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How to electrical test (PAT test) large transformers?

KC

Hello,

I need to PAT test a range of single-phase and three-phase transformers, from 15kVA up to 90kVA. These include both isolation transformers and step-down transformers.

While I have experience with general PAT testing and various PAT tester devices, I'm unsure of the best procedures when it comes to testing larger items like these.

For example, I understand that performing a leakage test using a traditional PAT tester isn’t feasible in these cases. Any guidance or best practices on how to approach testing of this kind, particularly in terms of safety checks, insulation resistance, earth continuity, and functional testing would be greatly appreciated.

Thanks in advance for your help.

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      I presume these are not really used like portable appliances and as you note the normal PA tester with a 13A socket on it won't quite cut it ;-)

    The first thing is to decide what needs testing, what failure modes you hope to pre-empt and then what the test limits should be.

    So for class 1 enclosures, the normal earth/ CPC connection can be verified but it needs to be good enough to operate the circuit fuse or whatever is in the supply - no good having an earth connection that breaks at a lower current than the supply fuse ;-) to that end certainly at the 90KVA end, I'd expect to see earthing of the same cross-section as the live cores, probably 25mm2 assuming that is a 3 phase unit, or rather more if its single phase. testing that with a meter is not really that easy or desirable - and if you do it will require a 4 terminal instrument. A quick buzz test with the lowest ohms range your normal meter can do,  and a thorough visual inspection may be more appropriate. Finding 'high' resistances on the scale of tens of milliohms is usually fraught with difficulty.

    Insulation primary to core/frame and from secondary to both core/frame and primary should be confirmed (except autotransformers and variacs, where primary and secondary are the same coil). Expect several megohms, but don't test at more than 500V DC without checking with the makers. It should do a lot better than that, but how much better is not clear.
    If you can measure the magnetising current without a load connected, and I realise that may not be so easy, where loads are wired in, as any increase in this between successive tests indicates a shorted turn or deterioration of the insulation between laminations.
    M.

  • 0

    Um, firstly are these really portable appliances and not part of the fixed electrical installation? What actually is the scope of work?

    Even if they are portable, to test it properly you'll end up with largely the same methods as fixed installs, so probably best starting your search / reading along those lines rather than trying to squeeze it into PAT. Others with more experience from the tools will no doubt be here soon to fill you in on methodology. I expect you'll need a new test device (or more than one)!

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