed at a branch circuit outlet to provide protection of appliance and extension cords. A likely form is a receptacle outlet. It may provide feed-through protection to cords connected to receptacles on its load side.
The Combination AFCI combines the functions of the B/F and OC AFCI in a single device. It is intended to protect branch circuit and feeder wiring and appliance and extension cords.
Figure C. Figure C shows a trace of current in a cord to a 10 A resistive load when the cord is cut by a "guillotine” similar to a paper cutter.
The Portable AFCI is a plug-in device intended to be connected to a receptacle outlet and which is provided with one or more outlets. It is intended to provide protection to connected appliance and extension cords.
The Cord AFCI is a plug-in device connected to a receptacle outlet, to provide protection to the cord connected to it.
The Standard does not specify the form of any of the types, which will allow them to emerge into the marketplace as needed for a specific application. This discussion will focus on the B/F AFCI, since it is the type the inspector may expect to see to meet the new National Electrical Code requirement. The B/F AFCI is presently available in the form of a circuit breaker that also incorporates the AFCI function. Figure 3 shows a typical circuit breaker AFCI unit.
Figure D. Figure D shows the current trace of four computers all energized at the same time and then reaching steady-state current.
These units provide all overcurrent protection functions and ratings of a standard circuit breaker and simply add the AFCI function. When a potentially hazardous arc is detected, the sensing unit signals the circuit breaker to trip and open the circuit breaker contacts. These units are intended to be installed in existing load centers in the position of a standard single-pole circuit breaker. Connections are similar to those of the circuit breaker and ground-fault circuit interrupter (GFCI). They have interrupting ratings, series ratings, SWD and HACR ratings similar to the circuit breaker of the same type.
AFCIs and the NEC
The new Section 210-12(b) of the 1999 National Electrical Code (NEC) reads:
"(b) Dwelling Unit Bedrooms. All branch circuits that supply 125-volt, single-phase, 15- and 20-ampere receptacle outlets installed in dwelling unit bedrooms shall be protected by an arc-fault circuit interrupter(s). This requirement shall become effective January 1, 2002.”
The revision cycle for the 2002 NEC has started with 20 proposals to revise Section 210-12 and two for other articles.
At this point in the development of the 2002 NEC, Code-Making Panel 2 has retained 210-12 much as it is in the 1999 NEC. They deleted the word "receptacle” from "receptacle outlets” resulting in a slight expansion of the requirement to include all bedroom outlets including lighting outlets. They also deleted the statement about the 2002 effective date. Panel statements make clear that they are not immediately ready to expand the requirement for this new technology for which little public experience exists. They were also clear in acknowledging the potential for reducing fire causes with AFCIs. A number of testimonials to AFCI effectiveness in detecting arcing faults appeared in proposal substantiations.
Section 210-12 with the revision requires AFCI protection of 15- and 20-ampere branch circuits, which means that the AFCI must be the Branch/Feeder type. This B/F type has been commercially available since 1998 and is presently the only type commercially available.
Of the two proposals not related to 210-12, one was for Article 230 to require AFCIs to be installed in service equipment when it is replaced in existing dwellings. CMP 4 has not accepted the Article 230 revision. The other proposal was for Article 550 to require AFCIs in bedrooms and living areas of mobile homes and manufactured homes. CMP 19 acted favorably on the Article 550 proposal but only to mirror the 210-12 requirement for protection of bedroom outlets.
Figure E. Figure E shows traces of a dimmer with a 1000-watt tungsten lamp load.
The state of Vermont has adopted Section 210-12 with an early effective date of January 2001. They are also requiring AFCIs in circuits feeding receptacle outlets in living areas of dwellings in addition to those of bedrooms.
The AFCI is still an emerging product. Arc detection is accomplished through recognition of arc characteristics in circuit waveforms. Ground-fault detection at 30 milliamperes is added by today’s manufacturers for excellent protection with avoidance of possible nuisance operation. Arc detection technology is becoming better understood by circuit design engineers which means that we can expect even better detection with experience. Indications are that the 2002 NEC will retain installation of the AFCI as a requirement for bedroom circuits in dwellings. There is still an opportunity for comment prior to the December Report on Comment meetings.
1 Fact-Finding Report on an Evaluation of Branch-Circuit Circuit-Breaker Instantaneous Trip Levels, (Underwriters Laboratories Inc., October 25, 1993).
As might be expected, arc current and voltage waveshapes are generally not simple sinusoids. This section shows several current and voltage traces from arcing conditions. Also shown are non-arcing conditions with characteristics that might seem similar to an arc. These are just examples of the countless loads and conditions that may be encountered in residences. An arc-fault circuit interrupter (AFCI) must accurately distinguish between an arc and needed energy if it is to be effective.
Resistive Load in Series with an Arc
Figures A and B show the simple case of arcing current in a resistive circuit. Notice that arc current is less than the ideal current due to the voltage drop across the arc. An arc looks much like a resistor element in a circuit and will frequently have a constant voltage drop of between 20 and 30 volts across it. Also, notice that "shoulders” appear on the current traces around the current zero locations. The arc ignites only after sufficient voltage across the gap returns following a current zero. It extinguishes when voltage drops below that needed to sustain the arc. Arc voltage is almost a square wave except for the transient near current zero. The choppiness of the voltage wave indicates another distinct characteristic of the arc, which is that of a high frequency voltage source.
With a series arc present, the rms value of current and I2t sensed by an overcurrent protective device in the circuit is less than it would sense without the arc. In other words, an overcurrent protective device would be less likely to operate effectively with a series arc in the circuit than without.
Figure C shows a trace of current in a cord to a 10 A resistive load when the cord is cut by a "”guillotine”" similar to a paper cutter. This action might simulate metal furniture cutting the cord, causing a line-to-neutral fault. Voltage shown is that across the cord ahead of the cut. Available short circuit current is 70 A.
The arc is not continuous. Once it starts, it is interspersed with segments of normal load current. Again, the rms value of current and I2t delivered is considerably less than that of a solid fault, especially when viewed over several cycles. The shoulders are again visible in the arc current trace of Figure C.
Switching-mode Power Supplies
Figure D shows the current trace of four computers all energized at the same time and then reaching steady-state current. Shoulders similar to those for arcing current are present in both start-up and steady state traces due to the high harmonic content of the load. Start-up current has characteristics similar to those of an arcing short circuit. Such loads as this must be allowed to start and run without activating an AFCI.
Figure E shows traces of a dimmer with a 1000-watt tungsten lamp load. This load waveform also has trace shoulders and high peaks with similarities to a sputtering arc.
Traces of arc current and voltage illustrate that:
1. Current with an arc in series has a lower rms value than current without the arc due to extinction and re-ignition around current zero.
2. When a sputtering arc exists in a line-to-line fault, the rms value or I2t delivered over several cycles is considerably less than that of a solid fault.
3. Voltage across the arc has characteristics of a square wave with some high frequency noise.
Information in this sidebar extracted from George D. Gregory and Gary W. Scott, "The Arc-Fault Circuit Interrupter, An Emerging Product,”IEEE Transactions on Industry Applications, September/October 1998, pp. 928-933.
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Posted By Philip Cox,
Saturday, July 01, 2000
Updated: Monday, February 11, 2013
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Over 4700 proposals were submitted to the National Fire Protection Association to amend the 1999 National Electrical Code. The 20 NEC Code-Making Panels met in January 2000, and took action on proposed changes to articles of the Code within the scope of each respective panel. Listed below is the first of a two-part brief summary of some significant changes accepted by the panels. The second part of the summary will be in the September/October issue. A more comprehensive coverage of accepted changes will be published following final action in 2001. The NEC is amended on a three-year cycle and follows the ANSI-approved National Fire Protection Association open consensus process of developing codes and standards. The code change process is open and anyone can submit proposals and public comments. Blank forms and information on submitting proposals and public comments are readily available for those wishing to participate. Code-Making Panels review and act on all proposals and public comments submitted to them.
