Arc-Fault Circuit Interrupters
What they are, why you should care, and how to install them.
Synopsis: Arc-fault circuit interrupters (AFCIs) are now required by most building codes. An electrician explains how these devices function and how they should be installed. Also included is a discussion of the costs vs. benefits of AFCI breaker design.
I could hear my garage light from across the room: a loud snap followed by a long frying sound. I removed the fixture to look at the wiring. It was a problem I’d seen before: another bad light fixture in which an arc had jumped through a loose, press-fitted rivet joint.
The danger is fire
Arcing is a luminous discharge of electrical current crossing a gap between two electrodes. According to Underwriters Laboratories (UL), arcing can involve temperatures of several thousand degrees Celsius. An unintentional discharge is called an arc fault, which can cause a fire in your home.
Arc-fault circuit interrupters (AFCIs), a part of the 2002 National Electrical Code, use available technology to help deter fires caused by arc faults. Currently, they’re required in all new 120v bedroom branch circuits — anywhere a cable enters the bedroom through the walls, floor or ceiling. This requirement affects receptacles, lights, fans and smoke alarms, and eventually may be expanded to the rest of the house. Because they’re so new, prices for AFCIs vary a lot; they can be found at electrical-supply houses starting at $40 and going up from there.
Three kinds of arc faults
There are three types of arc faults: hot to neutral, hot to ground and a series arc. The hot-to-neutral and hot-to-ground arcs — the two-wire direct shorts seen all the time — are called parallel arcs because they’re parallel with the electrical load. They generally involve higher current because they’re limited only by the available fault current of the circuit.
A series arc can occur when a loose or frayed wire makes intermittent contact with itself during use.
What is an AFCI?
Common fuses and breakers aren’t designed to prevent arcing unless the arc is of enough amperage or duration for the breaker to sense it. An arc at low current levels still can cause a fire. I’ve actually arc-welded with hot conductors without tripping the breaker. The current version of AFCIs works like a regular breaker in the service panel, but it also shuts off the circuit if it detects a hazardous arc. The sensitive internal electronics of an AFCI can distinguish the difference in wave form between a hazardous arc and the kind of ordinary arcing that occurs when, for example, you plug in a vacuum with its switch in the on position.
AFCIs not only are sensitive to parallel arcs of 75 amps or more, but they’re also sensitive to series arcs of 5 amps or more.
AFCIs detect series arcs indirectly. To trip the breaker, an AFCI depends on 30 milliamps to 50 milliamps (ma) of current leaking to ground. Indirect series-arc detection works only if a ground wire is present. It won’t work with old ungrounded wiring such as ungrounded NM (nonmetallic) cable or knob-and-tube wiring.
From Fine Homebuilding #153
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