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IAEI News provides educational forums, updates on electrical codes and reports of innovative research to facilitate the development and enforcement of practices designed to drive efficiency and compliance with the highest standards of product development and safety—for the public as well as for electrical personnel. The magazine reaches authorities with power of product specification, approval and acceptance. Published six times a year by the International Association of Electrical Inspectors.

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Metallic and Non-metallic Outlet Boxes Used in Fire-rated Assembly

Posted By Underwriters Laboratories, Friday, September 01, 2000
Updated: Tuesday, February 12, 2013

Question: Outlet box

How do I know if an outlet box can be used in a fire-rated assembly?


Metallic Outlet Boxes are Listed under the category Metallic Outlet Boxes (QCIT), in the UL Electrical Construction Equipment Directory. The Guide Information for QCIT specifies the installation requirements for use in fire-rated assemblies. Information about UL-Listed metallic outlet boxes, including the Guide Information, can also be found by accessing UL’s online product certification database

Metallic outlet boxes for use in firerated wall assemblies include single and double gang metallic outlet and switch boxes equipped with Listed metallic or nonmetallic cover plates. These outlet boxes are intended for installation in bearing and non-load bearing wood or steel stud gypsum wallboard walls with fire resistance ratings of 2 hours or less. The metallic outlet or switch boxes must be securely fastened to the studs. Openings in the wallboard facing are to be cut so that the clearance between the box and wallboard does not exceed 1/8 inch. The surface area of individual metallic outlet or switch boxes must not exceed 16 square inches. In addition, the entire surface area of the boxes must not exceed 100 square inches per 100 square feet of wall surface.

A minimum horizontal distance of 24 inches must separate metallic boxes located on opposite sides of walls or partitions. This minimum horizontal spacing may be reduced through the use of UL Classified Wall Opening Protective Materials (QCSN), commonly known as "putty pads” or "insert pads.” Further, metallic boxes cannot be installed on opposite sides of walls or partitions in staggered stud constructions unless putty pads or insert pads are installed with the metallic boxes in accordance with the Classification requirements for the protective materials.

Wall Opening Protective Materials are found under the category QCSN in the UL Electrical Construction Equipment Directory, and also under the category CLIV in Volume 1 of the UL Fire Resistance Directory, or online at UL’s product certification database.

Listed metallic outlet boxes with metallic or nonmetallic cover plates may be used in fire-rated floor/ ceiling and roof/ceiling assemblies with ratings not exceeding two hours. Such assemblies must be equipped with gypsum wallboard ceilings.

The metallic outlet boxes shall be securely fastened to the joists and the opening of the wallboard facing must be cut so that the clearance between the box and the gypsum wallboard does not exceed 1/8 inch. The entire surface area of the boxes must not exceed 100 square inches per 100 square feet of ceiling surface.

Information regarding the use of outlet boxes in fire-rated floor/ceiling and roof/ceiling assemblies can be found on page 4 in Volume 1 of the 2000 edition of the UL Fire Resistance Directory. This information was recently published to be consistent with those requirements specified by the International Conference of Building Officials (ICBO), the Southern Building Code Conference International (SBCCI), and the Building Officials and Code Administrators (BOCA) model building codes.

Non-metallic outlet boxes, which can be used in fire-rated assemblies, are Classified under the category Outlet Boxes and Fittings Classified For Fire Resistance (QBWY), in the UL Electrical Construction Equipment Directory, and under CEYY in Volume 1 of the UL Fire Resistance Directory. This same information can also be found by accessing UL’s online product certification database

This product category covers special purpose boxes for installation in floors, and nonmetallic outlet boxes for installation in ceilings, floor/ceiling or roof/ceiling assemblies, and in wall and partition assemblies in accordance with the National Electrical Code (NEC).

These boxes provide the required fire resistance when installed in specific fire-rated ceilings or walls described for each Classified company. These boxes have also been investigated and found to comply with UL’s electrical requirements. Any Listed metallic or nonmetallic cover plate is suitable for use with these nonmetallic boxes.

