A GFCI monitors the current balance between the ungrounded “hot” conductor and the grounded conductor. As soon as the current flowing throughthe “hot” conductor is in the range of 4 to 6 milliamperes more than the current flowing in the “return” grounded conductor, the GFCI senses this unbalance and trips (opens) the circuit off.

The unbalance indicates that part of the current flowing in the circuitis being diverted to some path other than the normal return path along the grounded return conductor. If the “other” path is through a human body, as , the outcome could be fatal.

UL Standard No. 943 covers ground-fault circuit interrupters.

• Class “A” GFCI devices are the most common. They are designed to

– trip when current to ground is 6 milliamperes (6/1000 of an ampere) or greater.

– not trip when the current to ground is less than 4 milliamperes (4/1000 of an ampere).

– may or may not trip when the current to ground is between 4 and 6 milliamperes.

– will open very quickly, in approximately 25 milliseconds.

• Class “B” GFCI devices are pretty much obsolete. They were designed to trip on ground faults of 20 milliamperes (20/1000 of an ampere) or more.

They were used only for underwater swimming pool lighting installed before the adoption of the 1965 NEC. For this application, Class “A” devices were too sensitive and would nuisance trip.

What a GFCI Does Not Do
• It does not protect against electrical shock when a person touches both circuit conductors at the same time (two “hot” wires, or one “hot” wire and one grounded neutral conductor) because the current flowing in both conductors is the same.

Thus, there is no unbalance of current for the GFCI to sense and trip.

• It does not limit the magnitude of ground-fault current. It does limit the length of time that a ground fault will flow. The GFCI should trip in about 25 milliseconds.

In other words, you will still receive a severe shock during the time it takes the GFCI device to trip “off.”

• It does not sense solid short circuits between the “hot” conductor and the grounded “neutral” conductor. The branch-circuit fuse or circuit breaker provides this protection.

• It does not sense solid short circuits between two “hot” conductors. The branch-circuit fuse or circuit breaker provides this protection.

• It does not sense and protect against the damaging effects of arcing faults, such as would occur with frayed extension cords.

This protection is provided by an arc-fault circuit interrupter (AFCI).

• It does not provide overload protection for the branch-circuit wiring. It provides ground-fault protection only.

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