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Re: Variac Safety



Howdy-



On Fri, 1 Dec 2000, John M. Clement wrote:

An excellent solution for the older lab is to have a portable GFCI outlet.

I have two questions/comments. First, say a series of equipment is wired
like this:

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

If there is a ground fault in the equipment, will the GFCI trip the
circuit? It doesn't in my room and it makes me nervous, but maybe the GFCI
is faulty. I should get a 220 GFCI, but they are expensive, and 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.

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

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.

They also can make the classroom really weird when you get to static
electricity. I live in New Orleans right now, but I used to work in
Houston in a school with GFCI circuit breakers. I was overly fond of the
Tesla Coil and used it every chance I could get. One day I decided to zap
the chalkboard, and blammo, every light went out.

Not just my light. No, the lights for many teachers went black.

What happened is that the wires for the overhead lights are not in Romex
plastic coated sheathing, but are in metal (armored) conduit where the
conduit itself is the ground. The conduit is strapped to the building's
internal metal frame with plummers' metal strapping tape. The chalkboard
is also attached to the metal frame of the building.

The tesla coil discharged into the board which discharged into the conduit
which ran into the GFCI. I am told that this isn't standard practice, but
in this system, in addition to monitoring the difference in current
between the hot and neutral wires, the GFCI monitored the current in the
ground wire. If there was any current in it, it killed the circuit. And
due to the tesla coil, there was.

The tesla coil sent current through many of these breakers until the
current was divided enough ways that there wasn't enough left to set off
any more breakers.

This incident helped to cement my reputation as being the most dangerous
(not too kids, though) teacher at the school.


---

Marc Kossover
marck9@mail.idt.net