This category includes Classifications for nonmetallic outlet and switch boxes for use in fire-resistive rated wall or partition assemblies. The information provided for each Classification includes the model numbers for the Classified products, a description of the rated assemblies, the spacing limitations for the boxes and the installation details.

Nonmetallic boxes are not to be installed on opposite sides of walls or partitions in staggered stud constructions, unless Classified for use in such constructions or unless Wall Opening Protective Materials (QCSN or CLIV) are installed with the nonmetallic boxes.

The minimum horizontal spacing between boxes located on opposite sides of walls of partitions may be reduced through the use of Wall Opening Protective Materials. In both cases, the nonmetallic boxes shall be protected as described in the Classification requirements for the protective materials for use on the specific box.

Nonmetallic outlet boxes Classified for use in fire-resistive designs will be marked on the box with the UL Classification Marking along with the hourly rating (Class 1 or 2HR); and the intended use "F” for floor, "W” for wall; "C” for ceiling; and "F/C” for floor/ceiling. Such boxes are Classified for use in specific fire-resistive designs when installed in accordance with the details described for each Classified company. Always refer to the Classification requirements in the UL Electrical Construction Equipment Directory and Fire Resistance Directories, and the installation instructions for proper installation guidelines.

Tags:  September-October 2000  UL Question Corner 

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Arc-Fault Circuit Interrupters

Posted By Clive Kimblin, Saturday, July 01, 2000
Updated: Monday, February 11, 2013


The objective of this paper is to increase inspector-awareness of arc-fault circuit interrupters. The significance of AFCIs is discussed in the introduction, and this is followed by a description of recent changes associated with the standard, with the National Electrical Code, and with the availability and application of the technology. Here there is a general discussion of AFCI availability, followed by a detailed description of the Cutler-Hammer line of residential miniature circuit breakers that incorporate branch/feeder AFCIs and a brief description of the technology involved. The operating experience has been excellent, relative to both fire protection and immunity from unwanted tripping. It is concluded that arc-fault circuit interrupters provide a significant fire-safety improvement for dwelling unit electrical distribution systems. They are the residential electrical safety technology of the future.


As explained in an earlier AFCI article in this magazine1, present residential overcurrent protective devices such as miniature circuit breakers (MCBs) are designed to protect circuit conductors by opening automatically before conductor damage is caused by excessive heating. For low current overloads, the breaker trips due to the heating of an internal bimetal. For high current overloads, the circuit breaker trips "instantaneously” due to internal magnetic forces. Because they prevent excessive or dangerous temperatures in the conductors or conductor insulation, these present-day circuit breakers reduce, to some extent, the incidence of residential fires associated with the electrical distribution system.

Data provided by CPSC2 show that about 10 percent of present-day residential fires are associated with the electrical distribution system. This percentage represents 41,600 fires and 370 civilian deaths. Further, these fires cause 1,430 civilian injuries and over $682.5 M in property losses. AFCIs have been specifically designed to supplement the protection afforded by overcurrent protective devices such as circuit breakers, and to make a significant reduction in these numbers.

An arc-fault circuit interrupter, as defined in the National Electrical Code3, is a device intended to provide protection from the effects of arc faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc fault is detected. In the embodiment discussed in this paper, the AFCI is integrated into the circuit breaker design, and the resulting integral circuit breaker (Listed to UL 489) and AFCI (Listed to UL 1699) combines conventional wire thermal-protection with the mitigation of arcing effects.

The previous paper1 was published in 1997. Since then, a UL standard has been published 4, AFCI issues have been addressed relative to the National Electrical Code 3, there have been advances in AFCI technology, and increases in both field experience and product availability. The objective of the present paper is to discuss these recent changes. In particular, the paper is intended to increase inspector awareness of this new fire-safety product.

Standards Situation (UL 1699)

The First Edition of UL 1699 was published in February 1999. This standard defines several types of AFCI’s:

• Branch/Feeder Arc-Fault Circuit Interrupter – A device intended to be installed at the origin of a branch circuit or feeder, such as at a panelboard. It is intended 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 device or a device in its own enclosure mounted at or near a panelboard.

