PSC higher than MCB breaking capacity

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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  • Gut feel is that 315A fuse is rather large to be directly feeding 3kA MCBs - it would be much more usual to have a lower rating (say 100A or less) in between (what's the rating of the boards the MCBs are in?)

    A 3kA MCB and a 100A fuse with a PFC of 7kA wouldn't worry me greatly - most consumer units are rated for 16kA when supplied by a 100A BS 88-3/BS 1361 fuse after all, even though that's well above the breaking capacity of the MCBs themselves.

    As for proof, you'd likely need the MCB manufacturer's data - they usually quote suitable backup fuse sizes for varying fault currents.

       - Andy.

Reply
  • Gut feel is that 315A fuse is rather large to be directly feeding 3kA MCBs - it would be much more usual to have a lower rating (say 100A or less) in between (what's the rating of the boards the MCBs are in?)

    A 3kA MCB and a 100A fuse with a PFC of 7kA wouldn't worry me greatly - most consumer units are rated for 16kA when supplied by a 100A BS 88-3/BS 1361 fuse after all, even though that's well above the breaking capacity of the MCBs themselves.

    As for proof, you'd likely need the MCB manufacturer's data - they usually quote suitable backup fuse sizes for varying fault currents.

       - Andy.

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