Guidance was provided to the 20 Code-Making Panels on several issues that are to be addressed during the 2002 NEC cycle. One item for action is that of placing international units (SI) of measure as the primary number and locating the standard inch-pound measures in parenthesis following the SI units. The general application rule in the conversion is that of using a "hard conversion” where the SI value is not an exact conversion, such as where "30″ (762 mm)” is revised to read "750 mm (30″).” Where the measurement has an impact on safety, a "soft conversion” is used where the SI values are a more exact conversion. An example of the "soft conversion” is changing "30″ (762 mm)” to read "762 mm (30″).”
Another item involves the use of the term "luminaire.” Action taken by affected Code-Making Panels and the NEC Technical Correlating Committee resulted in the acceptance of the term "luminaire” as a replacement of the terms "lighting fixture,” "lighting fixtures,” and "fixture” in appropriate locations in the Code. The term "fixture” has many other uses than that of describing a complete lighting unit. The term "luminaire” is used in both national and international product standards. The NEC is an international electrical code and, as such, it is appropriate to use this internationally recognized term. In a effort to make the conversion easier, the word "fixture” or "fixtures” will follow the term "luminaire” in parenthesis. The definition of "luminaire” is also being added to Article 100.
The 2002 NEC is scheduled to undergo a significant change in its style. The numbering system will change to be more consistent with other standards and continuing effort is being made to make the document more user friendly. The number of exceptions to the general rules is being reduced and many of these exceptions have been converted into positive text.
Many article numbers in Chapter 3 will be changed for the 2002 Code. This action is being taken to make the Code easier to use by grouping articles covering related wiring methods. Some articles in Chapter 3 are proposed to be relocated to other chapters of the Code because they appear to be better suited to the scope of those chapters.
Article 80—Administration and Enforcement:
Proposal 1-3:A new Article 80 was accepted to provide a standardized set of administrative rules that can be used by adopting jurisdictions. The proposed new article is similar to NFPA 70L but Article 80 is intended to be used with the NEC. While proposed Article 80 is to be included in the NEC, it is to be informative unless specifically adopted by the local jurisdiction. This set of administrative rules will be very helpful to jurisdictions that are either adopting the Code for the first time or have adopted it but want to revise the existing administrative provisions. This proposed new article will provide a basis for the adoption of more uniform administrative provisions across the country.
Article 90 – Introduction.
90-1(c). Proposal 1-5:The recommendation in this proposal is to delete the wording in 90-1(c). This section presently reads "(c) Intention. This Code is not intended as a design specification nor an instruction manual for untrained persons.”" The proposer contends that the provision covering design specifications is both misunderstood and inappropriately applied. Section 90-1(b) states, "This Code contains provisions that are considered necessary for safety.” It is evident by the multitude of provisions presently in the Code that contain design requirements that it has never been the intent of the Code to restrict the inclusion of such provisions. Section 90-1(c) has been cited many times as an argument against rules being adopted into the Code because they had design provisions in addition to safety rules.
90-1(d). Relation to International Standards.
Proposal 1-6:A new 90-1(d) and Fine Print Note (FPN) was added to read, "(d) Relation to International Electrotechnical Commission Standards. The requirements in this Code address the fundamental principles of protection for safety contained in International Standard "Electrical Installations of Buildings, IEC 60364-1, Section 131.
"FPN: IEC 60364-1, Section 131 contains fundamental principles of protection for safety that encompass: protection against electric shock, protection against thermal effects, protection against overcurrent, protection against fault currents, and protection against overvoltage. All of the above potential hazards are addressed by the requirements in this Code.”
The National Electrical Code is an international electrical code and so states on the cover of the 1999 edition. The NEC has been adopted and used in several countries around the world, but it is contended by some that IEC 60364 is the international standard. As pointed out in the last sentence of the Fine Print Note, the NEC includes provisions addressing those principles in IEC 60364.
Proposal 1-10:This proposal is a procedural follow-up on the action by the NEC Technical Correlating Committee to hold for further study Proposal 1-13 and Public Comment 1-18a of the 1999 Code cycle. Section 90-2(a) has been revised to read "90-2(a) Covered. This Code covers the installation of electric conductors, electric equipment, signaling and communications conductors and equipment, and fiber optic cables and raceways for the following:
1. Public and private premises including buildings, structures, mobile homes, recreational vehicles, and floating buildings.
2. Yards, lots, parking lots, carnivals, and industrial substations.
3. Installations of conductors and equipment that connect to the supply of electricity.
4. Installations used by the electric utility, such as office buildings, warehouses, garages, machine shops, and recreational buildings, that are not an integral part of a generating plant, substation, or control center.” The existing FPN following this section remains unchanged.
Section 90-2(b)(5) has been reworded to read, "Installations under the exclusive control of an electric utility where such installations consist of wiring for service drops or laterals or are located in legally established easements, right-of-ways, or on property owned or leased by the electric utility for the purpose of communications, metering, generation, control, transformation, transmission, or distribution of electric energy.”
Section 90-2(a) was editorially revised to more uniformly group similar types of installations together and to make it easier to read. Section 90-2(b)(5) was revised to better define the wiring identified as not covered by the NEC. The proposed wording in 90-2(b)(5) is an attempt to focus on more readily definable areas such as easements and specifically includes service drops and laterals. Local authorities, such as Public Service Commissions, generally have jurisdiction over dedicated areas for utility wiring. The wiring identified by ownership or lease and by purpose as not covered by the NEC is specified in 90-2(b)(5).
Proposal 1-93:The second paragraph of Section 90-4 has been revised by adding the wording "by special permission” at the beginning of the paragraph and will read: "By special permission the authority having jurisdiction may waive specific requirements in this Code or permit alternate methods where it is assured that equivalent objectives can be achieved by establishing and maintaining effective safety.”
This change has the effect of requiring the authority having jurisdiction to specify in writing variations from Code requirements given by that AHJ. Special permission is defined in Article 100.
Article 100 – Definitions:
Article 100 – Qualified Person.
Proposal 1-178: The definition of qualified person has been revised to read: "Qualified Person. One who has skills and knowledge related to the construction and operation of the equipment and has received safety training on the hazards involved.”
This revised definition is a better description of a qualified person. Being qualified is more than just being familiar with the construction and operation of the equipment and the hazards involved.
110-15. Flash Protection.
Proposal 1-235:A new section was added to read: "”110-15. Flash Protection. Switchboards, panel-boards, and motor control centers installed in other than residential occupancies shall be marked in the field to indicate the incident energy in calories per square centimeter for a worker at a distance of 18 in.”"
This new section addresses the concern over the protection of electricians who have to work equipment while it is energized. While it is necessary to work some equipment hot, the worker can be better protected against flash burns if the proper type of protective equipment is selected.
110-26(c).Entrance to Working Space. Proposal 1-260a:This title of Section 110-26(c) has been revised, the exceptions have been amended and located as part of the standard text, and the section has been restructured. The wording "Access and” has been removed from the title and it will now read "Entrance to Working Space.”" A new sentence has been added to the newly designated "(2) Large Equipment” to read, "Where the entrance has a personnel door(s), the door(s) shall open in the direction of egress and be equipped with panic bars, pressure plates, or other devices that are normally latched but open under simple pressure.”
This change addresses the concern for the safety of people who work on energized equipment described in this section and their ability to get out of personnel doors that provide access to and exit from that area during an emergency. The door opening devices that operate on simple pressure as required by this section will permit an injured worker to exit the area more easily than they could if they had to operate a standard door knob.
110-26(f). Dedicated Equipment Space. Proposal 1-271a:This section covering "dedicated equipment space” has been revised by deleting the existing wording in the exception under 110-26(f)(1)(a) and adding the wording, "Suspended ceilings with removable panels shall be permitted within the 1.8 m (6 ft.) zone.” The wording in 110-26(f)(b) has been revised to read: "(b) Foreign Systems. The area above the dedicated space required by 110-26(f)(1)(a) shall be permitted to contain foreign systems provided protection is installed to avoid damage to the electrical equipment from condensation, leaks, or breaks in such foreign systems.”