• Outlet Circuit Arc-Fault Circuit Interrupter – A device intended to be installed at a branch-circuit outlet, such as at an outlet box. It is intended 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 Arc-Fault Circuit Interrupter – An AFCI which complies with the requirements for both branch feeder and outlet circuit AFCIs. It is intended to protect downstream branch-circuit wiring and cord-sets and power-supply cords.

The standard also deals with cord AFCIs and portable AFCIs.

There is a great deal of commonality in the test requirements for the three types of AFCIs defined above. Thus they must all recognize parallel (line to neutral) arcing faults in circuits with available short-circuit currents of 75A and above. At these current levels, the intermittent arcs, see Figure 1, associated with damaged or abused cables and wires may be insufficient to trip the circuit breaker either thermally or instantaneously. The main difference between the branch/feeder requirements and the outlet requirements is that the branch/feeder is tested for parallel faults in both the installed wire (Type NM-B) and in commonly used appliance cord (Type SPT-2). The outlet device, however, is tested solely with Type SPT-2 cord.

Figure 1. Typical current waveform observed when a carbon-steeled blade cuts through 16 AWG SPT-2 cord to create a parallel arc.

There is also some commonality in the test requirements for responding to series arcs associated with arcing at a break in a line or neutral conductor. For branch/feeder AFCIs the tests are performed at current levels of 5A and above in Type NM-B cable. The criterion is that cotton above the break point internal to the cable must not ignite. For outlet AFCIs the tests are performed with Type SPT-2 cord, and the time for arc extinction must be less than specified arc-test clearing times.

For the three types of AFCIs, the standard contains similar tests to check that the devices are immune to unwanted tripping. These tests include immunity to (a) transient inrush currents, (b) the waveforms associated with electronic devices, (c) the arcing waveforms associated with the burn out of incandescent lamps, and (d) the waveforms of "safe” arcs associated with the normal operation of electrical devices. Further, there are similar tests to check that the devices will not be masked by circuit conditions.

It is noted that the Canadian Standards Association has a Technical Information Leaflet5 with requirements for AFCIs that closely parallel the UL standard for branch/feeder AFCIs. A more comprehensive CSA standard is presently under development.

The Cutler-Hammer AFCI is of the branch/feeder type and consequently addresses series and parallel faults in the installed wiring; Zone 1 of Figure 2. This is the origin of about 35 percent (6,7) of residential fires associated with the electrical distribution system. In addition, the branch /feeder AFCI detects parallel faults in Zone 2, which represents the appliance cords and loads beyond the outlet, and the parallel faults in Zone 3. It also responds to all arcs to ground in Zones 1, 2 and 3.

Figure 2. Division of the residential wiring into four zones.

Code Situations (NEC, Vermont and CEC)

Arc-fault circuit interrupters were included, for the first time, in the 1999 edition of NEC 70 (The National Electrical Code). They are referenced in Section 210-12 of Article 210 which deals with branch circuits. There we find:

210-12(a) Definition – An arc-fault circuit interrupter is a device intended to provide protection from the effects of arc faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc fault is detected.

210-12(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.

It must be noted that, in order to protect a complete branch circuit, as required by the 1999 NEC, the device must be located in or adjacent to the load center where the branch-circuit overcurrent protective device is located.

During the proposal stage for the 2002 edition of the NEC, there were many additional proposals to extend the application of branch/feeder AFCIs and to include AFCIs at the outlets. At the moment, it seems probable that branch/feeder AFCI protection will be mandated for all bedroom outlets rather than solely for bedroom receptacle outlets. Other proposals to extend branch/feeder coverage to additional dwelling unit rooms and, for example, to hotel guestrooms and limited care facilities will continue to be discussed through the comment period. From proposals submitted for the 2002 NEC, it also seems likely that branch/feeder AFCI protection will be extended to the bedroom outlets of mobile homes and manufactured homes.

With respect to the state of Vermont, branch/feeder AFCIs will be mandated for all 120V circuits serving receptacle outlets in dwelling unit living areas and bedrooms. The effective date is January 1, 2001. The application of branch/feeder AFCIs to dwelling unit bedroom circuits is also under active discussion relative to the Canadian Electrical Code, Part 1.

