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The Main source earth loop impedance is high(with earth fault IDMT) , will the sub-main switch protected by the main switch?

simonNg

Hello there, I have a question about installation new 63A MCCB when the existing main source earth loop impedance is over the requirement 0.53Ohm. 

I am wondering if the new switch may be able to be protected by the building main switch 400A MCCB with IDMT Earth Fault protection relay.

Sorry for my clumsy English, I have also attached an image for a better explanation. 

Cheers!~

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  • 0

    Ok, now the more difficult bit, which Mike mentioned. Are you happy with the likely disconnection times? You may be but a better design would be to use a 63A MCCB also with an Earth trip, and adjustable overload and Earth trip times. Disconnecting the whole installation because of a 40A Earth fault is surely very inconvenient, and the potential disconnection of a L-N fault is rather uncertain? What features do the other MCCBs have, because the design overall is looking a bit dodgy? Why is the 400A Earth loop so high? If it is a measured value I suggest you try another meter more suited to high current tests. This needs more work!

  • 0 in reply to David Stone

    I understand that earth-loop is bad, this is an existing design for one of the old buildings which my coworker is taking part in replacing the 63A MCCB work, and that 400A actually wasn't included in the work. And yes, we might redo a high current test again and tinkle a bit of the earth pit grounding, hoping to get the best result .

    I will do some more research about it, and thank you for your comment:)

  • 0 in reply to simonNg

    " earth pit grounding"

    Aha! is this installation TT ?

    That is to say there is no N-E bond at the consumer premises and  no earth wire back to the transformer ?. If so you are doing well to get a half ohm earth resistance at all. In that case a cascaded RCD approach with progressively tighter levels of earth leakage protection as the you get nearer the final loads is in order.
    Mike

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  • 0 in reply to simonNg

    " earth pit grounding"

    Aha! is this installation TT ?

    That is to say there is no N-E bond at the consumer premises and  no earth wire back to the transformer ?. If so you are doing well to get a half ohm earth resistance at all. In that case a cascaded RCD approach with progressively tighter levels of earth leakage protection as the you get nearer the final loads is in order.
    Mike

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