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 circuits 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.
Authors 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
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