Re: Isolation transformer

On Sat, 5 May 2007 17:40:59 +0100, "Andrew Holme" <andrew@xxxxxxxxxx>
wrote:


Aha. Now I see a glimmer of light. I understood that an IT was merely
a 1:1 transformer. That is, mains voltage in: mains voltage out. So
for the equipment to work, it still must have two wires entering that
are mains potential apart, I would have thought. These would now be
arranged to be + half mains potential and - half mains potential?

Yes, they are mains potential apart. It's not really meaningful to talk of
+/- half mains, because this is only relative to the voltage at the middle
of the secondary winding, which is floating relative to the rest of the
universe, unless you anchor it by grounding one end.

Very little current can flow if you touch one end of the secondary when
the
other end is open-circuit. If you connected the open-circuit end to
ground,
that would be a different story.

So you switch the equipment off after the IT? I understood the aim was
to work on the equipment while running.
So without the IT, the live wire can damage you, even when the neutral
is open circuit, whereas with an IT installed under the same
circumstances, the "live wire" out of the IT is almost impotent?

Yes; however, if you connect a ground to the isolated radio/TV (e.g. through
a 'scope probe) any live points in the circuit are just as dangerous as raw
mains; but the earthed chassis, which is the thing your hand is most likely
to touch, is safe.

In summary: you're safer with an isolating transformer; but not necessarily
safe.


Yeah, thanks, Andrew. It's starting to sink in, and I will be a full
bottle on the subject in a few days when I've absorbed all the great
advice I've received here. Pity about the abusive clown, but I suppose
every group has to have one. I've copied a bunch of his messages
including headers to his ISP and his news account. Perhaps they can
show him the error of his ways. Ever in hope, jack
.

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