Arc fault circuit interrupters

NFPA Journal, Mar/Apr 2001 by Bunker, Merton

The National Electrical Code requires AFCIs on branch circuits in dwelling unit bedrooms.

The 1999 edition of NFPA 70, the National Electrical Code (NEC), requires the use of arc fault circuit interrupters (AFCIs) on the bedroom branch circuits of dwellings.

What Is an arc fault?

An arc fault is a discharge of electrical current across an insulating medium that may occur when weakened insulation or a carbonized path between ungrounded conductors or between ungrounded conductors and grounded conductors allows current to flow between the conductors. Series arc faults can also occur. The core temperature of an arc fault ranges from 9,032 deg to 27,032 deg F (5,000 deg to 15,000 degC), well beyond the ignition point of most insulation.

An arc fault can create pressure that will spread hot gases and molten metal to surrounding areas, sometimes igniting combustibles. Smaller faults may heat the conductor until the resistance breaks down and sputtering occurs.

Arc faults can take anywhere from seconds to years to develop and may be sustained at 120 volts. Since most household wiring is hidden inside walls, some arc faults go undetected.

What is an AFCI?

AFCIs detect arcing faults that can cause wiring fires by recognizing characteristic wave forms unique to arcing and de-energizing the circuit when an arcing fault is detected.

Don't overcurrent devices provide protection from arc faults?

Not always. Overcurrent protective devices may not trip because the currents drawn during arcing faults may be low enough to keep them from tripping early enough to detect and prevent a fire.

What conditions are responsible for arc faults?

A loose or corroded connection, a staple that's punched through the insulation, damaged insulation between conductors, and pinched cords can all lead to arc faults.

Where does the National Electrical Code require the use of AFCIs?

As of January 1, 2002, Section 210-12 of the NEC requires AFCIs to protect branch circuits that supply 15- and 20ampere receptacle outlets in dwelling unit bedrooms.

Why are AFCIs only required in bedrooms of dwellings?

Bedrooms were selected by the code panel as a logical starting point for AFCIs. However, this requirement doesn't limit the use of ACIs elsewhere in the dwelling.

Why were these requirements added?

AFCIs are considered the best technology available to reduce electrical fires where arc faults are likely to occur.

What kinds of AFCls are available today?

Branch/feeder AFCI devices are readily available, as are individual cord protection devices. Products, such as outlet and combination AFCIs are in development. Most electrical equipment distributors carry these products.

Doesn't the switching of loads, such as turning appliances on and off, cause nuisance tripping?

Not usually. Operating light switches and disconnecting plugs often cause conditions similar to arc faults. However, AFCIs are designed to resist unwanted tripping during normal switching and to identify abnormal arcing. In fact, AFCIs are evaluated for this and other conditions as part of product certification testing.

What product standards apply to AFCls?

UL 1699, Standard for Safety Arc-Fault Circuit Interrupters currently provides product testing requirements for AFCIs for use on 120-volt circuits. Underwriter's Laboratories is currently developing additional requirements for AFCIs for use on 240-volt mulitwire circuits.

THE NEC IS ONE OF SEVERAL CODES BEING VOTED ON AT NFPA'S WORLD FIRE SAFETY CONGRESS AND EXPOSITION IN ANAHEIM IN MAY. FOR MORE INFORMATION ON THE NEC, CHECK OUT NFPA'S WEB SITE, WWW.NFPA.ORG

Merton Bunker, P.E., is NFPA's former chief electrical engineer.