This article was posted before 01/01/2011 and is most likely outdated.

GFCI's How They Work (01-25-2K)

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By Mike Holt, Published in EC&M Magazine

A ground-fault circuit-interrupter (GFCI) is the only protection device designed to protect persons against electric shock from an electrical system. The NEC defines a Ground-Fault Circuit Interrupter as "a device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds some predetermined value that is less than that required to operate the overcurrent protective device of the supply circuit."

A GFCI protection device operates on the principle of monitoring the imbalanced of current between the circuits ungrounded (hot) and grounded (neutral) conductor. An interesting point about these devices is that despite their name - they will operate on a circuitwithout a ground. In a typical 2-wire circuit, the current returning to the power supply will be equal to the current leaving the power supply (except for some small leakage). If the difference between the current leaving and returning through the current transformer of the GFCI protection device exceeds 5 mA (+ - 1 mA), the solid-state circuitry opens the switching contacts and de-energizes the circuit (Figure 1).

Author's comment: You can incorporate GFCI protection into receptacles, circuit breakers, cord sets and other types of devices.

Warning: Severe electric shock or death can occur if a person touches the hot and neutral conductor at the same time, even if the circuit is GFCI protected. This is because the current transformer within the GFCI protection device does not sense an imbalance between the departing and returning current and the switching contacts remain closed,

Danger: Typically when a GFCI protection device fails, the switching contacts remain closed and the device will continue to provide power without GFCI protection. According to a study by the American Society of Home Inspectors (published in the November/December, 1999 issue of the IAEI News) 21% of GFCI circuit breakers and 19% of GFCI receptacles did not provide GFCI protection. Yet the circuit remained energized!

The failures of the GFCI sensing circuits were mostly due to damage to the internal transient voltage surge protection (metal-oxide varistors) that protect the GFCI sensing circuit. This damage resulted from voltage surges from lightning and other transients. In areas of high lighting activity, such as Southwest Florida, the failure rate for GFCI circuit breakers was over 57%!

One manufacturer makes a 15A 125V GFCI receptacle you cannot reset if the GFCI circuit no longer provides ground fault protection. In addition, this particular GFCI receptacle has a built-in line-load reversal feature that prevents the GFCI from resetting if the installer mistakenly reverses the load and line connections.

One final thoughts on GFCI protection is that you should press the test feature of the GFCI protection device to insure it works! These are excellent devices when properly wired and functional. Don't assume that a GFCI protection device is operational unless you test it!