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A GFCI is only as good as its installation and inspection.
A ground-fault circuit interrupter (GFCI) is the only protection device designed to protect people against electric shock from an electrical system. Because of this, we need to understand what a GFCI is, how it works, and what its limitations are.
We need only to look to the NEC for the definition of a GFCI. The NEC defines it 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 imbalance of current between the circuit's ungrounded (hot) and grounded (neutral) conductor. It does not monitor the grounding conductor, and so it will still operate in a circuit without 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 5mA (51mA), the solid-state circuitry opens the switching contacts and de-energizes the circuit (see Figure, above). This will always happen as long as the GFCI is in working order. However, GFCIs fail more often than most people think. And they give no outward sign that they are no longer providing their protection.
Typically, when a GFCI protection device fails, the switching contacts remain closed, and the device continues to provide power - but no protection.
According to a study conducted by the American Society of Home Inspectors (published in IAEI News, November/December 1999), 21% of GFCI circuit breakers and 19% of GFCI receptacles tested did not provide GFCI protection. Yet, the circuit remained energized! In the examined cases, 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-lightning activity, such as Southwest Florida, the failure rate for GFCI circuit breakers was more than 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 thought on GFCI protection: Press the test feature of the GFCI protection device to ensure it works. These are excellent devices when properly wired. However, never assume a GFCI protection device is operational unless you test it!
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.