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Current Transformers

Former Community Member
Former Community Member
Hi,


Please can someone help settle an argument, we have 3 current transformer fitted on the incoming 3 phase supply at work. the current transformers are reading L1- 12A, L2 - 7.4A and L3 - 5.6A. I'm saying that all current transformers need earthing regardless of the current the transformers are reading. And the other electrical engineer is saying it only needs earthing when high current is being read. Who is right and who is wrong.


Andrew
  • 0
    Hi Andrew,


    Is it possible you are both wrong? I was brought up that you don't earth current transformers, though I am dredging my memory to try and remember the reasoning. If I remember rightly, if you have a broken wire on a current transformer circuit you can end up with a significant voltage from the CT. If one side is earthed then the other terminal will have a dangerous touch voltage while if there is no earth connection it is only dangerous if someone takes hold of both terminals simultaneously.


    I am sure that someone will be able to refute me/give chapter and verse on what should be done when and why.


    Alasdair
  • 0
    Which bit of the transformer is anyone  wanting to connect to earth, and why ?

    You may like the cores earthed if the bus-brs they are measuring are only single insulated, but if around something like meter tail maybe not.


    Are the 3 secondary circuits interlinked in star or delta? If they are there are very good reasons for being extra careful about how that is earthed if it is at all - it may be better floating, and indeed a lot  of metering designs assume it is.

    for things like the VIGI series of earth fault relays, you are supposed to let the pick up coils float, the sensing is all done relative to an internal reference


    What level of current is being earthed is not relevant - the makers instructions on the CT and for the kit which it is feeding into are more important.

    If it is a home rolled  resistor and meter movement, then you may decide to earth it, but if so just in one place though.


    M.

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