AFCIs target residential electrical fires

The National Electrical Code will soon require arc fault-circuit interrupters to protect bedroom receptacle circuits. Yet, they're largely unknown in protection officials.;

EVERY YEAR, ROUGHLY 40,000 RESIDENTIAL fires, or approximately 10 percent of all residential fires, are caused by problems with the electrical distribution system. (These fires result in 350 civilian deaths. An additional 30,000 fires annually are caused by electrical appliances.

A primary cause of these fires is arcing. Seen as a bright flash, an electrical arc is a momentary or continuous discharge of electricity across an insulating medium, usually air. The temperature at an arc's center ranges from 10,000 deg F to 25,000 deg F (5,538 deg C to 13,871deg C), hot enough to melt any known metal, and it releases energy in a pressure wave that carries with it molten metal particles or burning materials.

Both the intensely hot arc itself and the particles it expels can ignite surrounding materials.

The traditional circuit breaker can protect against hazardous arcing, which occurs at current levels above which they're designed to operate by shutting down the electricity to the affected circuit. Previously, there was no way to protect against electrical arcs that occurred at current levels below those at which circuit breakers were designed to operate until the recent development of the arc-fault circuit interrupter (AFCI), a device that allows the useful electrical current to flow until it detects hazardous arcing.

The following are just a few examples of the many circumstances in which electrical fires involving arcs may arise. We were able to obtain this evidence because someone was alert enough to call the fire department and take other action before a fire actually occurred. Without that action, however, these situations could have resulted in major fires.

Pierced insulation. Figure 1 shows evidence collected from a residential fire by the Bureau of Fire Prevention (BFP) in Cedar Rapids, Iowa, after firefighters disconnected three pieces of No. 12 AWG, Type TW copper wire that had been installed in a flexible metallic conduit in the house's ceiling. The overcurrent protective device (OCPD) hadn't opened, indicating that the current wasn't high enough or hadn't lasted long enough to cause it to operate. Although some of the insulation on all three wires had melted, the exposed conductors of the red and white wires indicated that the wires had arced to the conduit, not to each other. Apparently, the conduit had become hot enough to melt the wire insulation. Firefighters determined that sawdust next to the concealed conduit had begun to smolder, causing the smoke that triggered the alarm. An AFCI would be expected to detect the sputtering arcs from the wires to the conduit and prevent it from heating.

Wound wires. Figure 2 shows a second piece of evidence from the Cedar Rapids BFP The wound cord had apparently been connected to a window air conditioner that was left running unattended for some time. The normal current flow through the cord caused the insulation to melt, resulting in arcing between conductors. In this case, firefighters unplugged the unit before a major fire occurred. The OCPD hadn't opened because the current was at normal load except during the occasional burst of line-to-neutral arcing. An AFCI wouldn't have prevented the insulation from melting, but it would have detected the bursts of arcing and opened the circuit well before this degree of damage occurred.

High resistance connection. Figure 3 shows a basement receptacle that was used for 15 years to connect a water distiller. Although the grounding connection was present, it was never connected to ground because the circuit was an extension of a two wire circuit. Eventually, a hot, high-resistance connection led to surface tracking from the ground bracket to the receptacle jaws. Evidence of moisture was also present. Indications are that a line-to-neutral arcing fault occurred, igniting a fire that filled the house with smoke before the circuit breaker opened the circuit. The 2- by 4-inch (51- by 102-millimeter) board attached to the receptacle was blackened, but the fire self extinguished before firefighters arrived.

Surface tracking. Figure 4 shows an outdoor porch light damaged by arc tracking. The homeowner turned off the circuit when he noticed smoke accompanied by a buzzing sound, clear evidence of arcing on the screw shell from the energized tip of the light bulb as well as arcing to the grounded mounting screw in the socket base. The arc was tracking along the base surface of the bulb, much of which had eroded from intense heat. If the home owner hadn't turned off the circuit, the arcing could have ignited materials near the fixture. An AFCI would have detected either the line-to-neutral arc or the line-to-ground arc and opened the circuit.

History of the AFCI

The first experimental AFCI prototypes appeared in the form of circuit breakers in 1993. A year later, the Consumer Product Safety Commission studied AFCI prototypes developed by three circuit breaker manufacturers as part of its Home Electrical System Fires Project. In a report titled, "Technology for Detecting and Monitoring Conditions that Could Cause Electrical Wiring System Fires," Underwriters Laboratories (UL), which performed the product analysis, stated that the arc-fault circuit technology "is capable of detecting and responding to arcing-fault currents below normal load currents as well as above and, therefore, has the potential to monitor and detect precursory arcing conditions that may not constitute an immediate threat of ignition, but which could lead to ignition."