Product Availability

At the present time, branch/feeder AFCIs have been announced by four of the major US manufacturers of residential miniature circuit breakers (MCBs). These products are effective8. With respect to Cutler-Hammer, there is a complete line of UL Listed single-pole, 15A and 20A MCBs that contain the additional function of AFCI, see Figure 3. There are also single pole MCBs that contain both UL Listed AFCI and ground-fault circuit interrupter (GFCI) functions. Further, the AFCI function has been incorporated into two-pole Listed circuit breakers that are also classified for mitigating the effects of arcs, and which provide protection in shared neutral circuits.

Figure 3. Photographs of the line of MCBs with AFCI

It should be noted, as indicated in Figure 4, that these MCBs, with AFCI function, are completely interchangeable with conventional Cutler-Hammer MCBs. Their connection in the loadcenter is identical to present-day MCBs with GFCI function, namely with a wire connection to ground, and their cost is also similar to an MCB with integrated ground-fault protection.

Figure 4. Diagram showing the interchangeability between MCBs, with branch/feeder AFCI capability, and conventional MCBs

Operation of the Cutler-Hammer AFCI

A block diagram of a single pole AFCI appears in Figure 5. The load current sensor output is fed to an arc signature filter whose output is responsive to the magnitude of the arcing currents. Normal "non-arc” related current components are filtered out. The signal is then amplified and fed to a logic block that distinguishes between dangerous arcing events and normal circuit transients, including the waveforms associated with incandescent-lamp-burnout. In the event of a dangerous arc, the logic block provides an output that will trip the breaker. This logic block has a second input from a ground-current-sensor amplifier. If this logic input exceeds a preset 30 milliamp trip threshold, a trip signal is provided. This arc-fault circuit interrupter (listed to UL 1699) is also available in combination with ground-fault circuit protection (listed to UL 943). In this embodiment, the ground-fault sensor is set at five milliamps for personnel protection and a grounded neutral sensor is included. Individual test buttons are provided for both the AFCI and GFCI functions. It is noted that the value of a residential circuit breaker including both AFCI and GFCI technologies was recognized by UL in the Recommendations Section of their 1995 Report to CPSC9.

Figure 5. A block diagram of a single pole AFCI

A block diagram of a two-pole Listed circuit breaker also classified for mitigating the effects of arcs appears in Figure 6. The design basically consists of two single pole AFCI designs as described in Figure 5 with a common logic trip output signal. The response to arcs to ground is set at 30 milliamps. There are individual AFCI test circuits for each of the poles.

Figure 6. A block diagram of a two-pole Listed circuit breaker also classified for mitigating the effects of arcs

Operating Experience

More than 10,000 Cutler-Hammer MCBs with AFCI protection are presently operating in the field. The total operating time is more than 150 million hours. During that period there have been no reports of unwanted tripping issues. More importantly, there have been cases of reported fire prevention. For example, there have been cases of the detection of arc tracking at the base of two-wire lighting fixtures. These fixtures were typically located in the dining room, and arc tracking near the base of the bulb had led to a parallel arc. Other examples include the initiation of an arcing fault in a TV set, supplied by a two-wire cord, due to water dripping from a hanging plant, and the detection of arcing within junction boxes. Additional discussion of effective branch/feeder AFCI protection appears in a recent NFPA paper 10.

Future Applications

At the present time, branch/feeder AFCIs will provide protection to 120-volt dwelling unit bedroom circuits. These circuits are indeed associated with a large number of residential electrical distribution system fires11. However, other residential locations also have a high incidence of electrical fires11, and it is therefore to be expected that branch/feeder AFCIs will find application for many additional residential circuits.

The AFCI function can also be expected to extend to the higher voltages (e.g. 240 volt, 277 volt and 480 volt) associated with commercial and industrial electrical distribution systems.

It is also noted that the aerospace industry12 is interested in AFCI technology relative to the protection of onboard electrical wiring.


Arc-fault circuit interrupters represent the application of new technology to an old problem; namely, the need to improve fire safety in residential electrical distribution systems. Standards have been developed for the requirements of these devices, and branch/feeder AFCIs are available from many manufacturers. National, Canadian and state electrical code issues are being addressed, and the field experience with branch/feeder AFCIs has been excellent. AFCIs are the technology of the future with a fundamental focus on safety.


