What is Ground Fault Circuit Interrupting (GFCI)?

GFCI protection helps prevent shock or electrocution of people, especially in areas like kitchens, bathrooms, outdoors and in basements, where people are more likely to become a path to ground via plumbing fixtures, pipes, or damp floors that are in contact with earth or pipes. GFCI’s may also provide alternative safety for outlets lacking equipment grounding facility. Where the equipment ground would draw short-circuit current through the circuit breaker causing it to trip, the GFCI senses and interrupts resistive shorts from 6 milliamps and higher, making it more effective for personnel safety than the equipment ground it ‘replaces’ (see ‘Retrofit applications’, below).

GFCI and decorator style rocker switch in kitchen

GFCI’s open (turn off) a circuit in a fraction of a second, when current flowing to a load (ie: receptacle) is not balanced precisely on both wires supplying the load. Imbalances on the wires indicate unintentional current flow, for example, from a short inside a hair-dryer or washing machine, through a person touching the cord-and-plug appliance, and to ground via an accidental ground path, such as a faucet handle and/or copper plumbing pipes. Such a ‘ground fault’ may pass less than one amp of current through the person being shocked (failing to trip any breaker), and yet be sufficient to prevent the person from letting go of what is shocking them. Without operation of a GFCI protective device (set to trip when unbalanced current exceeds 6/1000 of one amp — ie: 6 milliamps), the person being shocked can become more and more nervous and sweaty, increasing skin moisture and conductivity until current steadily increases (but remains well below the 15- or 20-amp trip threshold of the circuit breaker), and conductivity in the victim’s body may extend along sweaty arms to the heart and trigger a fatal heart attack (electrocution).

Conventional circuit breakers protect buildings from fire; GFCI’s protect people from shock. Operating normally, a 20A breaker typical of circuits supplying bathrooms or kitchen counter receptacles may carry 100A of current for a minimum of 5 seconds and a maximum of 20 seconds. That’s more than 10,000 Watts of power flowing, for long enough to say ‘Ouch!’ more than a couple times (this is assuming your vocal cords aren’t being fibrillated).

The National Electric Code (NEC) governing electrical installations changes every three years, and GFCI devices have not always existed. Some locations where protective devices are required for new construction are ‘grandfathered’ in existing construction. Philadelphia housing code (for rental properties) and prudence (for others) mandates installation of GFCI protection at locations where the NEC requires it.

GFCI Receptacles in Retrofit Applications

GFCI receptacles may be installed on any safe circuit, whether grounded or not — so long as the receptacle’s wiring enclosure has enough room to accommodate the bulk of the GFCI device, most of which sits recessed inside the box. Many older circuits can benefit from GFCI-protection. These include:

Older kitchen circuits that pre-date GFCI technology, whether the circuits have equipment grounding facility or not.
Old circuits with no equipment grounding facility, where personnel safety modes equivalent to or better than equipment grounding may be desired as a retrofit alternative that does not require new wiring.