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Isolation transformer testing

 
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Isolation transformer testing

Post by Alistair D » Wed May 11, 2016 12:27 pm

While PATing a couple of isolation transformers recently I realised that as well as testing the earth continuity from the mains plug through to the output socket earth pin I should also be doing an insulation resistance test between the primary and secondary windings.

The attached file shows a lead I knocked up to simplify both the ECR and IR tests.

Please excuse the fact that the drawing shows US mains plugs and sockets. I have yet found or drawn TinyCad symbols for the UK versions

Al
Attachments
PAT_TX_test.png

 
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Re: Isolation transformer testing

Post by Cathovisor » Wed May 11, 2016 1:08 pm

Interesting; BBC practice was that the output earth pin on an isolating transformer was always disconnected, and the earth terminal made available separately. I think it was covered in a BBC Engineering Guidance Note as to why, but one reason not connected with safety was to separate mains and programme earths to break potential hum loops, especially on sound equipment.

I think the same practice was also carried out by the GPO/BT, as I have a 250VA transformer ex- the latter, which carries a large label instructing the user to make sure "the oscilloscope frame is adequately earthed."

 
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Re: Isolation transformer testing

Post by Alistair D » Wed May 11, 2016 2:09 pm

I have read(and may still have a copy) of that BBC engineering note, however, every newly purchased transformer I have seen always came with the output earth connected. I did read somewhere the part about disconnecting the earth and labelling it as such. Alas now I have no idea where.

Al

 
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Re: Isolation transformer testing

Post by Cathovisor » Wed May 11, 2016 2:33 pm

From memory, it was BBC EGN 4. Certainly when I've bought isolation transformers when working for Auntie I had to do the separate earth mod upon receipt: it's clearly a BBC-ism.

 
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Re: Isolation transformer testing

Post by Mark Hennessy » Wed May 11, 2016 4:41 pm

Yes, it's EGN 4 - you can get it from here: https://www.dropbox.com/s/56mw3oxzltqsc ... r.pdf?dl=0

(PDFs aren't allowed, but feel free to grab it and archive is somewhere)

Lifting of outlet earth connections might well be a BBC-ism, but it's for good reason (N-E swap on DUT - see Appendix 1). Why no-one else follows suite is the real mystery...

 
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Re: Isolation transformer testing

Post by Alistair D » Wed May 11, 2016 10:41 pm

I get the logic of Appendix 1. I have to say though that in 50 years of opening plugs I have never seen one with the earth and neutral leads swapped. I have come across live and neutral swapped and a 2 core lead wired between L and E.

One drawback of the BBC system is that not all equipment has somewhere to securely connect an earth lead to. A croc clip attached to the body of a BNC socket does not qualify as secure.

I have never seen a commercially made transformer with a separate earth connector so it would seem that transformer manufacturers did not give credence to this alternative earthing arrangement. I realise I could be wrong here and such transformers do exist. Remember I am only talking transformers fitted with 13A plugs and sockets.

Al

 
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Re: Isolation transformer testing

Post by Mark Hennessy » Sat May 14, 2016 11:02 am

Alistair D wrote:I get the logic of Appendix 1. I have to say though that in 50 years of opening plugs I have never seen one with the earth and neutral leads swapped.


I have :bba


Alistair D wrote:One drawback of the BBC system is that not all equipment has somewhere to securely connect an earth lead to. A croc clip attached to the body of a BNC socket does not qualify as secure.


Well, remembering that we're talking about bench maintenance, where the DUT will have covers removed, and will be in the care of a qualified engineer, so this is not an issue because suitable internal earthing points will be available. But when floating a DUT, it's done solely for the purposes of connecting test equipment, which will usually be earthed - the safety earth aspect is quite separate - indeed, once connected via the transformer, no current will flow into the earth anyway. In this context, calling them "safety isolating transformers" is a bit of a misnomer.

BTW, a non-isolating bench outlet with RCD protection is the default for bench maintenance; isolation transformers are only ever used in the very small number of cases when you do need to connect test gear to a live "earth" - such as the live side of a SMPSU. This is often at odds with people in the TV repair trade, who default to isolation transformers by default, and often make the mistake of connecting more than one DUT to the same transformer, or who even have the test gear on the same transformer! As many previous conversations on many different forums has amply demonstrated, even the best engineers seem to forget first principles when it comes to isolation transformers!

The other BBC use is performing artists, who bring in amplifiers, etc, in the most shocking states of repair. In theory, a PAT test should be done, but that would bring a halt to many/most performances. Even just taking the cover off the plug top to check the fuse isn't wrapped in foil gets into risky/litigious territory. So an isolation transformer with a circuit breaker and no earth connection carried through removes all risk while they are on BBC premises.

Outside of the workshop, other "isolation" transformers are available for other uses, such as site transformers (55-0-55). Someone I know uses (used, hopefully!) one that is 115-0-115, and it was obviously designed to reduce the magnitude of a shock. However, he'd lifted the earth, but maintained the secondary's centre-tap through to the earth pins of the outlets - which offered some "interesting" scenarios :bba


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