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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,

Parents

0 Alan Capon over 2 years ago

We do need to know some more details about the upstream fuse. You say BS88, but there are many parts to that standard. We need to know which part of BS88 the fuse complies with. The issue is also likely to affect a fault in the first few meters of cable, as its resistance will lower the fault current. That said, 10KA mcbs are readily available.

Regards,

Alan.
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0 Alan Capon over 2 years ago

We do need to know some more details about the upstream fuse. You say BS88, but there are many parts to that standard. We need to know which part of BS88 the fuse complies with. The issue is also likely to affect a fault in the first few meters of cable, as its resistance will lower the fault current. That said, 10KA mcbs are readily available.

Regards,

Alan.
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PSC higher than MCB breaking capacity

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