This revision should clarify that equipment not associated with the electrical equipment covered in this section is not permitted to be installed in the six-foot area above the electrical equipment. The question of suspended ceilings within the six-foot required clearance is also addressed by the addition of the new wording.
110-33. Entrance and Access to Work Space.
Proposal 1-291a:This section has been amended by adding the same wording regarding doors that open under simple pressure as that inserted in 110-26(c).
Article 200. Use and Identification of Grounded Conductors:
Proposal 5-9:The word "natural” has been deleted from this section and other sections in the Code where used as part of the term "natural gray.” A new Fine Print Note has been added to read, "FPN: The color gray may have been used in the past as an ungrounded conductor. Care should be taken when working on existing systems.”
This action will address the concern expressed by many that the term "natural gray” is not defined and there is no recognized color or tint that one can readily associate with that term. The Fine Print Note is cautionary because gray has been used in some cases as the ungrounded conductor.
Article 210. Branch Circuits:
210-6. Branch-Circuit Voltage Limitations.
Proposal 2-16:A new 210-6(e) has been added to read: "(e) Over 600 Volts Between Conductors. Circuits exceeding 600 volts nominal between conductors shall be permitted to supply utilization equipment in installations where conditions of maintenance and supervision ensure that only qualified persons will service the installation.”
This change clarifies that circuits exceeding 600 volts are permitted when maintenance and supervision ensure that qualified persons will service the installation.
210-7. Receptacle and Cord Connectors. Proposals 2-18 & 2-19:Existing Section 210-7 is proposed to be relocated to a new Article 406 covering receptacles, cord connectors, and attachment plugs and will be under a heading of "General Installation Requirements.” A new 210-7 has been created to read: "210-7 Branch-Circuit Receptacle Requirements. Receptacle outlets shall be located in branch circuits in accordance with Part C of Article 210. Specific requirements for receptacles are covered in Article 420.”
The relocation of this material is more appropriate because the proposed new Article 406 covers general requirement for how receptacles are installed and these requirements in existing 210-7 fall within that scope.
210-8(a)(7). Wet Bar Sinks.
Proposals 2-53 & 2-54:The last sentence of 210-8(a)(7) that reads: "Receptacle outlets shall not be installed in a face-up position in the work surfaces or countertops” is to be reworded and relocated to new Article 406 under the heading of "Receptacles in Countertops and Similar Work Surfaces in Dwelling Units.” This provision is more appropriate for Article 406 than it is for Article 210.
Proposal 2-57:A new 210-8(a)(8) has been added to read, "(8) Boathouses.”
Requirement for GFCI protection for receptacle outlets in residential boathouses in Section 555-3 has been deleted from that location and relocated to Article 210. This is the result of a change in Scope of Article 555. Article 555 will no longer cover single-family private residential docking facilities.
210-12. Arc-Fault Circuit-Interrupter Protection.
Proposals 2-102, 103, 110, 112, 113, 115, 116: Theterm "receptacle” and the last sentence in 210-12(b) have been deleted. Section 210-12(b) will now read, "(b) Dwelling Unit Bedrooms. All branch circuits that supply 125-volt, single-phase, 15- and 20-ampere outlets installed in dwelling unit bedrooms shall be protected by an arc-fault circuit interrupter(s).”
This change will extend the arc-fault circuit-interrupter protection to all outlets in dwelling unit bedrooms. The requirement previously applied only to the receptacle outlets.
Proposal 2-130:The word "allowable” has been added before the term "ampacity” in the first sentence of the Exception and a new last sentence has been added to read, "In no case shall the ampacity be less than the rating of the overcurrent device.”
Adding the word "allowable” clarifies that the ampacity of a conductor includes the application of adjustment factors if they apply. The last sentence requires the conductor ampacity to be not less than the rating of the overcurrent device for an overcurrent device that is listed for 100 percent operation.
210-19(d), Exception No. 1.
Proposal 2-133: The exception to 210-19(d) has been revised by adding a provision in the main paragraph requiring the tap conductors to have an ampacity sufficient for the load served. This will clarify that tap conductors must be properly sized.
210-23. Permissible Loads.
Proposal 2-142(a):210-23(a) has been revised to improve readability and has integrated language in 210-23(a)(1) to require cord- and plug-connected utilization equipment to be listed and marked to inform the user of the necessity for providing an individual branch circuit for such equipment that exceeds 80 percent of the branch-circuit.
210-52.Dwelling Unit Receptacle Outlets. Proposal 2-153(a): A new first sentence has been added to 210-52 to read, "This section provides requirement for 125-volt, 15- and 20-ampere receptacle outlets.”
The added first sentence clarifies that the required receptacles are to be 125-volt, 15- or 20-ampere configuration only. If other configurations are desired, they may be installed in addition to the required ones.
210-52(c)(5).Receptacle Outlet Location. Proposal 2-171: The maximum distance above a countertop that a receptacle serving that countertop is permitted to be mounted has been changed from 18 inches to 20 inches. This action was taken because some top cabinets are more than 18 inches above the base cabinet and this change would permit the installation of receptacles on the bottom of the top cabinets to serve the countertop spaces.
Read more by Philip Cox
Posted By David Young,
Saturday, July 01, 2000
Updated: Monday, February 11, 2013
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When we talk about mini- mum ground clearances of span conductors, we have to know the sag of the conductor because the minimum height of attachment on a structure to comply with the National Electrical Safety Code® (NESC®) is the minimum clearance plus the maximum sag of the conductor. The ground clearance for equipment cases and rigid live parts is very simple and yet often ignored. Too often someone says, "Mount it high enough so someone doesn’t bump his head on it.” This design "philosophy” is a gross violation of Rule 232B2 & 3 (page 72) of the 1997 NESC.
Equipment cases fall into two categories "ungrounded” and "effectively grounded.” "Effectively grounded” by definition on page 6 of the 1997 NESC is "”Intentionally connected to earth through a ground connection or connections of sufficient low impedance and having sufficient current-carrying capacity to limit the buildup of voltages to levels below that which may result in undue hazard to persons or to connected equipment.” If something is not "effectively grounded” then it is considered "ungrounded.”
Rigid Live Parts
Rigid live parts are rigid metal parts that are energized. Transformer bushings, transformer leads, jumpers and the energized parts of switches and fuses are some examples of rigid live parts. The minimum clearances specified in Rule 232B2 & 3 (Table 232-2) are a function of the normal activity anticipated under the equipment. For example, an effectively grounded equipment case mounted on a pole located in a parking lot must be at least 15 feet above ground. The same enclosure located in a park may be mounted at a lower level for accessibility provided the case does not unduly obstruct a walkway (Footnote 7 of Table 232-2). The clearances for a case mounted on a pole located within the limits of the highway right-of-way are the same for a pole located in the parking lot even though the pole may be well away from the travel lanes of the highway.
The metal beams that support the transformers in the following paragraph are considered part of the transformer cases. (see photo )
If this two-pole structure was located in a parking lot and the transformer cases were considered effectively grounded, then the beams would have to be at least 15 feet above ground. The high voltage bushings have to be 18 feet above ground for voltages up to 22kV phase to ground. For voltages greater than 22kV, the clearances have to be increased by 0.4 inches per kV above 22kV.
If you have general questions about the NESC®, please call me at 302-454-4910 or e-mail me email@example.com.
National Electrical Safety Code® and NESC® are registered trademarks of the Institute of Electrical and Electronics Engineers.
Read more by David Young
Posted By Leslie Stoch,
Saturday, July 01, 2000
Updated: Monday, February 11, 2013
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What should happen when the electrical utility or a utility customer decides to convert the ungrounded 600 volt, 3-wire supply to a 600/347 volt, 4-wire, solidly grounded electrical supply? Some commercial and industrial businesses still prefer to use an ungrounded 600 volt supply for service continuity reasons, or to avoid the costs of converting to a grounded 4-wire supply.
