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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
  • Ah  it is more complex.

    If (and only if) there is a silver stake fault very close to the MCB, so that there is a fault current greater than 3kA it is possible that the MCB may fail. Now without info we do not usually have, we do not know how,  - maybe  it will work anyway,  maybe it will open and never close again, maybe it will weld shut and never open, or maybe great jets of flame will emerge from the sides and seek out anyone nearby with a beard. We just do not know, just it is out of spec. But in a suitable enclosure, all of these possible outcomes may be  contained, and considered as "safe" potential outcomes (though some are inconvenient as you need a new MCB) so long as the fuse operates promptly before anything else, like say the wiring, catches light.

    Now that is not the normal adiabatic limit, that is about damaging PVC insulation so the cable life is shortened, but actually the real upper bound of instant cable damage, you can accept if you do not mind replacing the cable, is much higher,

    Now really you should look at the fuse data and what they do in the range 3000-7000A. I imagine that there will be a fairily constant I2t, but check.

    This represents the worst let through energy that the cables downstream may see - even if the MCB failed to open at all, and can be compared with cable ratings.

    The MCB I suspect stops getting faster at higher fault currents due to the mechanical limits of moving contacts and arc extinction, so unlike the fuse  the let-through energy rises ever more with increasing fault current.

    Then you need to consider which cables are credibly at risk more than a few tens of milliohms down the wire and the fault current is much reduced. If the first few metres are big chunky submains to other boards, and these are unlikely to be damaged while the fuse blows, that is OK.

    really thin stuff like lighring circuits you may decide woud be replaced anyway if accidentally drilled and accept that if the cable is 'lifed' during the last desperate act of blowing the fuse it is cheap enough to replace and you do not care.

    Or you may decide you really do care about all these things, and the consequences of the fuse doing the heavy lifting, ever, are just not acceptable in this case, and then you change the MCB for a large one, or fuse down into sub-circuits where the loss of any one is not such an issue.

    Mike.

    edit curse this forum - I have already answered this question and did so again as I did not see it.

Reply
  • Ah  it is more complex.

    If (and only if) there is a silver stake fault very close to the MCB, so that there is a fault current greater than 3kA it is possible that the MCB may fail. Now without info we do not usually have, we do not know how,  - maybe  it will work anyway,  maybe it will open and never close again, maybe it will weld shut and never open, or maybe great jets of flame will emerge from the sides and seek out anyone nearby with a beard. We just do not know, just it is out of spec. But in a suitable enclosure, all of these possible outcomes may be  contained, and considered as "safe" potential outcomes (though some are inconvenient as you need a new MCB) so long as the fuse operates promptly before anything else, like say the wiring, catches light.

    Now that is not the normal adiabatic limit, that is about damaging PVC insulation so the cable life is shortened, but actually the real upper bound of instant cable damage, you can accept if you do not mind replacing the cable, is much higher,

    Now really you should look at the fuse data and what they do in the range 3000-7000A. I imagine that there will be a fairily constant I2t, but check.

    This represents the worst let through energy that the cables downstream may see - even if the MCB failed to open at all, and can be compared with cable ratings.

    The MCB I suspect stops getting faster at higher fault currents due to the mechanical limits of moving contacts and arc extinction, so unlike the fuse  the let-through energy rises ever more with increasing fault current.

    Then you need to consider which cables are credibly at risk more than a few tens of milliohms down the wire and the fault current is much reduced. If the first few metres are big chunky submains to other boards, and these are unlikely to be damaged while the fuse blows, that is OK.

    really thin stuff like lighring circuits you may decide woud be replaced anyway if accidentally drilled and accept that if the cable is 'lifed' during the last desperate act of blowing the fuse it is cheap enough to replace and you do not care.

    Or you may decide you really do care about all these things, and the consequences of the fuse doing the heavy lifting, ever, are just not acceptable in this case, and then you change the MCB for a large one, or fuse down into sub-circuits where the loss of any one is not such an issue.

    Mike.

    edit curse this forum - I have already answered this question and did so again as I did not see it.

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