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Old 27th Jan 2023, 1:18 pm   #45
TonyDuell
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Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,190
Default Re: Isolator Transformer Query

Quote:
Originally Posted by stuarth View Post
For the case of the "testing machine" described above, isn't the transformer part of the installation? For transformers for specific applications, there are many ways you might want to connect an earth, and hopefully the designer of such equipment understands the issues and designs accordingly.
Agreed. After all the mains transformer in a valve audio amplifier is an isolating tranformer and it is common to earth the centre-tap of the HT secondary winding.

Quote:
Originally Posted by stuarth View Post

On a closely related subject, the same argument could/should be applied to the earth pin of the output socket on the general purpose isolating transformer, it should not have a fixed earth either. This is to guard against the very dangerous crossed N-E connection mentioned on this forum many times.
I've seen plenty of mains leads with L-N swaps, but never E swapped with anything else. It can happen, I guess, but it is not common. Also, I would recommend checking (visually and electrically) the wiring of anything before you power it up, even if you are going to use an isolating transformer. I always make sure the earth wire connects to what I expect, for example

If you have a mains filter with the traditional delta-connected capacitors in the device under test and are connecting an earth to some point in the 'mains' circuit of that device (e.g. by connecting a 'scope to it) then I feel it's a good idea to have the earth pin it connected to true earth. Otherwise you might get enough current flowing through said capacitors to give you a tingle, and more importantly enough current to do damage to circuitry on the output side if some point there happens to end up earthed.

Yes you can use a clip lead on the chassis to do this. But I find it easier to use the earth wire of the mains cable, it won't fall off at the wrong moment and touch something else.

My homebrew isolating transformer has a pair of 4mm sockets on the front of the box. One is connected to mains earth (via the input mains cable).The other is connected to the earth pin of the output socket. There's a link I can insert or remove as required.
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