Sometimes voltage conversions are carried out when one or more businesses in a multi-unit complex want to install 347 volt lighting or other single-phase loads, or when the electrical utility decides to upgrade its distribution system for other reasons. Unless notified, some utility customers may be completely unaware of the changes and the resulting consequences. Others may decide to convert without sufficient analysis, and without taking the correct steps to avoid problems. Without careful planning, unexpected problems can occur, including electrical equipment damage, injuries or worse. Such problems are easily avoided with some special precautions.
Switching over to a solidly grounded supply normally has advantages. The most obvious, a grounded electrical system is absolutely necessary where single-phase loads must be supplied. A solidly grounded system provides the best control over phase-to-phase and phase-to-ground voltages, preserving the life of electrical equipment and wiring by minimizing exposure to overvoltages and capacitive arcing faults. This usually more than offsets the inconvenience of an odd unplanned power interruption, since these can usually be minimized by a regular preventive maintenance program. Another thing to keep in mind, the available fault level is usually higher after the conversion. This means that the existing overcurrent protection may have become obsolete.
The most sought after benefit of ungrounded 600 volt systems, they don’t trip on a single-phase ground fault. Everything continues to run with a ground fault on one phase. However, a second fault on a different phase can cause massive equipment damage, injuries or worse. The Canadian Electrical Code requires that ungrounded systems must be equipped with ground indicating lights [Rule 10-106(2)] to warn of the presence of a ground fault, so that repairs can be done as quickly as possible. However if warning lights are frequently ignored or neglected, extended overvoltages combined with capacitive arcing faults will cause insulation damage and premature electrical equipment failures.
A number of electrical code requirements and safety precautions need to be observed when the electrical utility or the user converts from an ungrounded to a grounded electrical system. When 4-wire loads are to be supplied:
• The electrical utility’s 3-wire metering is replaced by 4-wire metering.
• Circuit-breaker and fuse protection throughout the facility must be carefully reviewed and, when necessary, upgraded to safely interrupt the available fault level, that will often be higher after the conversion [Rule 14-012(a)]. Combined with a higher possibility of ground faults, this could lead to more frequent damage and longer shutdowns if neglected.
• The electrical system should be checked over for weakened insulation and existing short-circuits before reconnection to the grounded supply.
• Remove the ground indicating lights. They could deceive someone into believing the system is ungrounded and making some wrong decisions.
• If the 600/347 volt electrical supply is 1000 amperes or more, ground-fault protection will be required by the electrical code (Rule 14-102). This could mean replacing fused main service switchgear with a circuit-breaker type or adding ground- fault protection to an existing main service circuit-breaker.
• To be compatible with the new electrical supply and in compliance with the electrical code, the main service equipment must be equipped with a neutral block to accept the electrical utility’s fourth wire, and for system grounding purposes. This also applies throughout the facility at points where single-phase loads are connected (Rule 4-026).
When the electrical utility changes the 600 volt supply to 4-wire, and there is no need for single-phase loads, at least the following precautions are necessary:
• Depending on the electrical utility’s practices and policies, metering may be converted to the 4-wire type or continue as 3-wire metering.
• Overcurrent protection through-out the facility should be reviewed and upgraded if necessary, since the electrical system fault level is usually increased.
• Any ground indicating warning lights should be removed to prevent confusion.
• In some jurisdictions the electrical utility’s neutral conductor may be connected to the service equipment enclosure. Otherwise, a neutral block in the main service equipment may have to be provided.
• Also, check whether the electrical inspection authority will require ground-fault protection should the 600 volt main service equipment have a rating of 1000 amperes or higher.
As with previous articles, you should check with the local electrical inspection authority for the most accurate interpretation of any of the above in each province or territory as applicable.
Read more by Leslie Stoch
Posted By Philip Cox,
Saturday, July 01, 2000
Updated: Monday, February 11, 2013
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Some members have voiced interest in expanding the scope of associate members of the IAEI in order to give those members a greater role in the operation of the organization. It is felt by some that because the majority of the members are classified as associate members and many of them work very hard for the organization, they should have the right to hold any office, vote on all matters, and represent the IAEI on committees involving other organizations. A previously published article on the value of the associate member emphasizes the importance of that membership and of the work those members do for the IAEI. They serve as members of the Board of Directors at the different levels of the organization and in many cases hold the office of secretary/treasurer, a key position in the operation of the IAEI. Associate members are very important to the success of the IAEI, and membership in the organization is at least equally beneficial to them. Associate members both give to and receive valuable benefits from the IAEI. Those benefits come not from being in a position to control, but by such things as establishing contacts, gaining familiarity with enforcement policy and procedures in local jurisdictions, providing information on products, making inspectors aware of the problems associate members experience in their profession, and enabling inspectors to become more familiar with actual products they will see in the field. Through this type of information exchange, many problems and the costs associated with them have been avoided. A lack of knowledge and the failure to communicate can be very costly.
Associate members of the IAEI include individuals who are electrical contractors, manufacturers representatives, engineers, electricians, utility representatives, etc. These individuals have organizations that represent the interests of their professions and do not have to rely on the IAEI for that purpose. Those organizations represent trades or professions in an effort to serve their interests and achieve their stated goals and they are controlled by the specific interest they represent. As an example, organizations representing electrical contractors, manufacturers, utilities, etc., do not allow those with other interests to serve in leadership roles or in positions of control. By the same token, the IAEI must remain as it is to adequately represent the interest of electrical inspectors.
The IAEI is the only international organization I know of that was established specifically to represent the electrical inspector’s interest. The electrical inspector’s voice and input into the affairs of the industry is vital. Those who formed the IAEI intentionally developed the rules to have inspector member lead the organization. Three stated purposes of the organization as included in the IAEI Articles of Association are directly related to those concerns. They are:
"(a) To cooperate in the formulation of standards for the safe installation and use of electrical materials, devices and appliances.
(c) To promote cooperation between inspectors, the electrical industry and the public.
(e) To represent the Electrical Inspectors in all matters which are dealt with nationally and internationally by the electrical industry.”
How would the IAEI be affected if the Articles of Association and Bylaws were changed to permit associate members to be eligible to hold all offices, to vote on all matters, and to serve on code making panels? It would effectively destroy the IAEI, both in purpose and in objective. How would it do that? Permitting associate members to vote on all matters and to hold all offices would change the organizational structure, objectives, concept, association with other organizations, perception by the public, and would effectively silence the voice of the electrical inspection community.
The IAEI relies heavily on its credibility. Inspectors are proud of their dedication to the development of good electrical codes and the application of those rules for the benefit of the public. The electrical inspector is in a position of trust as a representative of the public in electrical safety and that role must not be compromised. When an official position is taken on a subject, such as a Code proposal, it is the general voice of the inspector because only inspectors are involved in that decision. As an example, when an associate member processes a code proposal through the IAEI and it is endorsed, the submitter of that proposal has shown that a group whose sole interest is electrical safety supports the proposed code change.
Since a majority of IAEI members are not inspectors, it is likely that they would govern the organization if they were permitted to vote on all matters and to hold all offices. The positions of the majority would be supported and those of the minority would be defeated. If inspectors lost control of the operation of the IAEI, it would no longer be an inspector’s organization. It would simply be a broad-based organization representing a variety of segments of the industry. For that type of representation, organizations already exist.
In regards to codes and standards, the IAEI presently is privileged to have a principal and alternate member on all 20 National Electrical Code Code Making Panels. It also has membership on the NEC Technical Correlating Committee, NFPA 79, and several others. Should the IAEI change structure so that it did not specifically represent the inspector, it would lose all of those positions and would forfeit direct participation in the code development process. The IAEI would no longer be classified as an organization of enforcers and would not qualify for those positions. To allow associate members to vote and serve in all levels of elected positions would result in the removal of all IAEI representatives from NEC Code Panels and other committees.
These are just some of the reasons why the IAEI must represent electrical inspectors and remain as a forum which allows that voice to be heard. There is strong support from associate members for the IAEI to remain as it is. Most associate members recognize that it is to their advantage for it to remain so.
