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Current transformer rating

Richard Henderson
I wish to use a curren transfomer with a 10A primary and 5A secondary. The circuit is running at 3kV. The transfomer I whs to use is rated at 0.72/3kV. I feel the insulation is irrelevant as I will be passing the cable (with 10kV rated insulation intact) through the centre of the transformer and making no connection whatsoever (other than via a magnetic field which is proportional to current NOT voltage).


However, the manufacturer says that the transformer can only have up to 720V passed through it.


I think the person on th end of the phone is reading from a datasheet but doesn't actually understand it. That said, I am not familiar with CTs, so could be completely wrong.


Any ideas what the facts are here?
  • 0
    The manufacturer of the current ransformer has to assume that it could be used on a uninsulated cable. I guess that is the basis for the apparently conservative voltage rating.


    Ian
  • 0

    Ian MacDiarmid:

    The manufacturer of the current ransformer has to assume that it could be used on a uninsulated cable. I guess that is the basis for the apparently conservative voltage rating.


    Ian 




    Thanks Ian, that was the conclusion I'd come to so I've gone ahead with them.

     

  • Former Community Member
    0 Former Community Member
    Hi, I've only just picked this up, and you may already have found the right answer, but I would say you've got the wrong bit of kit. Looks like you are wanting a slipover CT, but in that case, not only is the insulation level determined by the existing equipment, but so is the current rating of the primary circuit. This type of CT is defined by the current rating of the secondary, and the number of turns of the winding. The primary is normally one turn (straight through) or if a flexible enough cable, you can wind it round a few times and work out what you are going to get.The primary current is normally not given, but for accuracy, you would use the turns ratio and aim to get close to the rating of the secondary.


    As you have given a current rating for the primary, this implies to me that the primary is wound, in which case both current rating and insulation level is definitely important. If you have got these figures from a rating plate, I would suggest there are either 2 secondary windings, or it is an interposing transformer. Or the figure could actually refer to the burden of the CT, which is usually given in VA. 10VA would be quite common for a metering CT with a 5A secondary.


    Hope this helps

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