AFCI - UL Test Requirements (9/17/01)
 

Arc-Fault Circuit Interrupters (AFCIs) -

Type and Performance Considerations

Copyright © 2001 Underwriters Laboratories Inc.

In February of 1999, UL published the First Edition of the Standard for Arc-Fault Circuit-Interrupters (AFCIs), UL1699.   According to the National Electrical Code (NEC), an AFCI is defined as 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.   The 2002 NEC will require all branch circuits that supply 125-volt, single-phase, 15- and 20-ampere outlets installed in dwelling unit bedrooms to be protected by an AFCI listed to provide protection to the entire branch circuit.

Branch Circuits

A branch circuit is defined in Article 100 of the NEC as the circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).   The length of a branch circuit can vary from several feet to several hundred feet, and include from one to several outlets.   Figure 1 shows a pictorial representation of a typical branch circuit that could be associated with a dwelling unit bedroom.

Figure 1 – Pictorial Representation of a Typical Branch Circuit (Not to Scale)


The branch circuit overcurrent protection is provided by a fuse or circuit breaker usually located in a centralized panelboard that is served with power by the local utility.   As the name implies, overcurrent protection protects the branch circuit against any currents that are in excess of the rated current or ampacity of the branch circuit conductors.   Overcurrents can be the result of overloads, short circuits, or ground faults. Overcurrent protection is provided to open the circuit if the current reaches a value that will cause an excessive or dangerous temperature in the branch circuit conductors or conductor insulation.

The branch circuit conductors are normally contained within a non-metallic cable (NM-B), armored cable (AC), or a metal or non-metallic raceway such as conduit or tubing.   Non-metallic cables and raceway systems contain a separate conductor for equipment grounding purposes.   Metal armor cables and raceways may contain an equipment grounding conductor, but in most cases the metal itself is permitted to serve as the equipment grounding path.

The branch circuit conductors extend throughout the building to outlets, which may be a receptacle outlet for connecting to cord- and plug-connected appliances, or to fixed equipment, such as a lighting outlet for a wall or ceiling mounted lighting fixture (luminaire).  

Receptacle outlets in the branch circuit provide for the connection of cord-connected appliances, which in a bedroom may include appliances such as portable lamps, clock-radios, and portable air heaters.   The cords attached to these appliances are generally referred to as power supply cords, as they supply the power from the branch circuit to the cord-connected appliance.   In some cases, a power supply cord is not long enough to reach from the intended location of the appliance to the nearest receptacle outlet.   In these situations, a cord set, often referred to as an extension cord, is used to extend the length of the appliance power supply cord to the electrical outlet.  

Cord sets and power supply cords are made from flexible cords that have designations such as SPT-2 which is often used on portable lamps and light duty extension cords.   Although flexible cords are not a substitute for fixed branch circuit wiring, they are tested for mechanical impact and flexural strength properties that are suitable for their intended application.   Flexible cords may or may not be provided with an equipment grounding conductor depending on the application or appliance involved.   Cord sets and power supply cords are not part of the branch circuit wiring, but since they extend power beyond the branch circuit, they can be subjected to the same overloads, short circuits, and ground faults as would the branch circuit wiring.

Branch Circuit Protection

The branch circuit overcurrent protective device (OCPD), (i.e. a fuse or circuit breaker), is specifically designed to protect electrical circuits, including the branch circuit conductors and flexible cords, against the unwanted effects of overcurrents.   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 (i.e. 15 or 20 amps), the OCPD will open the circuit before damage to equipment or a fire occurs.   However, an OCPD is not designed to protect the 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.

Arcing Faults

“Arcing” is defined as a luminous discharge of electricity across an insulating medium. The electrical discharge of an arc can involve temperatures on the order of several thousand degrees Celsius.   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 where the arcing is taking place.   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.    “Contact arcing” is arcing that involves direct or indirect physical contact between the conductors or "electrodes" where the arcing is taking place, such as arcing between closing or parting conductors making or breaking a circuit.

Arcing faults can occur in one of two ways, series arcing faults or parallel arcing faults.   A series arcing fault can occur when one of the current-carrying paths (e.g. a single wire) in series with the load is unintentionally broken.   For example, extreme flexing in an appliance power supply cord can cause one of the conductors to open and arc when flexed.   Series arcing faults are limited in current to the load current of the connected appliance or appliances in that circuit.   Parallel arcing faults occur when there is an unintentional conducting path between two conductors of opposite polarity, such as between a black and white conductor, or between a line conductor and ground.   Parallel arcing faults generally involve high currents, as they are limited only by the available fault current of the circuit.

AFCI Types

The UL1699 Standard addresses several types of AFCIs.   Each type of AFCI is intended for different applications and/or protection of different aspects of the branch circuit and extension wiring.   Three types of AFCIs for permanent connection to the branch circuit are identified in UL1699 as follows:

·       Branch/Feeder AFCI – This device is 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 (e.g. cord sets and power supply cords).   These may be a circuit-breaker type devices or a device in its own enclosure mounted at or near a panelboard.