I hope this gives a better understanding of why the rules governing the IAEI are as they are and why they must remain so.
Read more by Philip Cox
Posted By Underwriters Laboratories,
Saturday, July 01, 2000
Updated: Monday, February 11, 2013
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Recently, UL indicated that a new Standard for Arc-Fault Circuit Interrupters, UL 1699, had been published. How will UL 1699 affect new products currently in development?
In the March-April issue of IAEI News’ "UL Question Corner,” UL indicated that arc-fault circuit interrupters (AFCIs) were covered under the product category of Circuit Breakers, Molded Case, Classified for Mitigating the Effects of Arcing Faults (DIWL). Since then, the UL engineers responsible for AFCIs have pointed out that this category has been phased out and replaced by five new categories specifically for AFCI devices, now covered under UL 1699, Standard for Arc-Fault Circuit Interrupters.
The new categories are branch/feeder type (AVZQ); outlet circuit type (AWCG); portable type (AWDO); cord type (AWAY); and combination type (AWAH) AFCIs. These products will be included in the 2000 edition of the UL Electrical Construction Equipment Directory (green book), and in the 2000 edition of the UL General Information Directory (white book).
For more information on the types of AFCIs covered under UL 1699, see the related article on p. 80 in this issue of IAEI News. UL regrets this error.
Question: Listed and classified products
Does UL provide online information of Listed and Classified products?
Yes. UL’s new online certification and Registered firms database premiered in March. All information currently published in UL’s 16 product directories is now available at no charge on UL’s web site, atwww.ul.com.
However, UL will continue to publish print editions of its well-known product directories. Special search options have been added to assist regulatory authorities in their daily
routines. For example, code officials can use the Guide Information search option covered under a specific product category—such as installation requirements, building and electrical codes, UL Standards to which products are evaluated, related product categories, and information on identifying the appropriate UL Marks—when determining a product’s acceptance.
For more information about the UL certification database, contact the UL Data Services (ULDS) Group in Melville, NY, by phone at (631) 271-6200, ext. 22897; by fax at (631) 439-6018; or by e-mail firstname.lastname@example.org.
Question: Listed floor boxes
How are Listed floor boxes installed in accordance with NEC Section 370-27(b) identified?
UL Listed floor boxes include the complete assembly consisting of the cover, gasket and box. Receptacles may be included in the packaging or added to the assembly in the field. Products Listed for installation in accordance with NEC Section 370-27(b) are covered under the product categories metallic outlet boxes (QCIT), and nonmetallic outlet boxes (QCMZ), on pages 69 and 70 respectively, of the 1999 edition of the UL General Information Directory (white book).
Listed kits, consisting of the cover and gasket for use with specific Listed floor boxes, are identified in the installation instructions. The interchangeability of components is authorized only as specified on the product, or in the installation instructions included with the product.
UL Listed floor boxes bear the complete Listing Mark, including the UL in a circle, the word "Listed,” the product name, and the control number. The Listing Mark consisting of the UL in a circle can be found on the product itself; the complete Listing Mark is usually located on the smallest unit container in which the product is packaged, indicating that the entire assembly has been evaluated for compliance with applicable UL Standards and NEC Section 370-27(b). Also, the product identity "floor box” is placed adjacent to the Listing Mark.
Receptacle and cover assemblies complying with NEC Section 370-27(b), are covered under the product category Receptacles for Attachment Plugs and Plugs (RTRT), on pages 85 and 86 in the 1999 white book. These products are Listed as display receptacles, including the receptacle, flush device cover place or outlet box cover, and closure plugs. They are intended for use in show window floors and similar locations where the device is not subjected to scrub water. UL Listed receptacles and cover assemblies are not intended to be used as substitutes or in place of floor boxes.
UL Question Corner
Posted By Michael Callanan,
Monday, May 01, 2000
Updated: Monday, February 11, 2013
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The Hazards of Electricity
Figure 1. A good job safety will help determine the appropriate PPE for the task.
The electrical inspector and electrician are no different from any other craftsman. As they accumulate more experience and expertise in their field they begin to achieve a level of comfort with the tasks they perform on a regular and routine basis. Unfortunately, as the comfort level increases, the potential for complacency can begin to set in. In the electrical industry, there is no room for complacency. "Ordinary” and "routine” tasks, such as verifying voltage, taking current readings, and even visual inspection of live or energized parts, can subject the inspector and electrician to the multiple hazards of electricity.
For the past several years, an increasing amount of information has emerged concerning the multiple hazards of electricity. For many years, we assumed the electrical shock was the only hazard to be considered. Recent studies have shown that additional concerns must include the damaging effects of arc-blasts and arc-flashes. Consideration must be given to the devastating forces generated in arc-blasts when molten copper expands to 67,000 times its original value as it vaporizes. Likewise, arc temperatures can reach 35,000°F causing fatal burns at distances up to 10 ft and the pressure wave generated by the blast can reach upwards of 2,000 lbs/sq.ft., certainly enough pressure to rupture eardrums or even collapse the lungs. It is especially important to consider that as our electrical distribution systems continue to grow in size and capacity, the potential for higher and higher available fault currents and significantly greater arc-blasts and arc-faults rises sharply. For these reasons, electrical inspectors, electricians and anyone who works on or near energized circuits or equipment must be on constant guard that even the routine tasks they perform on a regular basis are done in a manner which affords the highest possible degree of personnel protection.
First Step: Are You Qualified?
The OSHA electrical safety-related work practices standard establishes guidelines for both the "qualified” person and the "unqualified.” Unfortunately, the OSHA General Industry, Electrical Standards, give very little direction in determining and defining what skills are necessary to be considered "qualified.” Fortunately, NFPA 70E-1995, Standard for Electrical Safety Requirements for Employee Workplaces defines a qualified person in Section 2-2.1. as one "trained and knowledgeable of the construction and operations of equipment or a specific work method, and be trained to recognize and avoid the electrical hazards that might be present with respect to that equipment or work method. Such persons shall also be familiar with the proper use of special precautionary techniques, personal protective equipment, insulating and shielding materials, and insulated tools and test equipment. A person can be considered qualified with respect to certain equipment and methods but still be unqualified for others.” As a first step, employers should evaluate all employee’s skills to determine if they have the necessary knowledge and training to perform work on or near energized electrical circuits and equipment. Those that have inadequate training or knowledge must maintain minimum approach distances in the direction of live parts to ensure their protection. At a minimum they should be trained in and be very familiar with proper approach distances for unqualified persons.
Figure 2. Potential high energy and available fault currents require specialized PPP.
To protect electricians and other workers exposed to the hazards of electricity, the Occupational Safety and Health Administration (OSHA) established guidelines which must be followed to protect against the damaging effects of electricity. In general, these are referred to as "Electrical Safety-Related Work Practices.” These work practices were originally developed under direction from OSHA in NFPA 70E, Standard for Electrical Safety Requirements for Employee Workplaces. In general, both NFPA 70E and OSHA 1910 Subpart S, Electrical Standards require electrical circuits and equipment to be de-energized before work is performed on or near them. Note that "de-energized” is defined as being placed into an electrically safe work condition by locking out and tagging the circuit and equipment. For circuits 50-volts and above, work is not permitted to be performed on or near live parts unless the employer can demonstrate that de-energizing the circuit or equipment introduces additional or increased hazards or is infeasible due to equipment design or operational limitations. Note that the definition of "infeasible” does not include considerations such as cost or convenience. Infeasibility is intended to apply to equipment operational limitations. For example, measuring voltage or taking current readings is not possible with the circuit or equipment de-energized and would require that the task be performed with the circuit in an energized condition. Too often, accidents occur when electricians fail to de-energize or request that circuits be de-energized first. Typically, accident reports indicate that work was performed in an energized condition because it would have been "inconvenient” or would have cost too much to de-energize the circuit or equipment.
Figure 3. Routine tasks, such as current readings, require appropriate PPE.
