Ground-Fault Circuit Interrupters (GFCI)
A ground-fault occurs when there is a break in the low-resistance grounding
path from a tool or electrical system. The electrical current may then take an
alternative path to the ground through the user, resulting in serious injuries
or death.
The ground-fault circuit interrupter, or GFCI, is a fast-acting
circuit breaker designed to shut off electric power in the event
of a ground-fault within as little as 1/40 of a second. It
works by comparing the amount of current going to and returning
from equipment along the circuit conductors. When the amount going
differs from the amount returning by approximately 5 milliamperes,
the GFCI interrupts the current.
The GFCI is rated to trip quickly enough to prevent an electrical
incident. If it is properly installed and maintained, this will happen as
soon as the faulty tool is plugged in. If the grounding conductor is not
intact or of low-impedance,
the GFCI may not trip until a person provides a path. In
this case, the person will receive a shock, but the GFCI should
trip so quickly that the shock will not be harmful.
The GFCI will not protect you from line contact
hazards (i.e. a person holding two "hot" wires, a hot
and a neutral wire in each hand, or contacting an overhead power
line). However, it protects against the most common form of
electrical shock hazard, the ground-fault. It also protects
against fires, overheating, and destruction of wire insulation.
For construction applications,
there are several types of GFCIs available, with some
variations:
Receptacle Type
- The Receptacle Type
incorporates a GFCI device within one or more receptacle outlets.
Such devices are becoming popular because of their low cost.
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Portable Type
- Portable Type GFCIs
come in several styles, all designed for easy transport. Some are
designed to plug into existing non-GFCI outlets, or connect with a cord
and plug arrangement. The portable type also incorporates a
no-voltage release device that will disconnect power to the outlets if any
supply conductor is open. Units approved for outdoor use will be in
enclosures suitable for the environment. If exposed to rain, they
must be listed as waterproof.
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Cord-Connected
Type
- The Cord-Connected
Type of GFCI is an attachment plug incorporating the GFCI module.
It protects the cord and any equipment attached to the cord. The
attachment plug has a non-standard appearance with test and reset buttons.
Like the portable type, it incorporates a no-voltage release device that
will disconnect power to the load if any supply conductor is open.
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Because GFCIs are so complex, they require testing on a regular
basis. Test permanently wired devices monthly, and portable-type
GFCIs before each use. All GFCIs have a built-in test
circuit, with test and reset buttons, that triggers an artificial
ground-fault to verify protection.
Ground-fault protection, such as GFCIs provide, is required by
OSHA in addition to (not as a substitute for) general
grounding requirements.
Additional Information:
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www.osha.gov