·       Outlet Circuit   AFCI – This device is 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 – This is 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.

The NEC and AFCIs

During the revision process for the 2002 NEC there were several proposals to revise Sec. 210-12 to require both a branch/feeder and outlet circuit AFCIs in branch circuits required to be protected (bedrooms).   This would provide protection to both the branch circuit wiring, as well as cord sets and power supply cords that extend beyond the branch circuit.   The Code Panel did not accept these proposals.   There was also a proposal for the 2002 NEC to permit either a branch/feeder AFCI located at the origin of the branch circuit, or a new type of AFCI designated an “outlet branch circuit” type located at the first outlet receptacle.   A proposed revision to UL1699 would include this new type of AFCI defined as follows:

·       Outlet Branch Circuit AFCI – A device intended to be installed as the first outlet in a branch circuit.   It is intended to provide protection to downstream branch circuit wiring, cord sets and power-supply cords against the unwanted effects of arcing. These devices also provide protection to upstream branch circuit wiring.

The final language agreed upon by the Code Panel for the 2002 NEC for Sec. 210.12 will indicate the following:   “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 listed to provide protection to the entire branch circuit.”


AFCI Tests

As the UL1699 Standard continues to develop and address different product types and technology enhancements, it is important to understand how each type of AFCI is suitable for protecting various regions of the entire circuit against arc faults, and the extent and conditions under which this protection will be provided. Four different arc-fault tests are identified in UL1699 as shown in Figure 2.

Tests

Branch/ feeder

AFCI

Combination

AFCI

Outlet branch circuit

AFCI

 

Carbonized path arc ignition test             <series>

     
 

NM-B insulation cut

X

X

X(+)

 

<new>   NM-B w/o gnd insulation cut

   

X(+)

 

Carbonized path arc interruption test      <parallel>

     
 

SPT-2 insulation cut

X

X

X

 

NM-B insulation cut

X

X

X

 

Carbonized path arc clearing time test    <series>

     
 

SPT-2 insulation cut

 

X

X

 

<new>   NM-B insulation cut

   

X(+)

 

Point contact arc test                              <parallel>

     
 

SPT-2 insulation cut

X

X

X

 

NM-B insulation cut

X

X

X

(+) – also includes an upstream insulation cut

Figure 2 – AFCI Arc-Fault Tests

The Carbonized Path Arc Ignition Test is a non-contact arcing test conducted with NM-B cable with a series insulation cut.   Tests are conducted with arcing currents of 5 A, 10 A, rated current, and 150% rated current.   The Branch/Feeder, Combination, and Outlet Branch Circuit AFCIs are subjected to the Carbonized Path Arc Ignition Test.   The Outlet Branch Circuit AFCI is subjected to the Carbonized Path Arc Ignition Test with the arcing occurring upstream, to represent series arcing in the branch circuit wiring between the origin of the branch circuit and the first outlet receptacle.   The Outlet Branch Circuit AFCI is also subjected to the Carbonized Path Arc Ignition Test using NM-B cable without a grounding conductor, as may be found in some older homes built over 40 years ago.

The Carbonized Path Arc Interruption Test is a non-contact arcing test conducted with NM-B cable and SPT-2 flexible cord with a parallel insulation cut.   Tests are conducted with arcing currents of 75 A and 100 A.   The Branch/Feeder, Combination, and Outlet Branch Circuit AFCIs are subjected to the Carbonized Path Arc Interruption Test, however, the Outlet Branch Circuit AFCI is not tested with this parallel arcing occurring upstream from the device.

The Carbonized Path Arc Clearing Time Test is a non-contact arcing test conducted with SPT-2 flexible cord with a series insulation cut. Tests are conducted with arcing currents of 5 A, 10 A, rated current, and 150% rated current. The Outlet Branch Circuit and Combination AFCIs are subjected to the Carbonized Path Arc Clearing Time Test.   The Branch/Feeder AFCI is not subjected to this series arcing test with flexible cord as found in many cord sets and power supply cords. The Outlet Branch Circuit AFCI is subjected to the Carbonized Path Arc Clearing Time Test with NM-B cable and the arcing occurring upstream, to represent series arcing in the branch circuit wiring between the origin of the branch circuit and the first outlet receptacle.

The Point Contact Arcing Test is a contact arcing test conducted with NM-B cable and SPT-2 flexible cord with a parallel insulation cut.   Tests are conducted with arcing currents of 75 A through 500 A. The Branch/Feeder, Combination, and Outlet Branch Circuit AFCIs are subjected to the Point Contact Arcing Test, however, the Outlet Branch Circuit AFCI is not tested with this parallel arcing occurring upstream from the device.

Further Information

For more information on AFCIs, see the Regulators Page on the UL Web site at:

http://www.ul.com/regulators/afci/index.html

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