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Old 15th April 2011, 07:19 PM   #1
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Default Help me ground my Variac

I just completed my first build: a Grounded Grid Pre. I'm pretty stoked about it and ready to fire it up for the first time.

I purchased a vintage Power Stat from ebay and it appears to be in good shape. It has a 3 prong line cord, but a 2 prong output. I have partially opened the case and verified that the input is attached to a grounding lug, I now need to ground the output.

I am considering using a 3 prong cheater plug with the ground tab screwed to the case. This screw would then be attached to the input grounding lug using a ring terminal, a length of wire and some solder.

Q's

Is this approach correct?
Is it safe?

Thanks for helping a noob. Happy building....
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Old 15th April 2011, 07:26 PM   #2
AndrewT is offline AndrewT  Scotland
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NO !
Both outputs of the Variac must be treated as if they were Live.

The earth of the new equipment must be connected to PE in the supply to the Variac.
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Old 15th April 2011, 07:35 PM   #3
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Ok, I must not have been clear.

The grounding tab on the cheater plug would be connected to PE via a screw through the case. The screw would then have a ring terminal attached to it (on the inside of the case), with a wire connecting to the input ground (PE).

AndrewT: Thanks for the reply. I've learned a lot from your posts.
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Old 15th April 2011, 09:52 PM   #4
AuroraB is offline AuroraB  Norway
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Unless it is explicitly termed as an isolation transformer and variac, most variacs are simple autotransformers. If it's wired correctly, and used in a TN circuit (with live/neutral/ground), the cold end of the variac will be connecteed to neutral, which in turn should be connected to ground at the main fuse cabinet. It is correct, though, that neutral should also be concidered active, i.e. not as a ground. It is still acceptabel to feed the ground through, as described, but only if ground is required, and the intention of the variac is to provide a variable output voltage, and not isolation.
If the variac has an isolated input winding, the ground loop is somewhat more questionable, as the whole idea of isolation, is exactly as decribed - full isolation. Iso-variacs like this are/were common in repair labs, particularly with old TVs, where the chassis was connected to neutral ( or one phase for balanced 220V IT grids ). Isolation may still be a good idea in a lab....
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Last edited by AuroraB; 15th April 2011 at 09:55 PM.
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Old 15th April 2011, 11:37 PM   #5
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The case of the Powerstat should be grounded via the third wire of the line cord. Anything you power with it that also needs ground should also be tied through the case to ground. If I understand you correctly it sounds like you're going to do it correctly.

Neutral and ground connect at the breaker box, but the two shall never be connected downstream from that point. The Powerstat (and Variacs) provide no isolation at all, so IMHO it's even more important to continue ground protection through the Powerstat housing to the DUT. No floating chassis'.
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Old 16th April 2011, 02:30 PM   #6
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Thanks for the reply's. I think I'm actually getting somewhere!
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