The report went on to say that "arc-fault detection appears to be a very promising technology, especially when added to residential branch-circuit breakers...It's recommended that additional research be considered to better define the nature of residential electrical ignition sources, levels of arc-fault protection needed, and standardized test methods to verify effectiveness of practical products that would utilize this technology:"

UL did a major part of the additional research the report recommended in a subsequent study under contract with the National Electrical Manufacturers Association (NEMA), producing another report entitled, "Report of Research on Arc-Fault Detection Circuit Breakers" in March 1996. A year later, the first commercial AFCIs were available.

That same year, three proposals were made that would have allowed the 1999 National Electrical Code (NEC) to require AFCIs in 15- and 20-ampere residential branch circuits. At the time, however, AFCIs weren't generally available. Nor was solid field experience of their performance. Nonetheless, the panel considering the three proposals recognized the potential of these devices and added a requirement in Section 210-12 that AFCIs be installed in branch circuits to bedroom receptacles. This meant that AFCIs would gain valuable experience protecting a residential area in which a large percentage of fires originate, especially those resulting in civilian deaths. The panel made the requirement effective in 2002.

UL publishes a standard

In February 1999, further UL research resulted in UL 1699, Standard for Safety for,4rc-Fault Circuit Interrupters, which evaluates each AFCI in a broad range of tests, which includes performance under extreme environmental conditions, during a voltage surge, with electrical noise, and during abnormal electrical conditions. The three test areas most closely related to the AFCI's protection function are arc fault detection; unwanted tripping, or nuisance; and operation inhibition.

The arc fault detection tests determine an AFCI's ability to detect an arc under various standard conditions and to open the circuit within a prescribed period. The unwanted tripping tests evaluate the AFCI's ability to avoid nuisance operation when using a variety of equipment that's likely to have signals resembling an arc, such as light switches and motors with arcing brushes, or that's likely to produce wave forms that look like electrical noise, such as computers, dimmers, and shop tools. The operation inhibition test verifies that the AFCI can correctly identify an arc even if it's in a circuit with other equipment that might hide the arc signal.

UL 1699 also identifies five potential forms of AFCI. One, the branch/feeder AFCI, is meant to be installed at the origin of a branch circuit or feeder to protect the fixed wiring system, as well as cords and extension wiring. So far, the 15- and 20-ampere branch/feeder AFCI in circuit breaker form is the only AFCI that's commercially available.

Another form of AFCI is the outlet circuit AFCI, intended for use at an outlet, perhaps in receptacle form. It's meant to protect cord sets and power supply cords. From the testing standpoint, there's an important difference between a branch/feeder and an outlet circuit AFCI. The branch/feeder AFCI is tested to determine how well it will protect construction wire and cable, as well as two or three wire cords for arcs of 75 amperes and greater. It's also tested for arcs to ground at 5 amperes and above in the same wires and cables. Though the outlet circuit AFCI isn't tested to determine how well it will protect construction wires and cables, it's tested for arcs in two wire cords at 5 amperes and above with no arc to ground involved. This test wasn't included for the branch/feeder AFCI because the potential for nuisance operation is greater with the 5-ampere detection level, and UL wanted to avoid nuisance operation at the branch circuit.

Another form of AFCI is the combination AFCI, which marries the capabilities of the branch/feeder and outlet circuit AFCI. It's intended to protect fixed wiring systems, cord sets, and power supply cords. Though these devices aren't commercially available at this time, they can be expected in circuit breaker and other forms to protect an entire circuit.

The fourth and fifth types of AFCI are both plug-in devices. The cord AFCI is intended to protect the power supply cord connected to it, and the portable AFCI is provided with one or more outlets. UL 1699 carefully avoids identifying a specific form for each type of AFCI to avoid limiting the types of devices that may emerge as the technology matures.

In choosing among these different types of AFCIs, one should look at where fires originate. The majority of fires in the electrical distribution system occur in the fixed system, rather than extension cords, and the only device that addresses the fixed system is the branch/feeder AFCI. However, the branch/feeder AFCI provides limited protection for plugs, cords, and appliances, which are better suited to the outlet circuit AFCI when it becomes available. The portable and cord AFCIs will provide more focused protection for specific appliances, although none of these AFCIs has yet emerged.

Future

AFCIs have now been installed in some homes for more than two years with no reports of nuisance operation, and there have been good reports of hazardous arc detection. If this new technology is as effective in addressing electrical fires in residences as it currently appears to be, we can expect to see a reduction in home fires in the future.

George Gregory, P.E., is manager of Industry Standards at Square D Company in Cedar Rapids, Iowa.

Copyright National Fire Protection Association Mar/Apr 2000
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