Re: Debate on Isolation Transformers

Dave Casey

You should be aware that GFCI will not protect you on the secondary side of
an isolation transformer, or any transformer for that matter if the
secondary is not referenced to earth ground. It may not trip even if your
secondary IS referenced to earth ground.

Dave Casey

On Tue, Mar 14, 2017 at 2:36 AM, Jan Bottorff jcbottorff@...
[TekScopes] <TekScopes@...> wrote:

So, oh wise Tek wizards, who among you dreams about isolation transformer
grounding in your sleep?

Not me, but being aware of the issues was enough to make me fork out the
$300 for a differential scope probe (battery powered) that’s good up to
1400V. I now feel free to probe power supplies and such with little concern
about what lead is connected to what.

I also tracked down a USB isolator, as I was reading about how connecting
even a battery powered device with a USB port to a laptop computer could
make things ground referenced. USB probably has no danger issues, but
signal integrity and equipment damage are issues.

I suppose this doesn’t help keep ME safe from devices that do bad things.
One possible strategy for questionable devices might be to plug it into a
computer UPS running on battery power (disconnected from the outlet). I’d
have to research if this is a good strategy or not.

I also do have the outlets in my home lab protected with a combination
AFCI/GFCI (which now come in outlet form), so it should not be possible to
get a shock between ground and the hot or neutral power leads. The GFCI
disconnects if the current flow between the hot and neutral are not
balanced, like some is flowing to ground. If I should ever plug in some
device and it immediately trips the GFCI, I will consider that a warning to
perhaps ban the device from my lab.

A few years ago I also forked out $100 for a device (made by Amprobe) to
measure GFCIs timing and trip current, and things like the impedance of
different legs of power outlets. I was unhappy to find a few of the outlets
in my 30 year old home had a high impedance on the ground leg, so if there
ever was a short to ground, the current might not trip the breaker. That
got fixed immediately. I believe the device basically generates a short
circuit to different legs for a very brief moment, and measures the current
or perhaps something about the waveform, and can display what the impedance
is and the short circuit maximum current. I feel safer knowing the ground
pin really is solidly connected to ground and if needed can shunt a few
hundred amps to ground. It’s probably good for MOV surge suppressors to
have a good ground too.

It would be useful to understand if there are still cases were an
isolation transformer might provide safety above what I currently have.


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