How a circuit breaker trips, and what an AFCI adds

A circuit breaker is a switch that throws itself. Its job, in CPSC’s words, is to protect the electrical branch-circuit wiring and reduce the risk of fire from overheating of installed wiring due to overloads or faults. It does this with two separate mechanisms inside one small plastic body, each tuned to a different kind of trouble. The arc-fault breaker adds a third.

Two ways to trip

The thermal trip, for overloads. An overload is the slow kind of trouble: the appliances and lighting on a circuit regularly demand more current than the wiring can safely deliver. Inside the breaker, a bimetal strip heats up when the current exceeds the rating. The strip bends as it warms, and when it bends far enough it unlatches the contacts. CPSC’s technical paper notes that the response depends on the product of the square of the current and the time, which is a precise way of saying a small overload takes a long while to trip and a big one trips fast. A space heater and a hair dryer on the same 15-amp bathroom circuit is the classic thermal trip.

The magnetic trip, for short circuits. A short circuit is current taking a shorter or different path than intended: a hot wire touching neutral or ground, a nail through a cable. The current surges to many times the rating in an instant. The breaker’s magnetic (instantaneous) trip reacts to rapid increases in current of at least 8 to 12 times the rating, throwing the latch before the thermal strip has time to warm.

Both mechanisms exist to protect the copper in the wall, which is why the breaker’s rating is matched to the wire gauge, not to what you want to plug in.

A breaker that keeps tripping

CPSC’s home wiring guide is clear about what repeated trips mean: if fuses or breakers continue to trip, keep track of which branch circuits are affected and which appliances are in use when it happens, and consult a qualified electrician to resolve the problem. The one thing never to do is put in a higher-ampere fuse or breaker. CPSC: these overrated devices allow overloading of your branch circuits that can lead to fire. A 20-amp breaker on wire sized for 15 amps turns the breaker from a guard into a bystander.

Resetting one

There are two styles. The first simply moves back to ON. The second must be pushed fully to OFF first, then to ON; a tripped handle that sits in the middle will not reset until you do. Both types, CPSC says, must be reset by hand after the problem has been eliminated. Reset once. If it trips again on the same load, that is the warning above, not an invitation to try a third time.

Signs of wiring trouble

CPSC’s list of things to report to an electrician: lights that dim often, parts of the electrical system that are hot (parts may be warm but should never be hot), and the odor that burned or overheated wires often give off. A breaker that is warm to the touch belongs on the same list.

What an AFCI adds

Conventional breakers respond only to overloads and short circuits. They do not protect against arcing conditions that produce erratic current flow, and arcing is how a lot of electrical fires begin. An arc fault is dangerous sparking from damaged, overheated or stressed wiring or devices: old wires that have become frayed or cracked, a nail or screw driven into a wire behind a wall, a damaged outlet or cord. The current in an arc is too small and too brief to trip a breaker, but hot enough to ignite insulation and wood.

An arc-fault circuit interrupter uses current-sensing circuitry to discriminate between normal arcing, like the tiny spark when a switch flicks or a motor brushes, and unwanted arcing. Once it detects the unwanted kind, its control circuitry trips the internal contacts and de-energizes the circuit. AFCIs address fire hazards; GFCIs address shock hazards. They are different devices, though combination units that do both exist. Since the 2008 edition, the National Electrical Code has required AFCI protection beyond bedroom circuits to additional living areas of the home, including dining rooms and family rooms. States and cities adopt NEC editions on their own schedules and some have amended the AFCI rules, so a house built or rewired since then usually, but not always, has AFCI breakers in the panel.

AFCIs get a monthly test, like GFCIs: with the power on, press the TEST button on the breaker, which should trip it; then reset it. If the device does not trip when tested, CPSC says it is defective and should be replaced, and ESFI says to have a licensed electrician do that.

What is yours, and what is an electrician’s

Yours: resetting a tripped breaker once, testing AFCIs monthly, noting which circuit and which loads were involved when something trips, and keeping the panel’s door closed and its front clear. An electrician’s: anything behind the panel cover (CPSC: do not remove the service-panel cover; this is a job for a qualified electrician), a breaker that trips repeatedly, replacing a breaker of any kind, adding AFCI or GFCI protection, and any warm, hot or smelly part of the system.

Related

• Almanac: every month (standing list), May, National Electrical Safety Month
• How a GFCI trips and how power gets into and around the house
• Glossary: circuit breaker, overload, short circuit, AFCI, service panel, wire gauge