1. Arc-Fault Circuit Interrupters: New Technology for Increased Safety, J. C. Engel, R. J. Clarey, and T. M. Doring, IAEI News, October 1997.

2. 1996 Fire Loss Estimates, US Consumer Product Safety Commission Report, 1998

3. National Electrical Code, NFPA 70, 1999.

4. Arc-Fault Circuit Interrupters, UL 1699 Standard for Safety, First Edition, February 28, 1999

5. CSA Publication of Technical Information Letter (TIL) No. M-02, Interim Requirements for Arc-Fault Circuit Interrupters, September 30, 1999.

6. What Causes Wiring Fires in Residences?, L. Smith and D. McCoskrie, Fire Journal, pp. 19-24, January/February 1990.

7. The U.S. Home Product Report, 1992-1996, (Appliances and Equipment), K. Rohr, NFPA Fire Analysis and Research Division, February 1999

8. Preventing Home Fires: AFCIs, Consumer Product Safety Review, Volume 4, #1, Summer 1999

9. "Technology for Detecting and Monitoring Conditions that Could Cause Electrical Wiring System Fires”, Report Prepared by Underwriters Laboratories (UL Project Number NC233, 94ME78760) for the Consumer Product Safety Commission (Contract Number CPSC-C-94-1112), September 1995

10. AFCIs Target Residential Electrical Fires, G. D. Gregory, NFPA Journal, pp. 69-71, March/April 2000.

11. Residential Electrical Distribution Fires, L. Smith and D. McCoskrie, US Consumer Product Safety Commission Report, December 1987.

12. Arc-Fault Circuit Interrupters, J. McCormick, M. Walz, J. Engel, P. Thiesen and E. Hetzmannseder, Proceedings of the Conference on Advances in Aviation Safety, Paper 2000-1-2121, Daytona Beach, Florida, April 2000.

Read more by Clive Kimblin

Tags:  Featured  July-August 2000 

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Lab Data

Posted By David Dini, Saturday, July 01, 2000
Updated: Monday, February 11, 2013

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

Tags:  Featured  July-August 2000 

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Overcurrent Protection for Air-Conditioning and Refrigeration Equipment

Posted By Michael Johnston, Saturday, July 01, 2000
Updated: Monday, February 11, 2013

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).

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

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Arc Detection with the AFCI

Posted By George Gregory, Saturday, July 01, 2000
Updated: Monday, February 11, 2013

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:

  • Line-to-neutral,
  • Series (such as in a broken or frayed wire or at a loose connection),
  • Line-to-ground.

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.

Arc Recognition

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.

Wrap Up

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).

Arc Characteristics

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.

Sputtering Arc
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.

Read more by George Gregory

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Proposed Changes for the 2002 NEC

Posted By Philip Cox, Saturday, July 01, 2000
Updated: Monday, February 11, 2013

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:

Article 80.

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.

90-2. Scope.

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).

90-4. Enforcement.

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.

210-8(a)(8). Boathouses.

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.

210-19(a), Exception.

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

Tags:  Featured  July-August 2000 

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Just Don’t Bump Your Head On It

Posted By David Young, Saturday, July 01, 2000
Updated: Monday, February 11, 2013

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

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

Photo 1

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

National Electrical Safety Code® and NESC® are registered trademarks of the Institute of Electrical and Electronics Engineers.

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Tags:  July-August 2000  Other Code 

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Delta-Wye Conversions

Posted By Leslie Stoch, Saturday, July 01, 2000
Updated: Monday, February 11, 2013

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

Tags:  Canadian Code  July-August 2000 

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The IAEI and the Electrical Inspector

Posted By Philip Cox, Saturday, July 01, 2000
Updated: Monday, February 11, 2013

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.

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Tags:  Editorial  July-August 2000 

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Five New Categories for AFCI Devices Under UL 1699

Posted By Underwriters Laboratories, Saturday, July 01, 2000
Updated: Monday, February 11, 2013

Question: AFCIs

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,

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

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.

Tags:  July-August 2000  UL Question Corner 

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