V.High PFC at switchfuse

When the BS3036 fuse holders were being designed all the modern subtleties of PFC/PSSC were not high on the list. Unless marked otherwise assume it will break no more than 1kA totally safely if inserted live onto a fault, and rather  more without damage with the cover fitted, and quite a bit more without actually damaging anything but itself internally.


For a higher official rating it needs to be the kind that is either shrouded or makes it hard to push a re-wired fuse in onto a fault, in some designs, that is done by interlocking the switch to make sure it can not turn on until the cover is in place.

The problem that limits the breaking capacity is that in some more open versions, it can be that as the wire is vapourised by the fault, hot metal is ejected from the holes/gaps in the fuseholder over the  finger tips and hand pushing it in, not that it fails so violently that it demolishes the building.

Assuming the BS88 is a sensible rating, then to a degree it will energy limit if there is a gross fault close to the BS3036 fuse, which the same thinking that allows 6kA MCBs in such a case and in practice in this case the danger is not great, so long as when the fuse is changed it is not refitted with the load switched on.


As an aside, 19kA is quite high for this sort of thing (usually its the other way about, electric showers dim the lights etc) unless the transformer is in the basement below, and before panicking, I presume you have verified that your meter leads are in good order, correctly nulled and the meter is in cal and not low on battery.

Also, look at your MFT's precision specification: percentage or number of significant digits etc.

Be careful of the risk of asbestos being present in the fuse carrier.

Thanks for response guys, the switchfuse is a metal one which needs to be switched off before the cover can be removed therefore no chance of inserting a fuse in onto a fault. As s kindly pointed out, any explosions due to a future fault will be limited by the metal housing so I guess its deemed as safe for continued operation.. Nevertheless I will still need to record the breaking capacity of the switchfuse on a certificate and im not sure what value to enter ?? The BS88 fuse supplying the switchfuse will likely limit the fault current but to what value im not sure; this is a good learning curve for me !


The Ze of 0.01 is not helped by the fact that earth from the switchfuse is via the metal trunking, there is no earth cable to speak of. So I cannot disconnect any parallel paths easily. Now im contemplating the breaking capacity of the BS88 fuses that supply the switchfuses (which are also dated 1970's I think, and of porcelain) these are 500v I expect as dist board is 3ph; would BS88's of that vintage have a 80KV rating and therefore reduce the need for downstream devices to be so high. Cheers

Given the submain is T and E and that means a reduced earth core, 60 milli-ohms seems entirely credible for a distance of 20m.


The BS88 fuse will not limit the current exactly in the same way that a resistor would. Rather it cuts the fault current off early - so the total damage done, which is related to the energy dissipated at contacts and other pinch  points. The parameter I²t is actually joules per ohm, and relates to the energy that is dissipated. Reducing 't' clearly reduces the peak energy, so volume of metal vaporised or melted, damage to insulation etc.

Luckily we do not need to calculate this exactly, as the fuse makers have done this for us.  an explanation here for example.

Thanks for link, I will try to work that out.

If you get stuck, do please come back, but have a read first - this sort of thing has been asked on this forum and it's predecessor a no. of times, but it is easier if you can say ' I understand as far as X but not Y '  and are at least familiar with the terms used so there is not too much misunderstanding when folk start talking about let through energy, bullrush diagrams and so forth.