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 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.

I have to admit to not having read App. 10 previously. Why does the same principle not apply to ring final circuits?

Why does the same principle not apply to ring final circuits?

I suppose the same principles are applied for fault protection - as the cables are about 2/3rds the rating of the protective device, the protective single device is adequate for fault protection. Overload protection is a slight modification of the 'equally divided' rule, to not worse than two-thirds, which the designer is meant to ensure by layout etc. - so the underlying theory is fairly similar.  We have had discussions in the past about using separate overload protection in each leg, for situations where it's difficult for the designer to be reasonably confident the loads can be spread evenly.

    - Andy.

i've sat with a pen, paper and calculator , doodling  various made up scenarios ...   for more than two conductors it can get a little tricky to say the least   - or, it proved my lacking mental ability at least  :-)      As BS7671 App.10  suggests,  especially for more than two conductors,  linked devices should be a serious consideration and one can see why !     For two parallel conductors, especially when overload protection is being provided (as well as fault), there is not much of an issue to worry about in most  circumstances (single phase particularly)  with one protective device as has been suggested (and it is like a RFC) ...     i'm still trying to hack the maths if a fault would clear two separate devices  (two parallel conductors) in various scenarios .     

As an aside,  are there any [online] tools for parallel conductor 'maths'   (I presume that Amtech stuff would do it)  ...

Thank you all for the previous comments.

Yes it's a mind-bender. In many practical cases, though, it's somewhat easier to specify the cables and the installation method such that current sharing is sufficiently even that a single upstream device can provide overload and fault protection. And of course then make sure that the installation method is actually followed through on site.

psychicwarrior said:

I presume that Amtech stuff would do it

Heck no. It's not much more than a glorified spreadsheet, albeit one that is useful for speed on conventional circuit designs. It is possible to specify parallel circuits but there's no means of calculating current sharing factors etc, at least on the version we run.

Also, one thing that's not been mentioned (and I don't think is excplicitly covered in appx 10) is that if you have separate devices for fault protection on each parallel conductor, the fault withstand capacity of each still ought to be the total circuit prospective fault, because you'll see that in some scenarios they won't all clear at the same time, leaving the one "turning out the lights" having to break the full Ipfc.

What is the context?

Is this really a problem in, for example, singles in conduit, or doubling up cores in SWA? Or are we really thinking about industrial installations where a single very large cable would be too difficult to handle, so is divided into smaller conductors?

And how well balanced are the routes - the reason we don't need to bother bother with the neutral cores in split concentric, even at the level in the larger sizes when any one core would fuse let alone overheat, if it carried the  full current, is that the routes of the individual insulated wires  are very well balanced indeed by the design of the cable and the fact they are in thermal contact,

I suspect there are very very few cases indeed where parallel feeds are the best solution and need to be done in a way that a common fuse or breaker is not enough, and where they are one really needs to be thinking about fusing and L-N current balance as well.Mostly folk would be able to use a larger cable or the run is controlled enough that a dangerous and undetected fault is acceptably unlikely.

Mike.

This discussion is intriguing me, I’m not really understanding why you would install two MCBs to supply the same circuit with paralleled conductors rather than commoning them into one MCB.

But if you must there is a third option, use a Double Pole MCB, so both legs of the circuit are isolated together, but terminated separately.