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Old 19th May 2012, 04:19 AM   #1
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Default Isolation Transformer

Don't be shocked, but I have an ancient Stancor P-6415 isolation transformer that I really should be using on the bench. It has a 2 wire, ungrounded plug. Should I add a grounded cord? If so, would the ground wire connect to the case?
And does it make a difference whether a Variac is plugged into it or vice-versa?
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Old 19th May 2012, 07:29 AM   #2
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the whole purpose of an isolation transformer is so that you can "float" whatever you're working on.

you should be able to grab either side of the 120vac line after the isolation transformer and NOT get shocked.

if you ground the output of an isolation transformer it pretty much defeats the purpose of having one.
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Old 19th May 2012, 01:16 PM   #3
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Of course. My question is for the input side of the transformer. I've seen newer versions that are grounded and I want to know if this is necessary.
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Old 19th May 2012, 02:26 PM   #4
AndrewT is offline AndrewT  Scotland
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The Earth connection is needed to blow the mains fuse if the transformer or it's connections become faulty.


If you can "see" that the isolation transformer is not faulty and cannot send mains voltage downstream to the following equipment then one does not need the Earth connection.

BUT,
that is a big risk if something goes wrong that you can't see !
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Old 19th May 2012, 02:47 PM   #5
jitter is offline jitter  Netherlands
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Grounding, or better: connecting protective earth (PE) (aka: safety earth) to the case of the transformer would not change the fact that the output is isolated from the input. It's only the mains part (L and N) that needs to be isolated and of course PE would only be connected to casings. Hence the reason why many isolation transformers have PE available on both in- and outputs.

This wiki article states that a grounded elctrostatic shield inside an isolation transformer prevents capacitive coupling between primary and secondary windings.
The schematics symbol for isolation transformer should also give a clue (first image).
Look at page 22 for type e of this pdf. Your transformer has an electrostatic shield which, according to the document, is "grounded to the core internally".

So yes, I would connect the case of the transformer to PE.

If your variac is an autotransformer, it is not isolated (second image) and I would use it behind the isolation transformer. If your variac is truly isolated (look for the two circle image on the case, similar to the third image), then the isolation transformer would not be needed on that variac.

Edit: if you're unsure if the variac has fully isolated windings, always assume for safety reasons that it hasn't.
Attached Images
File Type: gif Isolation transformer.gif (1.9 KB, 220 views)
File Type: gif Autotransformer2.gif (1.3 KB, 226 views)
File Type: gif Variable voltage transformer.gif (1.6 KB, 227 views)

Last edited by jitter; 19th May 2012 at 02:56 PM.
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Old 19th May 2012, 03:05 PM   #6
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The Primary or Input side should be grounded.

The Secondary or Output side must NOT be grounded.
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Old 19th May 2012, 04:26 PM   #7
jitter is offline jitter  Netherlands
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Agreed for testing purposes the output of the isolation transformer should be left floating. But beware, simultaneously touching live (L) and neutral (N) wires at the isolated end of the transformer remains just as dangerous! Only the (more common) mistake of touching L and e.g. case is not dangerous anymore.

Having said that, there are situations in which it is necessary to make an isolated supply act as a normal one. PE and N are interconnected on the secondary side (e.g. on a boat that gets an isolated power supply from shore to prevent electrolytic corrosion, see image). PE on primare side and secondary side are NOT interconnected, though. I didn't make that clear in my first post.

But for testing purposes, only use L and N to power a device, do not connect PE!
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Old 19th May 2012, 09:30 PM   #8
cihtog is offline cihtog  United States
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Having or not having a third ground wire is irrelevant to the operation of the isolation transformer, as far as isolating goes. Newer models have the ground connected to a shield within the unit, to head off any capacitive coupling between the primary and secondary windings. If the windings in yours were to fail badly enough that they would be exposed to the case, then the current would have most likely blown the main breaker anyway - as the neutral (white wire) on the primary side AC line is already connected to earth/ground, at the breaker box.

If it makes you feel better to add a 3-wire cord and connect it to the case, go right ahead. But I would be more worried about the safety of old iron/copper in general - as this unit may not have an electrostatic shield built-it, or may have been modified by someone else, that felt it more bothersome to change the outlets in their house to 3-prong.

In reference to the ground passing all the way through the unit - this defeats nothing, as most AC transformers (with the exception of variacs) are "isolation" transformers. They just step up or step down, but the ground is still there, and technically bypasses the TF altogether. If it were me, I'd at least pop the covers off and take a peak, and secondly, it's your safety in question - when in doubt, throw it out.

As far as the variac goes, I haven't used mine since discovering the "dim bulb". Most will say it does not matter, but I prefer mine connected after the iso - just because the only thing on the primary side, is the primary itself - every switch, button, knob, and dial after that, is isolated.

Does yours have a single cord connected to the secondary, or 3 outlets on the case? i.e. 110, 115, 125...

Last edited by cihtog; 19th May 2012 at 09:38 PM.
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Old 19th May 2012, 09:44 PM   #9
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by cihtog View Post
- as this unit may not have an electrostatic shield built-it
It does (see post #5).
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Old 19th May 2012, 10:16 PM   #10
cihtog is offline cihtog  United States
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Quote:
Originally Posted by jitter View Post
It does (see post #5).
OK... then he still should be able to connect it to a grounded 3-wire cord, as well as 3-wire secondary, but it should already have 3 taps on the case.

Last edited by cihtog; 19th May 2012 at 10:29 PM.
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