Author Topic: Variac Isolation  (Read 273 times)

Offline abstruse1

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Variac Isolation
« on: April 26, 2022, 04:54:15 AM »
I have a 10 A. Variac.  To properly isolate it from mains power/ground, do I need an isolation transformer, or can I just not connect the 3rd pin (ground) to the mains plug?

An adequate torodial isolation transformer (I assume this kind will work) with adequate capacity sells on eBay for $225.
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Offline Mads Barnkob

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Re: Variac Isolation
« Reply #1 on: April 26, 2022, 08:29:05 AM »
I made a video about the differences on variacs, auto transformers and isolation transformers

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Offline abstruse1

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Re: Variac Isolation
« Reply #2 on: April 28, 2022, 04:03:41 AM »
Thanx, Mads.  Good video and great collection of variacs.

But it doesn't answer my question, can merely cutting off the 3rd (ground) pin from my Variac is adequate to prevent safety mishaps, or do I need an isolation transformer?

Thanx.
Abstruse1
   Been working with electricity for 60 years and still getting a tingle!

Offline Hydron

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Re: Variac Isolation
« Reply #3 on: April 28, 2022, 07:51:22 AM »
I think there is a more fundamental question - what "safety" do you think you're gaining with an isolation transformer? They are useful tools in certain circumstances, but sometimes can actually make safety worse, e.g. when they end up defeating RCD protection.

And to actually answer the question, the Variac offers no isolation and cutting off the ground pin is almost never the answer.

Offline klugesmith

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Re: Variac Isolation
« Reply #4 on: April 28, 2022, 08:15:44 AM »
The usual meaning of "isolated output" is high or infinite resistance between the output terminal pair and the input terminal pair.
Safety benefit: with one hand on any output terminal, and other hand on any grounded object, you won't get a shock.
You can run a wire from any output terminal to any grounded object, and not get a spark or blow a fuse.

Disconnecting the plug's ground pin does not provide that isolation.
It would remove one kind of shock hazard: touching a hot wire and the variac case at the same time.
It would add a different kind of shock hazard, that's more sneaky: Accidental contact between a hot wire and case.   
If the case is properly grounded, a fuse would blow and whole circuit would turn off.   If the case is not grounded, the accidental contact with hot wire would make the case "live" and dangerous to touch.
« Last Edit: April 28, 2022, 09:16:15 AM by klugesmith »

Offline Hydron

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Re: Variac Isolation
« Reply #5 on: April 28, 2022, 09:20:00 AM »
The usual meaning of "isolated output" is high or infinite resistance between the output terminal pair and the input terminal pair.
Safety benefit: with one hand on any output terminal, and other hand on any input terminal or grounded object, you won't get a shock.
You can run a wire from any output terminal to any input terminal or grounded object, and not get a spark or blow a fuse.
Yet if you end up grounding the "isolated" side anywhere e.g. via a scope ground lead, then you're back to the safety situation of the non-isolated case, except worse as a switchboard RCD won't help you due to the transformer in the way.

There are totally cases where it does make sense to use an isolation transformer, and they can increase safety if used appropriately, but the OP's original post and question about removing the earth pin didn't inspire confidence that this would be one of these cases, or that the implications of the transformer are fully understood. My question was really to try to get an idea of what the OP thought they were gaining from the transformer, and how it would be connected/used.

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Re: Variac Isolation
« Reply #5 on: April 28, 2022, 09:20:00 AM »

 


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