Sizing of protective conductors - what am I missing here

Why is it ok that potentially I could have a conductor with a K2S2 less than the I2T of the protective device in the case of a 1mm2/1mm2 cable just because it has been selected rather than calculated?

I'm with you on this one - there's certainly something that doesn't stack up (and we've discussed this in the past). Similar situations can appear using a 1.5mm² c.p.c. on a ring with a B32 protective device where fault currents might be large-ish.

I suspect it's mostly historical accident - the UK has a long history of both ring final circuits and reduced c.p.c.s and/or small (6A) lighting circuits which conspire to mean relatively small conductors and relatively high capacity single phase supplies so potentially high fault currents (up to 16kA on paper). Whereas EN standard MCB specifications seem to have been were dreamt up with more European situations in mind (low fault currents in domestics, no rings, full size c.p.s.c on small circuits, min 1.5mm² conductor size). Yet UK practice continued and not much harm appeared to occur.

There was an attempt a while back to make the minimum conductor size 1.5mm² rather than 1.0mm² - which would help avoid some of these problems (and similarly align with continental practice) but was pretty much defeated by UK custom and practice (there had to be an exception for "lighting circuits").

So much guidance (e.g. the OSG) is rather caught between a rock and a hard place - BS 7671 says what needs to be achieved, but that's neigh on impossible using BS EN circuit breakers and the small cables we're used to. There have been valiant attempts - using favourable manufacturer's data rather than generic values from BS EN 60898, and using the actual fault current rather than the rated breaking capacity of the MCB, but it's all still a bit of a kludge to my mind.

Such thing were much simpler with fuses - as their energy let-though doesn't significantly increase with increasing fault currents, unlike MCBs, It was safe to assume that if the device provided overload protection and it had suitable breaking capacity, then it would naturally provide fault protection (to the conductors).

   - Andy.

Why is it ok that potentially I could have a conductor with a K2S2 less than the I2T of the protective device in the case of a 1mm2/1mm2 cable just because it has been selected rather than calculated?

I'm with you on this one - there's certainly something that doesn't stack up (and we've discussed this in the past). Similar situations can appear using a 1.5mm² c.p.c. on a ring with a B32 protective device where fault currents might be large-ish.

I suspect it's mostly historical accident - the UK has a long history of both ring final circuits and reduced c.p.c.s and/or small (6A) lighting circuits which conspire to mean relatively small conductors and relatively high capacity single phase supplies so potentially high fault currents (up to 16kA on paper). Whereas EN standard MCB specifications seem to have been were dreamt up with more European situations in mind (low fault currents in domestics, no rings, full size c.p.s.c on small circuits, min 1.5mm² conductor size). Yet UK practice continued and not much harm appeared to occur.

There was an attempt a while back to make the minimum conductor size 1.5mm² rather than 1.0mm² - which would help avoid some of these problems (and similarly align with continental practice) but was pretty much defeated by UK custom and practice (there had to be an exception for "lighting circuits").

So much guidance (e.g. the OSG) is rather caught between a rock and a hard place - BS 7671 says what needs to be achieved, but that's neigh on impossible using BS EN circuit breakers and the small cables we're used to. There have been valiant attempts - using favourable manufacturer's data rather than generic values from BS EN 60898, and using the actual fault current rather than the rated breaking capacity of the MCB, but it's all still a bit of a kludge to my mind.

Such thing were much simpler with fuses - as their energy let-though doesn't significantly increase with increasing fault currents, unlike MCBs, It was safe to assume that if the device provided overload protection and it had suitable breaking capacity, then it would naturally provide fault protection (to the conductors).

   - Andy.