Determining device designation of rewirable fuses

Do you really need to? If they're in a consumer unit then the whole thing is usually rated at 16kA anyway.

   - Andy.

It is really a measure of the containment of the flying metal when they blow, and is rather a nebulous quantity, as you do not know how well they have been rewired. Generally threy will all open at a higher pssc than rated, what varies is how much of a clean up is needed before it can be rewired and re-energised.  As hot metal can be shot out of the wire threading  holes as well as the inspection well in the ceramic, the higher rated (4000A ) designs have a convoluted path for the hot gas so it does not get out so easily, to protect the finger tips of someone putting a fuse in against a fault The older 1kA ones generally do not..


If not sure assume 1000A, or fit the cartridge fuse holder versions in the hole instead.

The designation refers to the breaking capacity of the fuse carrier. Final domestic circuits will almost certainly be the lowest rating. I just remembered a copy of BS3036-2 I have and the 1,2, and 4 are 1000th of the breaking capacity. Used ones are usually copper plated inside, and as Mike says should be cleaned up but from experience, this is very difficult. A C3 would probably be the correct thing, they have always been considered old but fine in operation. A C2 would indicate a change of fusegear, and there are a lot still giving good service. Unless the PSCC is very high I would choose C3.

Thanks for the replies gents, much appreciated.


F

To answer the original post, if it doesn’t say it’s rated at more than 1 kA on it, then assume it’s not.


Andy Betteridge

I have seen some which are marked,  but not many.

Got to be careful here, while the BS3036 is much loved, time marches on, and you can get a PSCC higher than a 1ka fuse can take. Modern, or even not so modern DNO services, even cable sheath, can have a higher rating at 240 volts for a short service.


Regards, UKPN

There's a Bill 20 amp BS3036 fuse on EBay at the moment which is marked S2A

Agree, and the safe course is to assume anything unmarked is the lowest category -s1 = 1000Amps. If your pssc at the board is more than this, then there is a risk if there is a fault close to the board, if the fault is not cleared before refitting the fuse holder.

The board will not blow off the wall or anything like that, we are not talking mega joules !  **

Nor will the ceramic crack, but there is a risk of burns to someone replacing a fuse, if the fault is very low impedance and not cleared.  (though if the fault is just 10m along some 2.5mm T &E that extra impedance will significantly reduce the fault current and the risk) Take great care to identify and fix the fault before replacing the fuse wire.

regards

Mike



**As an aside, even if supply side PSSC was infinite,  the energy would be limited to the resistance of the fuse wire -at most a few milliohm - times the I2t of either this fuse or the company fuse which ever blows first. I2t is not normally quoted for BS3036 fuses but you can estimate it from the I/t curves.)