[Test-Equipment] Isolation transformer

[Eric Lemmon]

Blair,

First, let's establish what kind of transformer you have.  An
"auto/isolation" transformer cannot exist; either it is an autotransformer
which, by definition has no isolation, or it is an isolation transformer
which, by definition cannot be an autotransformer.

I assume it is a true isolation transformer, with completely independent
windings, for this discussion.  The primary winding may be 120 VAC, 208 VAC,
240 VAC, or 480 VAC- depending upon your power source.  An equipment
grounding wire (green wire) should connect the frame of the transformer, and
the Faraday shield if one exists, to the grounding bus in the distribution
panel.

Now, an isolation transformer is considered to be a "separately derived
source" in the National Electrical Code.  One side of the secondary winding
must be grounded at the transformer to one or more available grounding
electrodes as defined in Article 250 of the NEC.  However, if this
transformer is to be portable, then the only available grounding electrode
is the frame and the enclosure which, as noted previously, is connected to
the grounding bus in the panelboard.  And this is exactly what the inspector
wanted.

The grounding method just described is the safe and correct method of
connecting an isolation transformer.  It does not short out anything;
indeed, it prevents the formation of ground loop currents by establishing an
independent (isolated) power source.  If you are looking for an even
"quieter" power source, use an isolation transformer that has a
center-tapped 120 VAC secondary.  The NEC allows such a power system for
installation in testing labs and recording studios (among others) to almost
eliminate 60 Hz hum, since the equipment sees a balanced power source.
Pretty slick.

73, Eric Lemmon WB6FLY


-----Original Message-----
From: test-equipment-bounces at mailman.qth.net
[mailto:test-equipment-bounces at mailman.qth.net] On Behalf Of Rasputin
Novgorod
Sent: Tuesday, January 03, 2006 10:34 PM
To: Test Equipment
Subject: [Test-Equipment] Isolation transformer

I have a small 115vac Isolation transformer that I power
projects with, expecially transformerless ones,
for safety's sake. This is an older transformer that has
been repaired. How do I test it, with an ohm meter,
to know that it is wired correctly and safely? The input
has a three pin plug, the output is a standard three-pin
outlet. I presume:

1) continuity between input and output grounds?
2) no continuity between either output lead to output ground?

I ask because I had an argument with an inspector about
my 15 amp/220vac Sola auto/isolation transformer for the 
whole shop. He wanted all grounds and one side bonded
thru, which seemed to me to short circuit the isolation...

Please don't guess, but if you really know how these things
should be wired, I'd be grateful for direction...

Sincerely
/blair


		
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