When the employer demonstrates that it would create an additional or greater hazard or it is infeasible to de-energize the circuit, other safety-related work practices must be employed to protect workers when work on or near energized circuits or equipment must occur. These safety related work practices vary with the specific task and hazards associated with the job and must be suitable for the conditions under which the work is to be performed and for the voltage level of the exposed electric conductors or circuit parts. Typically they may include, the use of all necessary personal protective equipment, insulating blankets, shields or barriers, insulating tools and protective clothing. These provisions apply to all circuits and equipment which operate at 50-volts and over. Because of the multiple hazards associated with this type of work, a complete job safety analysis should be completed before performing the work. This analysis should include careful consideration of the specific hazards associated with the task. The proper selection of the appropriate safety-related work practices and personal protective equipment is determined from this analysis.
Unfortunately, many electricians today fail to follow or completely adhere to the OSHA regulations and NFPA 70E recommendations. Instead, they choose to ignore the minimum safety-related work practices and fail to utilize proper personal protective equipment. Frequently, electrical workers and other personnel who work on or near energized electrical circuits and equipment, without the necessary PPE become statistics, highlighting the dangers associated with electrical work. A decision to work circuits and equipment in an energized condition should only be made after a determination has been made that it would be infeasible or that it would create a greater hazard to de-energize. Once that demonstration has been made, a job hazard analysis should be performed to evaluate all of the possible hazards associated with the tasks. The specific safety-related work practices, personal protective equipment and insulating tools necessary for the task should be apparent after the job hazard analysis is complete. Finally, a job briefing should be conducted prior to beginning the task to ensure that each of the employees involved with the task understand his or her responsibilities and the hazards associated with those responsibilities.
Read more by Michael Johnston
Posted By IAEI,
Monday, May 01, 2000
Updated: Monday, February 11, 2013
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Figure 1. Grounding electrode
A conducting element used to connect electrical systems and/or equipment to the earth. [See figure 1]
For many applications, grounding electrodes provide the essential function of connecting the electrical system to the earth. The earth is considered to be at zero potential. In some cases, the grounding electrode serves to ground the electrical system. In other instances, the electrode is used to connect noncurrent carrying metallic portions of electrical equipment to the earth. In both situations, the primary purpose of the grounding electrode is to maintain the electrical equipment at the earth potential present at the grounding electrode.
Another essential function of the grounding electrode is to dissipate over-voltages into the earth. These over-voltages can be caused by high-voltage conductors being accidentally connected to the lower-voltage system such as by a failure in a transformer or by an overhead conductor dropping on the lower-voltage conductor. Over-voltages can also be caused from lightning.
Figure 3. Grounding Electrode System
In Section 250-24(c), we find a requirement to connect the equipment grounding conductors, the service-equipment enclosures, and where the system is grounded, the grounded service conductor to a grounding electrode. The conductor used to make this connection is the grounding electrode conductor.
Grounding electrode system
The NEC in Section 250-50 requires that, where available on the premises at each building or structure served, all grounding electrodes including "made” electrodes be bonded together to form the grounding electrode system. This includes metal underground water pipes, metal frames of buildings, concrete-encased electrodes, and ground rings. The general requirement is that a bonding jumper must be installed between the grounding electrodes to bond them together. A grounding electrode conductor is run from the service enclosure to one of the grounding electrodes that are bonded together. The NEC also provides for the option of running a grounding electrode conductor to each grounding electrode individually. [See figure 3]
Where the interior metal water pipe is used as a part of the grounding electrode system or as a conductor to bond other electrodes together to create the "grounding electrode system,” Section 250-50 requires that all bonding take place within the first 5 feet from the point the water pipe enters the building. This section does not require that the interior water pipe be used for the purpose of interconnecting other electrodes to form the grounding electrode system. Any of the other electrodes, such as the metal frame of the building, concrete encased electrode or ground ring, can be used for the purpose of interconnecting the other grounding electrodes. Where these other electrodes are used for this purpose, no restrictions are placed on where the connections are permitted to be made or how far inside the building they are permitted. Section 250-68(a) requires grounding electrode conductor connections to grounding electrodes to be accessible except for connections to a buried, driven, or concrete encased electrode.
Grounding electrodes required to be used
Figure 4. Grounding electrodes that must be used
All of the identified grounding electrodes are required to be used where "available on the premises at each building or structure served.” The Code does not define what is meant by "available” nor does it require that the electrodes be made available where they are not. For example, if the building has encased concrete reinforcing rods when the electrical system is installed, it is not required that the rods be exposed for connection. On the other hand, the concrete reinforcing rods must always be used when "available.” Several electrical inspection agencies require that a concrete-encased grounding electrode be connected to the system before approval of the service for utility connection is granted. The grounding electrodes are not listed in an order of preference nor is it optional to choose which ones to use. [See figure 4]
Electrodes that must be used, in addition to any "made” electrodes that exist or are installed at the building or structure served, where "available” are as follows:
1. Metal Underground Water Pipe.Defined in Section 250-50(a) as "A metal underground water pipe that is in direct contact with the earth for 10 feet or more including any metal well casing that is effectively bonded to the pipe.” There is no minimum or maximum pipe size given. Types of metal, such as steel, iron, cast iron, stainless steel or even aluminum are not distinguished. Different types of water pipes such as for potable water, fire protection sprinkler systems, irrigation piping, etc., are also not defined. As a result, all of these metal underground water pipes must be used where "available at each building or structure served.”
Continuity of the grounding path of the water pipe grounding electrode or the bonding of interior piping systems cannot depend on water meters or on filtering devices or similar equipment. See Section 250-50(a)(1). Where a water meter or filtering equipment is in this metal water piping system, a bonding jumper must be installed around the equipment to maintain continuity even if the water meter or filter is removed.
2. Metal Frame of the Building.Section 250-50(b) requires the metal frame of the building to be used as a grounding electrode where it is effectively grounded. "Grounded effectively” is defined in Article 100 and means that the metal frame of the building is, "Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or to persons.”N
To be an effective grounding electrode, the metal frame of the building must have a sufficiently low-impedance contact with the earth to pass current when called upon to do so and to maintain the electrical system at or near the electrical potential of the surrounding earth. The building steel can be connected to the earth by bolted or welded connection to reinforcing steel in foundations or footings that are in turn encased in concrete. Also, building structural steel may itself be encased in concrete that is in contact with the earth. In both of these cases, the concrete that encases the building steel or reinforcing steel must be in direct contact with the earth.
Certain back-fills such as gravel or vapor barriers may render the building steel an ineffective electrode. Building steel that is connected to concrete footings or foundations by only "J” bolts are not considered "effectively grounded” unless these "J” bolts are in turn connected to structural members such as reinforcing steel. The reinforcing steel needs to be near the base of the footing or foundation.
Figure 6. Size of bonding jumper for grounding electrode system
The structural steel should be tested with an earth resistance tester if in doubt about its resistance to ground and adequacy as a grounding electrode.
3. Concrete-Encased Electrodes.Section 250-50(c) defines this grounding electrode as one or more steel reinforcing bars or rods that are not less than 20 feet in length and ½ inch in diameter or 20 feet or more of bare copper conductor not smaller than No. 4. These electrodes must be located within or near the bottom of the foundation or footing and be encased by at least 2 inches of concrete. A single 20 ft. length of reinforcing bar is not required. Reinforcing bars are permitted to be bonded together by the usual steel tie wires or other effective means like welding. Where subjected to high currents such as lightning strikes, welding is preferred.
Reinforcing rods must be of bare, zinc galvanized or other electrically conductive coated steel material. Obviously, insulated reinforcement rods would not perform properly as a grounding electrode. Some complaints have been made that lightning surges, that are dissipated through this electrode, break out chunks of concrete where the surge exits the footing.
This grounding electrode is commonly referred to as the "Ufer ground” after H.G. Ufer who spent many years documenting its effectiveness. Additional information on the development and history of the concrete-encased electrode is available in the Appendix of Soares Book on Grounding.
