October is Fire Safety Month, and a good time to take stock of the devices in your home that protect against costly and potentially deadly conflagrations. For most people that means changing the batteries on their smoke detectors or checking the pressure on a fire extinguisher, if they have one, without a lot of thought given to actual causes of home fires.
Yet every year, electrical malfunctions cause nearly 48,000 residential fires, 418 deaths, and 1.4 billion in property damage. About half of those fires can be traced to shorts, also known as arc faults, caused by electricity crossing a gap between frayed or broken wires and between loose or corroded connections in walls, appliances, and cords. These arcs can reach temperatures of 10,000 degrees F., hot enough to throw off sparks and ignite wood, fabric, and debris. Fires that start this way are particularly dangerous because they often occur in wall cavities, attics, and other places where a smoke alarm can't detect them until it's too late. You can't depend on a standard circuit breaker to stop arc faults; they are usually too brief and at a current level too low to make the breaker trip. But now, thanks to the Arc-Fault Circuit Interrupter (AFCI), we have a device that can detect and automatically shut down an arcing circuit.
AFCIs have proven so effective at preventing electrical fires that the National Electrical Code (NEC) requires AFCIs to be installed in almost every room in newly built houses. Bathrooms, garages, and unfinished basements—areas defined as non-living spaces—are among the few exceptions. The NEC also requires the installation of AFCIs in existing homes whenever an addition, an extra circuit, or even an extra outlet is added. If your wiring is old, or you want the best available protection for your electrical system, you might consider a phased replacement of the key outlets in each circuit.
They're different from GFCIs
AFCIs look similar to ground-fault circuit interrupters (GFCIs), which have long been required for outlets in kitchens, bathrooms, garages, and outdoors, and other damp locations. Both GFCIs and AFCIs have two buttons: one for testing the device and one for resetting it after it trips. But a GFCI performs an entirely different function: it shuts down current flowing outside a circuit to the ground, guarding against shocks that can injure or even kill a person.
According to Bill Grande, Senior Director of Product Management at Leviton, a major manufacturer of electrical control devices, AFCIs have a much different job. They're engineered to sense minute, fleeting fluctuations in the frequency and amplitude of the electricity's 60-cycle-per-second waveform. Such fluctuations occur all the time when current flows. But an AFCI has to differentiate between the common background fluctuations and detect only the ones that exhibit the signature of a dangerous arc fault—either a parallel arc fault (figure 1) between two conductors or a series fault (figure 2) where there's a break in a single conductor.
Two types of AFCIs
AFCIs can be installed in circuit-breaker panels or in outlets. The circuit-breaker types, known as combination AFCIs, are typically more expensive, may not fit in older circuit-breaker panels, and should be installed by an electrician. The outlet types, known as Outlet-Branch Circuit (OBC) AFCIs, are as easy to install as a regular receptacle. They cost about $30, and they're also easy and convenient to test and reset if they trip. Leviton's 15-amp AFCI, for instance, has a green LED that tells you it's actively monitoring the circuit (figure 3). When that light goes out, you know the AFCI has tripped. (This device is also tamper resistant, as required by the NEC.)
OBC AFCIs do not need to be installed in every outlet in a room. When one is placed in the first outlet on a branch circuit (figure 4), it will protect everything downstream from that location—including any device, appliance, or fixture plugged into the outlets on that circuit—from both parallel and series arc faults.
If you don't know which outlet is first in the branch, contact an electrician or follow this procedure, recommended by Leviton's Grande. Shut off the breaker to the circuit and remove the outlet that you suspect is the first in the circuit. Cap the wires and turn the breaker back on. Now test the other outlets by plugging in a work light. If you chose correctly, they will all be dead. If you chose incorrectly, put the original outlet back in and repeat the process after removing another outlet.
What to do when an AFCI trips
It is not wise to push the “reset” button and hope the problem will go away. The device is signaling that there's an arc somewhere in the circuit, and it should tracked down and fixed. In many cases, Grande says, arcing occurs in an appliance, cord, or light that's plugged into the AFCI-protected circuit. In that instance, disconnecting the faulty item from the receptacle should restore the circuit's safety.
But if an AFCI trips even after all plugged-in items are removed, then the problem is likely to be with the wiring inside the walls. Call an electrician for a diagnosis. He or she will inspect all the connections, and, if necessary, replace the entire circuit.
AFCIs can last for decades, but like other sensitive electronics, Grande says, their performance degrades as they're exposed to power surges and humidity. Underwriter Laboratories (UL) recommends that an AFCI be tested monthly to make sure it's still in working order. Pushing the “test” button creates an intentional fault inside the device that should shut the circuit down. If you push the
'test" button and the AFCI doesn't trip, then it's time to replace it.
A monthly check of every AFCI may sound like a nuisance, but it's a small price to pay for the protection it provides you and your family against house fires.
For more information on AFCIs, please go to leviton.com/afci.