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Re: GFCI +- isolation transformer



It is instructive to know how a GFCI works. It works by detecting an
imbalance between the current in the hot wire and the current in the neutral
wire. Both wires pass through a coil. If the currents in the hot and
neutral are equal but opposite, no emf is generated in the coil. If the
currents are not equal/opposite an emf is generated in the coil and this
signal is used to shutdown the circuit.

The presumption is, if there is more current in the hot than in the neutral,
that current is getting to ground by some path that it shouldn't be taking.
That path could be through a person.

If the GFCI powers an isolation transformer, the GFCI still provides
protection for a grounded person who touches anything that is hot because of
being connected to the hot wire from the GFCI. On the other side of the
isolation transformer those secondary wires have no potential difference
with respect to ground unless one of them is grounded, which would defeat
the purpose of the isolation transformer. So a properly-wired isolation
transformer itself offers some protection against grounded people touching
things that are live because of the transformer. That's why it is an
"isolation" transformer.

The GFCI in front of the isolation transformer is worthless for preventing
faults after the transformer because the GFCI continues to see the hot &
neutral balanced as far as it is concerned.

Also note, via the description of the GFCI, that it does not protect you
from touching hot and neutral. If you hold a paper clip in one hand and put
it in the hot slot of a GFCI, then put a paper clip in your other hand and
stick it into the GFCI neutral slot, everything will be balanced, the GFCI
will just think you are the device being powered, and you will be
electrocuted. The GFCI specifically detects when the hot current fails to
completely return via the neutral. That typically happens when there is a
fault (alternative path) to ground... hence "ground fault."


Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817