Several electrical inspection agencies require that a concrete-encased electrode be installed or connected to the service prior to authorizing electrical service due to its effectiveness in most any climatic and soil condition.
4. Ground Ring.Section 250-50(d) recognizes a copper conductor, not smaller than No. 2 and at least 20 feet long, as a ground ring grounding electrode. The conductor must "encircle” the building or structure and be buried not less than 2½ feet deep. Ground rings often are installed at telecommunication central offices, radio and cellular telephone sites. Where available on the premises served, ground rings must be used as one or more of the grounding electrodes making up the grounding electrode system.
Figure 7. Size of individual grounding electrode conductor
Section 250-50(a)(2) requires that where the only grounding electrode available and connected at the building or structure served is a metal underground water pipe, it be supplemented by another grounding electrode. Electrodes suitable to supplement the metal underground water pipe include: the metal frame of the building, a concrete-encased electrode, ground ring, other local metal underground systems or structures, rod and pipe electrodes, or plate electrodes. This supplemental grounding electrode is required since, often, metal underground water pipes are replaced by plastic water services or the system continuity is interrupted by nonmetallic couplings or repairs. The effectiveness of the water pipe grounding electrode would thus be lost.
Specific locations are provided where the supplemental grounding electrode is permitted to be connected. Where an underground metal water pipe is the only grounding electrode, the supplemental grounding electrode is permitted to be connected to only the grounding electrode conductor, the grounded service-entrance conductor, the grounded service raceway or to any grounded service enclosure. An exception to this requirement permits the bonding connection to the interior metal water piping in a qualifying industrial or commercial plant to be made at any location if the entire length of interior metal water pipe that is being used as a conductor is exposed.
Figure 8. Made electrodes
Often, changes, repairs or modifications are made to the metallic water piping systems with nonmetallic pipe or fittings or dielectric unions. In this case, it is possible to inadvertently isolate portions of the grounding system from the grounding electrode conductor. This is another in several steps that has been taken over recent years to reduce the emphasis and reliance on the metal water piping system for grounding of electrical systems.
With a change to the 1999 NEC, where the supplemental grounding electrode is of the rod, pipe or plate type, it is now required to meet the 25-ohm-to-ground rule in Section 250-56. This means that the supplemental grounding electrode must have a resistance of not more than 25 ohms or a second supplemental grounding electrode must be used. This has the effect of the system being served by only the supplemental grounding electrodes in case the underground metal water pipe grounding electrode is interrupted for any reason. [See figure 6]
Size of bonding jumper for grounding electrode system
The bonding jumper used to bond the grounding electrodes together to form the grounding electrode system must be sized in accordance with Section 250-66 based on the size of the ungrounded service-entrance conductor. The conductor that connects the grounding electrodes together is a bonding conductor and not a grounding electrode conductor. The bonding conductors are not required to be installed in one continuous length as grounding electrode conductors are. Also, the exceptions for sizing the grounding electrode conductor in Section 250-66 apply for the sizing of the bonding jumpers. [See figure 7]
Figure 9. Installation of made electrodes
For example, if the service-entrance conductor is 500 kcmil copper, the minimum size of bonding jumper is determined by reference to Section 250-66 and Table 250-66, including the rules in Sections 250-66(a), (b) and (c) are as follows:
- To metal underground water pipe and metal frame of a building; No. 2 copper or No. 1/0 aluminum conductor. (From Table 250-66.)
- To "made” electrodes as in Section 250-52(c) or (d) such as pipes, rods or plates; that portion of the bonding conductor that is the sole connection to the made electrode; No. 6 copper or No. 4 aluminum. The term "sole connection” means that the bonding conductor is not connected to the made electrode being considered and then another grounding electrode is connected to it. See Section 250-66(a).
- To a concrete-encased electrode as in Section 250-50(c); that portion of the bonding conductor that is the sole connection to the concrete-encased electrode; No. 4 copper conductor. See Section 250-66(b).
- To a ground ring as in Section 250-50(d); that portion of the bonding conductor that is the sole connection to the ground ring is not required to be larger than the ground ring conductor. See Section 250-66(c).
Note that aluminum is not permitted to be installed as a grounding electrode conductor where in direct contact with masonry or the earth or where subject to corrosive conditions. Where used outside, aluminum or copper-clad aluminum grounding conductors are not permitted within 18 inches of the earth. See Section 250-64(a).
No sequence for installing the bonding jumper or jumpers is given. However, the minimum wire size required to the various grounding electrodes must be observed. In addition, the point where the grounding electrode connects to the grounding electrode system must provide for the largest required grounding electrode conductor. For example, it would be a violation to connect a No. 4 bonding conductor from a concrete-encased grounding electrode a building steel grounding electrode which would require a 3/0 grounding electrode conductor. The installation would be acceptable if the 3/0 copper grounding electrode conductor connects to the building steel and a No. 4 copper bonding jumper extends to the concrete-encased electrode. In addition, the unspliced grounding electrode conductor is permitted to run from the service equipment to any convenient grounding electrode.
Alternately, individual grounding electrode conductors are permitted to be installed from the service equipment to one or more grounding electrodes rather than the electrodes being bonded together in a circular or "daisy-chain” manner. See Section 250-50. The minimum size of each grounding electrode conductor to the individual grounding electrode is shown in Figure 6-5. Note that a grounding electrode conductor is permitted to "supply” or "serve” any number of grounding electrodes but must be sized for the largest grounding electrode conductor required. For example, a bonding conductor is permitted to be run to a concrete-encased electrode and then to the underground metal water pipe. The bonding jumper must be sized for the largest grounding electrode conductor required for the grounding electrode or electrodes served.
Where the electrodes described in Section 250-50 are not available at the service location, a grounding electrode must be "made” or installed. The made electrode as provided for in Section 250-52 may be local metal underground systems or structures, driven pipes, or rods or buried plates conforming to the following requirements:
(b) Local systems.Local metallic underground systems as piping, tanks, etc. These objects must have the metal in direct contact with the earth. Protective coatings may render them ineffective as a grounding electrode.
(c)(1) Pipe electrodes.Pipe or conduit electrodes shall be not less than 8 feet in length nor smaller than ¾-inch trade size and if of iron or steel, shall be galvanized or metal-coated for corrosion protection.
(c)(1) Rod electrodes.Electrodes of steel or iron shall be at least 5/8 inch diameter. Rods of nonferrous metal or stainless steel that are less than 5/8 inch in diameter shall be listed and be at least ½-inch in diameter.
(d) Plate electrodes.Electrodes shall have at least 2 square feet of surface in contact with exterior soil. If of iron or steel, the plate shall be at least ¼-inch thick. If of nonferrous metal, they shall be at least 0.06 inch thick.
Note that underground metal gas piping systems are not permitted to be used as a grounding electrode. This does not eliminate the requirement that interior metal gas piping systems be bonded. For additional information on bonding of metal piping systems, see Soares Book on Grounding, 7th edition, Chapter 8.
Installation of made electrodes
Where practicable, made electrodes must be installed below permanent moisture level. This is a key ingredient in establishing an effective made electrode. They also are required to be free from nonconductive coatings such as paint and enamel. [See figures 8 and 9]
Rod and pipe electrodes must be installed so at least 8 feet is in contact with the soil. They must be driven vertically unless rock bottom is encountered. If rock bottom is encountered which prevents the rod from being driven 8 feet vertically, the rod is permitted to be installed at an oblique angle of not more than 45 degrees from vertical or it can be buried in a trench that is at least 2½ feet deep. See Section 250-52(c)(3).
The upper end of the rod must be flush with or below ground level unless the aboveground end of the rod and the grounding electrode attachment are protected from physical damage. This, of course, requires that a ground rod longer than 8 feet be used if any of the rod is exposed above ground level. For an eight-foot ground rod or pipe, the ground clamp must be listed for direct earth burial as the electrode must be driven to its full length.
Plate electrodes are required to be buried not less than 2 1/2 feet in the soil.
