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

Question

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!~

mapj1 · Accepted Answer

I am hoping I have understood your question. If not, please come back and correct me. 
 This is what I think is the situation. The loop impedance is > 0.53 ohms, so (assuming it is a 230V system in your place ?) the maximum fault current would be something rather less than ~ 500A . A 63 A breaker will trip on this, but not always instantly. Depending in the actual fault current it it may take several seconds. But the 400A MCCB has some sort of earth fault detection. 
 What is the tripping current of that earth fault detector ?( not the 400A but the earth current) 
 When it is tripped is it instant or is there a delay ? - Sometimes earth fault relays have a delay so that faults on smaller circuits have time to operate their own trips without cutting out the whole installation. 
 If I understand it right, then yes the MCCB will trip for earth faults above its tripping limit, that do not cause something else to trip first. This may be an example of a good economic design, or it may be very unwise, that all depends on what else loses power when the big trip operates and how important it is that the rest of the installation has power. 
 Do come back with more info if you can. 
 Mike

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

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