Probably unusual to need individual fuses for only two parallel conductors, but going with it as an academic exercise (and if we find the right approach, it should scale to larger number of parallel conductors)

can one 'simply' consider the distrib circuit fuses as one fuse (each summed as such)  ?

Certainly sounds sensible, at least as a first approximation (in the old days it was common practice to twist lower rated fuse wire together to "make" a higher rated fuse link), I might suspect though that 2off 32A fuses don't behave entirely identically like a 64A (or rather 63A) fuse. so I'd probably think more in terms of one 32A fuse but half the prospective fault current. Not sure if that help much in practical terms though, I²t won't be just a quarter, as t will likely be larger for the lower current, but it might give you something to look up.

  - Andy.

p.s. just a random thought - if we have parallel line conductors and need parallel fuses, presumably the same problem exists with the N conductors...so you'd need fuses on the parallel Ns as well ... but we don't fuse Ns (at least not with ordinary fuses that can open without simultaneously opening all the other conductor)..

The 3 fuses are kind of there for different purposes, The final circuit fuse is there to proected the final load, but it conviently puts an upper bound on the maximum load that may be drawn.
The fuses in the parallel lines are presumably there to protect them against overload, and had it been 2 lines on a  16A fuse each I;d have been more twitchy. A fault on any one of the parallel lines to earth will blow all the supply side fuses, as current will flow to that fault from both sides, unless each line is fused at both ends, and then we are getting quite complicated. So the condition to guard against is only one line going open circuit, and the others then seeing an overload that might damage the cable. How serious that is  depends on the cable size and the reason for the parallel conductors - if it is voltage drop, then it is likely there is no real overload risk, and if it is flexibility for a high current over a short distance, then the 'one line open' may be an incredible condition not worth guarding against.

Fused neutrals, we would not do, ever. yet there are plenty of cases where there are many insulated parallel paths. Anything supplied by this stuff for a start.

Mike.

The 3 fuses are kind of there for different purposes, The final circuit fuse is there to proected the final load, but it conviently puts an upper bound on the maximum load that may be drawn.
The fuses in the parallel lines are presumably there to protect them against overload, and had it been 2 lines on a  16A fuse each I;d have been more twitchy. A fault on any one of the parallel lines to earth will blow all the supply side fuses, as current will flow to that fault from both sides, unless each line is fused at both ends, and then we are getting quite complicated. So the condition to guard against is only one line going open circuit, and the others then seeing an overload that might damage the cable. How serious that is  depends on the cable size and the reason for the parallel conductors - if it is voltage drop, then it is likely there is no real overload risk, and if it is flexibility for a high current over a short distance, then the 'one line open' may be an incredible condition not worth guarding against.

Fused neutrals, we would not do, ever. yet there are plenty of cases where there are many insulated parallel paths. Anything supplied by this stuff for a start.

Mike.

The fuses in the parallel lines are presumably there to protect them against overload

My understanding was that the arrangements in appendix 11 with fuses on multiple parallel conductors are mostly for fault protection, since we're permitted to assume that parallel conductors of the same length, c.s.a., disposition, etc will share the current equally enough (433.4.1/523.7) - i.e. one connection coming loose (like faults between different circuits) isn't something we're obliged to consider. And that way of thinking works just as well for a dozen conductors in parallel as it does for two.

For faults though it gets trickier - since a conductor of a given rating is usually good for fault protection with a fuse of two or maybe three times it's rating - things start to fall down when the fuse to conductor ratio is higher than that - so with a dozen conductors in parallel, one fuse (of around 12x the rating of any individual conductor) isn't going to provide adequate fault protection at all. So you end up with individual fuses on each conductor - often one on each end (two per conductor) as faults can be fed from both ends. And, yes it gets more complicated.

You might need to provide overload protection to individual conductors, but only when the current sharing won't normally be equal, which should be the exception rather than the usual case.

Split con still does have a few questions to answer...

  - Andy.