BS88 Fuses

70,000 seconds is a very long time in the thermal world - the usual adiabatic equations are only good to around 5s - beyond that heat starts to be lost from the cable and you actually need a much smaller conductor than k²S² would suggest. If you applied the adiabatic to 160A and infinite time it would suggest you need an infinite c.s.a. cable - which clearly isn't correct or helpful.

Generally relatively small overcurrents like this are regarded as overloads rather than fault currents - and in most cases simply ensuring that the cable's Iz is at least In is sufficient to ensure it's protected - there being suitable margins in the tables and construction of the cables to ensure reasonable safety (thermally at least). Small corrections might be needed for BS 3036 fuses and buried cables, where the margins are otherwise a bit thin.

   - Andy.

Hi Andy, thanks for getting back.

It was the small overcurrent scenario I was thinking off.

I didnt realise it only applied to short duration faults and not the small overcurrent scenario, which take can take hours to blow the fuse.

Some good analogies above, for although the energy transferred for a small overcurrent may be large, it it the time overwhich it occurs keeps temperature rise down.

Thanks, Derek

Hi Andy, thanks for getting back.

It was the small overcurrent scenario I was thinking off.

I didnt realise it only applied to short duration faults and not the small overcurrent scenario, which take can take hours to blow the fuse.

Some good analogies above, for although the energy transferred for a small overcurrent may be large, it it the time overwhich it occurs keeps temperature rise down.

Thanks, Derek