An arc-fault occurs when loose or corroded connections make intermittent contact and causes sparking or arcing between the connections. This translates into heat, which will break down the insulation of the wire and can be the trigger for an electrical fire.
Unlike a short circuit, that is a hot wire coming into contact with a ground or neutral wire, arcing may not trip the circuit breaker. If you’ve ever heard a switch buzzing, hissing, or popping, you’ll know what I’m talking about.
In order to protect your home, an arc-fault circuit interrupter can be used to detect just such a problem.
Arc-fault circuit-interrupters provide protection from the effects of arc faults, which includes arcing of switches and the like. The device then shuts the circuit down when arcing is detected.
The 2008 National Electrical Code states in article 210.12(A) that the definition of arc-fault circuit-interrupter is, "A device intended to provide protection from the effects of arc faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc fault is detected."
So the question now focuses on where these safety devices are to be placed in dwelling units. According to article 210.12(B), "All 120-volt, single phase, 15- and 20-ampere branch circuits supplying outlets installed in dwelling unit family rooms, dining rooms living rooms, parlors, libraries, dens, bedrooms, sun rooms, recreation rooms, closets, hallways, or similar rooms or areas shall be protected by a listed arc-fault circuit interrupter, combination-type, installed to provide protection of the branch circuit."
The exceptions to the rule are stated in articles 760.41(B) and 760.121(B) for power requirements for fire alarm systems. Also, see the National Fire alarm Code in the NFPA 72 - 2007 edition where 11.6.3(5) tells of information related to secondary power supply requirements for smoke alarms installed in dwelling units.
AFCI's vs. GFCI's
AFCI's are different than GFCI's. Ground fault circuit interrupters are required in homes, especially where water is present. There has long been a discussion, for lack of a better word, about whether the National Electrical Code says that you need to install GFCI outlets or just have GFCI-protected circuits. I and some electrical inspectors have discussed this in detail in some cases. One even told me that his interpretation of the rule is that you cannot feed off the load side of the GFCI and count that regular outlet as a GFCI-protected outlet.
GFCI receptacles are a great choice for areas like bathrooms, kitchens, exterior receptacles, and those around water. However, that can also be said for GFCI breakers for these areas. The thing to consider is what may be connected to the GFCI protected circuit. If the circuit is only for protecting the receptacles in that area, then maybe a breaker is the better choice. The GFCI breaker could be protecting many outlets, which would then be a cost saver. However, if there is going to be receptacles that don't need to be protected on that circuit, maybe a GFCI receptacle is the better choice. In older homes that have receptacles without a ground connection, GFCi's are a great replacement for these outlets.
They can sense trouble in the wiring and shut the circuit down before trouble can start.
So whether you need a GFCI to sense a difference of potential on the line or an AFCI to detect arcing, like on a lighting circuit, you can rest easy with the safety devices that have been developed for your home.