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Subject: TWL: GFCI's (was Which Galvanic Isolator)
From: Andy Woods (awoods@XXX.XXX)
Date: Sun Aug 25 2002 - 08:28:41 EDT
Hello Bob,
I did not refer to a Ground Fault Circuit Interrupter. A Ground Fault
Circuit Interrupter is not a Galvanic Isolator. They are different devices
doing different jobs.
Sorry if I misled you or anyone else. I was referring to ground faults. A
ground fault is a fault or short between the phase conductor (line) and the
grounding conductor. The grounding conductor should always be a green or
bare conductor. In a cord it is a green conductor.
A Ground Fault Circuit Interrupter (I think the name is misleading) or GFCI
is a device that measures the current of the phase conductor (line) and the
current of the neutral conductor (not the grounding conductor). If the
current in the line conductor is 5 milliamps more than the current in the
neutral conductor, the GFCI assumes the current unaccounted for is going
somewhere it shouldn't, and it trips. The Ground Fault Circuit Interrupter
does not even care if a ground wire is connected to it. The ground wire
actually has nothing to do with the GFCI, and the GFCI will work perfectly
well without it. That's why I think the name is misleading. A GFCI
receptacle is easily recognized by the two buttons on the face marked "test"
and "reset". The National Electric Code allows the GFCI receptacle to be
installed as a replacement for the old two wire receptacles commonly found
in older houses. As you probably know the old two wire receptacles had no
ground wire. If the receptacle were to break or need replacement, many
people would simply install a new grounded receptacle, but there was no
ground wire to hook up. This is a code violation. You could run a ground
wire to a grounded cold water pipe, but most people would not do that. The
GFCI does not care if the ground wire is not present, it will protect people
from leakage current without the ground. If the current goes somewhere else
other than on the neutral wire (like through your body) it will trip. If a
ground wire is not present the device must be marked as such, according to
the Code. The GFCI receptacle is NOT a circuit breaker. It only cares
about the difference in the current, it does not care about the value of the
current.
A GFCI circuit breaker is a hybrid device that also measures the difference
in the line current and the neutral current and will trip if the difference
is greater than 5 milliamps. The GFCI circuit breaker also cares about the
actual value of the current, and will trip when the current exceeds the
rated
value of the circuit breaker. There are GFCI circuit breakers rated a 30
milliamps that are used for equipment protection in industrial
installations, generally heat trace and related equipment. These 30
milliamp breakers are NOT for personnel protection. Generally they are not
readily available at places like Home Depot, only the 5 milliamp breakers
are readily available, but you must be aware of what you buy. I have never
seen a GFCI receptacle rated at 30 milliamps.
Again, I apologize for any confusion I may have created.
Best Regards,
Andy
Andy & Linda Woods
Grand Folly
1970 Grand Banks 36 Classic
Georgetown, MD
awoods@XXX.XXX
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