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Old 29th Dec 2018, 12:54 pm   #7
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,241
Default Re: Variac first or Isolation first ?

It's not clear-cut.

Suppose you go TX -> Variac

On the assumption you are using the isolation TX in the usual workshop way - that is, to allow the connection of earthed test equipment to the "live" parts of a DUT - then you will not be connecting through the earth connection. The reasons for that have been covered here many times in the past.

But as a result, the metal body of the variac will now be floating. Or will it? What happens when you connect your earthed test equipment to some point of the DUT? Or suppose there is a fault in the variac or the DUT?

It all feels rather risky to me. When I do this - which I admit to doing fairly often - I'm always very aware that something could go wrong with this, and I avoid touching the metalwork of my variac. Easy to do, as it's mounted on a lump of timber. Or I actively ground it when I know that the DUT is definitely Class 2.

OK, suppose you go Variac -> TX

This is better because it provides soft-start for the TX and allows the body of the variac to be correctly earthed, but what happens if you wind the variac up past 230V? The core of the transformer will saturate, causing buzzing and overheating or worse. Unless you pay extra for a low-flux design, all standard-grade transformers made today are designed to be on the limit in normal operation - copper and iron has never been more expensive - and don't like being over-run.

Anyway, I've arrived at the conclusion that there is no 100% safe way to reliably run a variac and isolation transformer as two separate boxes, and have been planning to rebuild my isolation transformer and variac into a nice metal case that combines both functions safely. I have the box and everything else I need (including some nice metering) - I just haven't had the time to do it.
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