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PSC higher than MCB breaking capacity

E95

Hi all,

Looking for a practical answer here;

I've read a couple of threads on this but to no clear solution:

  • MCB rated at 3kA breaking capacity
  • 7kA recorded
  • Upstream is a BS88 315A fuse

434.5.2 effectively says something around you should consider the supply side protective devices without giving any further advice (that I understand at least)

After reading discussions on here it seems that the downstream MCB will only be damaged if it tries to operate whilst the fault is present.

Am I correct that to prove this I need to find the worst case operating times at 3kA for the fuses and that should be bigger than the best case operating time for the MCB for this to be safe?

Regards,

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  • 0

    I wonder where the 7KA came from. I doubt you could have confidence in any of the standard MFTs giving an accurate reading at those relatively  low levels of impedance (resistance really).

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  • 0

    I wonder where the 7KA came from. I doubt you could have confidence in any of the standard MFTs giving an accurate reading at those relatively  low levels of impedance (resistance really).

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