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 ďa 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.Ē
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).
You can incorporate GFCI protection into receptacles, circuit breakers, cord sets and other types
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. Donít assume that
a GFCI protection device is operational unless you test it!