HOW GFCIs WORK

A ground-fault circuit interrupter (GFCI) is specifically designed to protect people against electric shock from an electrical system. A GFCI protection device operates on the principle of monitoring the imbalance of current between the circuit’s ungrounded (hot) and grounded (neutral) conductor. An interesting point about these devices is that despite their name, they will operate on a circuit or without an equipment grounding conductor. 100-ground-fault ci 02 no grn ok.cdr

During the normal operation of 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 very 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. The mA used above stands for one thousands of an amp, so 5 mA is equal to 5/1000th of an ampere.

Author’s Comment: GFCI protective devices are commercially available in 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 (International Association of Electrical Inspectors magazine), 21 percent of GFCI circuit breakers and 19 percent 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 protectors (metal-oxide varistors) that protect the GFCI sensing circuit. This damage resulted from voltage surges from lightning and other transients. In areas of high lightning activity, such as southwest Florida, the failure rate for GFCI circuit breakers was over 50 percent!

At least one leading manufacturer markets a listed 15A, 125V GFCI receptacle you cannot reset if the GFCI circuit no longer provides ground fault protection. As an added safety improvement, 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 thought on GFCI protection is that you should press the test feature of the GFCI protection device to ensure that it works by turning the power off to the connected load. Do not assume that a GFCI protection device is operational unless you properly test it!

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