Power supply for testing: Variac or isolation transformer first?

[Guineafowl]

TL;DR

As title.

This is for safely powering up devices being tested/repaired. Supply is 240V rms single phase, protected by 32A MCB and 30 mA RCD/GFCI, then fused at 13A.

There doesn’t seem to be a consensus online, and my EE and repair books just say I need both, without specifying order.

I think the isolation tx should go first, since it is designed to input/output 240V, and that is exactly what it’ll be doing. That way, the variac will take in 240V and output a variable voltage, again as per design.

If you put the variac first, the isolation transformer will be given a variable input voltage, from 0-270V.

Perhaps someone could confirm if I’m right, wrong, or it doesn’t matter. Mains Earth will of course be taken straight through without interruption.

 

[berkeman]

I would put the Iso transformer first. The 240Vac mains has a tolerance on it (10%?), so if you put the Variac first, you could get more than 270V into the Iso. (It still would probably be fine.)

BTW, 13A at 240V is a mongo Iso transformer, no? How big is that thing?

 

[essenmein]

In a past life working on large telco SMPS, the iso transformer was first, then variac, I think from memory it was that way because the variac could provide somewhere like +20% or 25% voltage so we could test over voltage operation etc.

 

[berkeman]

Supply is 240V rms single phase, protected by 32A MCB and 30 mA RCD/GFCI, then fused at 13A.

BTW, 13A at 240V is a mongo Iso transformer, no? How big is that thing?

Does your Iso transformer look something like this?

https://www.ato.com/3-kva-isolation-transformer

 

[DaveE]

You can do it either way. I would probably put the transformer first. That way the magnetizing current from the transformer (that's the primary current that flows in addition to the currents caused by the load) won't have to go through the variac. Although I suppose you could make the same argument about the viriac's magnetizing current too.
As others have said the variac being downstream will keep the transformer from seeing the extra 10-20% over-voltage when the variac is turned all the way up (if it does that). However, I' sure the transformer would be ok, it will just get a little hotter.
If either is rated for much larger power than the other, I would probably put that one closer to the source.

 

[Guineafowl]

Thanks all. Sounds like iso first.

I would put the Iso transformer first. The 240Vac mains has a tolerance on it (10%?), so if you put the Variac first, you could get more than 270V into the Iso. (It still would probably be fine.)

BTW, 13A at 240V is a mongo Iso transformer, no? How big is that thing?

Ah, no - the supply is UK standard, but the isolation transformer is a rewound battery charger transformer, probably 300 VA or so. It’s just for powering up devices for testing, so I can scope them and flip the boards around without too much danger.

This and the variac (which is 2.4 kVA) will be used as part of my test supply panel:

Master switch turns everything off; second one toggles mains/isolated, and isolation tx can be turned off; next section allows a series bulb or complete bypass (‘missile’ cover so I don’t accidentally...); next three switches add in parallel bulbs to allow more current through in limit mode.

Finally, a decent socket with DP switch and a kill-a-watt type meter so I can measure what’s going on.

This will be for careful powering on of valve(tube) radios, and general solid-state boards with faults. This panel has been tested to UK standard:
Continuity/polarity.
Earth bond (send 30A through each Earth path (including switch bodies) to check integrity).
Insulation resistance (500Vdc between L+N and E).

Hopefully a useful addition to the bench!

 

[berkeman]

Looks like a useful setup!

probably 300 VA or so

In that case, I'd probably downsize the fuse for your setup...

then fused at 13A