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Re: GFCI +- isolation transformer



At 10:59 PM 12/1/00 -0500, Marc Kossover wrote:

Wall current --> GFCI --> 110 V to 220 V transformer --> equipment

Is it true a GFCI on the other side of an isolation transformer is
useless?

Right, it is almost useless in the indicated configuration.

If there is a ground fault in the equipment, will the GFCI trip the
circuit?

In the indicated configuration, a single fault on the isolated side ought
to be undetectable (and also harmless) as discussed in more detail below.

It doesn't in my room and it makes me nervous, but maybe the GFCI
is faulty.

There is no reason to believe there's anything wrong with the GFCI module.

I should get a 220 GFCI,

Maybe. OTOH, in the indicated configuration, the protection provided by
the isolation transformer is most ways _better_ than anything a GFCI can
provide; the GFCI adds nothing (except in rather obscure cases).

It is more informative to draw some semblance of a circuit diagram.


blue
$ $-------------|
$ $ |
black $ $ equipment
----G/ -------------$ $ |
F $ $ |
C white $ $ red |
----I/ -------------$ $-------------|

green
------------------------------------


There are some things a GFCI can do, and some things it cannot.
1) It can detect a short from black to green; that is its main job. It
trips when the fault current is a few milliamps.
2) In most cases it can detect a short from white to green.
3) A short from white to black is not a ground fault. It is
indistinguishable from the load. If it is an overload, the GFCI will trip
like a normal circuit breaker, but this requires many amps, not
milliamps; it is more than enough to start fires and/or kill you directly.
4) For reasons similar to item (3), a short from blue to red is
indistinguishable from the load, and trips only for high-current overloads.
5) Because of the isolation function, the GFCI cannot detect a short
from blue to green, or red to green.

The role of the transformer is rather similar:
6) An isolation transformer provides near-perfect safety in case of an
inadvertent connection from red to green. This is a short, but it is not a
short circuit; there is no complete circuit and no current flows through
the inadvertent connection.
7) Similarly, an isolation transformer provides near-perfect protection
for a single fault connecting blue to green.
8) IN NO CASE does the transformer provide any protection in the case of
a single fault shorting blue to red. You are relying on something upstream
of the primary to trip in case of overload. Again, in this case there is
more than enough power to cause great harm.
9) The worst case is when you have a double fault: there is a short
from red to one part of the chassis, and from blue to another part. If you
grab those two parts, you get well and truly zapped.
10) You might be tempted to assume that a double fault is vanishingly
improbable. But don't be too sure. The problem is that the first fault
might go unnoticed for a very long time. The first sign of trouble comes
when the second fault occurs, which is an ugly scenario. Ideally you would
have a GFCI on the secondary, with some additional trickery to detect the
first fault.



I know from experience that a GFCI rated for 110 V releases all the magic
smoke* that makes it work when trying to trip a 220 circuit.

Hee hee.

Second, GFCI circuit breakers are expensive, but they can be cheaper than
rewiring every outlet, especially if the boxes are too small to
acommodate the larger GFCI.

A) In my house, I put all the GFCIs are in the breaker box in the garage.

B) In my parents' house, that option wasn't available, so I used the GFCIs
that are integrated with a socket.

Note: Even if you decide to go with plan B, you don't need to upgrade
every old socket with a new integrated GFCI socket. It turns out that the
integrated sockets allow you to wire other (plain) sockets to the protected
side of the GFCI module. So if you upgrade one or two strategically-chosen
outlets, you can get lots of protection with disproportionately little work.

=============================

Bottom line:
*) GFCIs are not magic. There are some things they can do, and some
they cannot.
*) Isolation transformers are not magic either.
*) When in doubt, draw the circuit diagram and ponder it.