In a Schneider CU, yes.
100A mcb
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Never mind that, have you checked the curves of a 100A fuse and a 100A MCB to make sure there's a useful level of discrimination between the two? The answer will be a resounding no. I am aware that even a 6A B type MCB doesn't have discrimination with a 100A cutout fuse in all situations, but those are only in unlikely situations.
I suppose it all depends upon what you would regard as being a "useful level of discrimination" but, in terms of the sort of modest overloads the OP is considering/contemplating, I personally wouldn't say that it would necessarily be a 'useless' amount of discrimination.
Having said that, I would think one would have to try quite hard to blow a service fuse by installing 'heavy appliances' - so it could well be that, if one observed apparent 'discrimination' (in that the MCB operated), the fuse may well not have blown even in the absence of the MCB!
Kind Regards, John
If you are connecting such heavy loads that are likely to risk blowing the cutout fuse you should be contacting the DNO asking for permission before connecting the large loads to the supply, and if the loads exceed the current supply capacity a supply upgrade will be needed.
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The loads could be approaching the limit if all are on at the same time, not go over. I know about diversity, etc, which says you can put on larger loads as the probability of them all being on simultaneously is low. Best to be sure and put in a 100A mcb than can be user reset.
If the DNO gives a 100A supply then keeping under it does not need their involvement.
If the DNO gives a 100A supply then keeping under it does not need their involvement.
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You seem to be ignoring the time factor. To blow a 100A service fuse you would have to have loads totally considerably more than 100A 'all being on simultaneously' for an appreciable period of time, which is exceedingly unlikely with any 'ordinary domestic loads' (and most 'not-ordinary' ones, too!). It should take half an hour or more for 200A worth of load ('all on, all the time') to blow a 100A fuse.
Kind Regards, John
Primarily to protect against a (L-N or L-E) fault, if the fault is upstream of the CU or if the installation's protective devices doesn't clear it - and that fault current will potentially be many hundreds of amps. In a TN-C-S installation, even if the Ze is the 'maximum' of 0.35Ω, a fault near the origin of the installation would result in a current of almost 600A; if the Ze were, say, 0.15Ω (not unusual for TN-C-S), then a similar fault would result in a current of over 1,500A.
The way we label fuses is fine for those who understand what the labelling means. A 32A MCB will typically allow about 46A flow for around an hour before tripping - do you think that we "might as well label it 46A"?
Kind Regards, John
Even a 100A MCB will allow 145A to flow for about an hour without tripping. Anyone contemplating using a current which averaged over 145A for at least an hour would presumably be doing something not intended to be done in domestic premises (and would certainly need DNO permission to do whatever they were doing).
Kind Regards, John
I agree. It would only go over 100A in some sort of fault condition.
Quite - and, as I understand it, that's not what we are talking about (i.e. the reason this 'backup' 100A MCB was proposed).
If a fault causes the service fuse to blow, either the fault was upstream of the installation's CU etc. (hence very probably also upstream of the postulated 100A MCB), or else the installation's protective devices must have failed to clear the fault. Neither of those situations has got anything to do with 'heavy loads' being connected to the installation, nor would the presence of such 'heavy loads' make one iota of difference to the situation.
Kind Regards, John
Well, one obviously should not load a fuse beyond its 'rating' i.e. 100A in the case being discussed.
However, what I'm saying is that (even if one is 'breaking the rules') unless one contemplates having a load which averages more than 145A for at least one hour (which one should not do with a 100A fuse, and should not do without DNO permission), there is no point in having the 100A MCB, since it should never trip.
Kind Regards, John
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