Section 250-10 requires that ground clamps or other fittings be approved (acceptable to the authority having jurisdiction) for general use without protection or be protected from physical damage by metal, wood or equivalent protective covering.
Common grounding electrode
Section 250-58 of the NEC requires that a common grounding electrode be used for all alternating-current system grounding in or at a building. In addition, where more than one service supplies a building, the common grounding electrode must be used for all services. This section recognizes that where two or more grounding electrodes are bonded together, they are considered to be one electrode.
Interestingly, no distance between electrodes is given beyond which the electrodes do not have to be bonded together. Buildings of "large area” are permitted by Section 230-2(b)(2) to have more than one service. However, nothing in the Code defines the dimensions of a "large building.” Some inspection authorities use voltage drop of major feeders for guidance in determining when a building is one of "large area.” Where feeder conductors would have to be increased in size unreasonably to maintain voltage regulation, one or more additional services are permitted.
Section 250-58 requires the grounding electrodes for the multiple services be bonded together no matter how far apart they are in the same building. This is important so there is not more than one earth potential impressed on equipment in or on the building. Section 250-50 requires the bonding jumper(s) used for this purpose to be sized from Table 250-66 and be installed in accordance with Sections 250-64(a), (b) and (e). For additional information on installation of grounding electrode conductors, see Soares Book on Grounding, 7th edition, Chapter 7. It is permitted to use the steel frame of a building that is effectively grounded or a concrete encased grounding electrode to bond grounding electrodes from other services together.
Section 250-58 also requires that a common grounding electrode be used to ground conductor enclosures and equipment in or on the building and that the same grounding electrode be used to ground the system. This does not mean that one cannot use more than one grounding electrode. But, if more than one is used, then all the grounding electrodes must be bonded together to form a common grounding electrode. Where multiple grounding electrodes are bonded together as cited above, such multiple grounding electrodes become, in effect, a common grounding electrode system.
Earth return prohibited
No mention is made in the Code to providing a low-resistance, low-impedance, common grounding electrode for clearing ground faults. Reference to Figure 6-10 will show that the grounding electrode is in the earth return circuit. Even if the grounding electrode resistance to earth was very low, it would have little affect on the clearing of a ground fault, because the reactance of the earth and the soil resistance in the return circuit is very high. Where a parallel path exists through the earth and through a grounded service conductor, about 95 percent or more of the ground-fault current will return to the source over the grounded service conductor. A low-resistance, common grounding electrode is valuable, however, in holding equipment close to earth potential. It simply is not effective in clearing a line-to-ground fault.
Section 250-2(d) and 250-54 make it clear that grounding electrodes are not permitted to be used instead of equipment grounding conductors. The earth is not to be used as the sole or only equipment grounding conductor. However, grounding electrodes are permitted to supplement equipment grounding conductors.
If a ground fault should develop as shown in the upper drawing in Figure 6-11 where two separate grounding electrodes are used, the fault current flow will be through the service conductor then through the impedance of the ground fault, the grounding electrode conductor, the grounding electrode, the path through the earth to the grounding electrode at the transformer and finally through the grounding conductor to complete the circuit to the transformer. It would be a rare case where that circuit resistance would add up to less than 12 ohms (while the impedance would be higher). At best, therefore, the fault current would not reach a value high enough to operate a 15-ampere overcurrent device on a 120 volt-to-ground circuit. (120 ÷ 12 = 10 amperes).
Considering resistance only, the circuit shown has two grounding electrodes in series. Compared to the much lower resistance parallel path of the grounded circuit conductor, a resistance ratio between the two parallel paths is about 50 times for a 100 ampere service, to well over 100 times for the larger services. When impedance of the two paths is considered, the ratio will be higher. Thus, almost all the current from a line-to-ground fault will return to the transformer over the grounded service conductor.
Under normal operating conditions some unbalanced current will flow in the neutral. Some unbalanced neutral current will thus flow through the earth, but it will be small in comparison to that which will flow through the grounded service conductor.
Any belief that the circuit to the grounding electrode can be depended on to clear a ground fault is clearly erroneous no matter how large a grounding electrode conductor is used or how good a grounding electrode is. However, when the high-impedance earth path is short-circuited by installing the grounded circuit conductor as shown in the lower drawing in Figure 6-11, a low-impedance path is established as required in Section 250-2(d). This will allow a large current to flow over the equipment grounding and service-grounded conductor to allow the branch-circuit, feeder or service overcurrent device to clear the fault and thus provide the safety contemplated by the Code.
Resistance of grounding electrodes
There is no requirement in Article 250 that the grounding electrode system required by Section 250-50 (consisting of metal underground water pipe, metal frame of the building, concrete-encased electrode or ground ring) meet any maximum resistance to ground. No doubt it is felt that the grounding electrode system will have a resistance to ground of 25 ohms or less.
Figure 12. Supplemental electrode
The rules change for "made” electrodes. The Code states, in Section 250-56, that where a single rod, pipe, or plate electrode does not achieve a resistance to ground of 25 ohms or less, it shall be supplemented by one additional electrode. This means that where driven ground rods are utilized, two ground rods would be the maximum required under any condition. There is no requirement that additional made electrodes such as ground rods or plates be installed until the 25 ohm-to-ground resistance is obtained.
In general, metallic underground water piping systems, metallic well piping systems, metal frame buildings and similar grounding electrodes may be expected to provide a ground resistance of not over 3 ohms and, in some cases, as low as 1 ohm.
However, from a practical standpoint, no grounding electrode, no matter how low its resistance, can ever be depended upon to clear a ground fault on any distribution system of less than 1,000 volts.
If a system is effectively grounded as pointed out in the Code under Section 250-2(d), a path of low impedance (not through the grounding electrode) must be provided to facilitate the operation of the overcurrent devices in the circuit. See Chapter 11 of Soares Book on Grounding "”Clearing Ground-fault Circuits on Distribution Systems.”"
The lowest practical resistance of a grounding electrode is desirable and will better limit the voltage to ground when a ground fault occurs. It is more important to provide a low-impedance path to clear a fault promptly, for a voltage to ground can only occur during the period of time that a fault exists. Clearing a ground fault promptly thus will enhance safety. [See figure 12]
Even though the grounding electrode has low resistance, it is a part of a high-impedance circuit and plays virtually no part in the clearing of a fault on a low-voltage distribution system. This is due in part to being a higher resistance path through the earth than through the grounded service conductor. In addition, the remote path through the grounding electrode and earth is a high-impedance path compared to the circuit where the grounded service conductor is installed and routed with the ungrounded phase conductors.
The Code in Section 250-6 recognizes that conditions may exist which may cause an objectionable flow of current over grounding conductors such as the grounding electrode conductor, other than temporary currents that may be set up under accidental conditions. We should recognize that grounding conductors are not intended to carry current under normal operating conditions. They are installed for and are intended to carry current to perform some safety function.
The Code does not define what is meant by "objectionable” currents. Clearly, any current over a grounding electrode conductor that would prevent it from maintaining the equipment at the earth potential would be objectionable. Since every conductor has resistance, current flow through the conductor will produce a voltage drop across it. Any voltage drop on a grounding conductor that would create a shock hazard certainly would also not be acceptable.
Section 250-6(b) permits the following corrective actions to be taken where there is an "objectionable” flow of current over grounding conductors:
1. If due to multiple grounds, one or more, but not all, of such grounds may be discontinued,
2. The location of the grounding connection may be changed,
3. The continuity of the grounding conductor or conductive path between grounding connections may be suitably interrupted, or
4. Other means satisfactory to the authority enforcing the Code may be taken to limit the current over the grounding conductors.
The Code points out that temporary currents that result from accidental conditions such as ground-fault currents, that occur only while the grounding conductors are performing their intended protective functions, are not considered the "objectionable” currents covered in these sections.
Section 250-6(d) points out that currents that introduce noise or data errors in electronic equipment are not considered to be objectionable currents. Electronic data processing equipment is not permitted to be operated ungrounded or by connection to only its own grounding electrode.
Reprinted fromSoares Book on Grounding, 7th edition.
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