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Old 28th Jan 2023, 6:04 pm   #53
Lucien Nunes
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Join Date: Apr 2008
Location: London, UK.
Posts: 2,508
Default Re: Isolator Transformer Query

Inter-winding breakdown of that kind ought to be vanishingly rare in a panel transformer. It's a reminder that when using a transformer to provide an electrically separated supply, it is not really sufficient simply to know that the primary and secondary can withstand an insulation test between them. The whole thing should be made to a recognised standard, e.g. BS3535, which will impose minimum standards for many parameters of the insulation.

This thread is mainly about 230-230V transformers for bench use in maintenance work, but the requirements for isolation are common to ELV applications too. In @Roger Ramjet's machine, the ELV side appears to have been what we would now consider SELV, so the transformer would need to be made as a Safety Isolating Transformer to the above standard. By converting the output to PELV, some of the requirements are relaxed.

Did you find the root cause of the insulation breakdown? Some USA-made transformers run very much on the border of saturation on 50Hz and if the voltage is also high (e.g. when 208V equipment is supplied from a 230V stepdown) might get undesirably hot.
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