the context is (was originally) in the OP.  i

it is about the principle of [mainly] fault protection in parallel cores   I  suppose ...   and what is written in App10 ... but you already know that   from your previous reply

in my reply above (to myself) -  i think it had already come to a conclusion at least for me.

i've learned a bit so its been good.

cheers

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.

Mostly it's a matter of fault protection. For simplicity think of fuses to start with - generally a fuse of two or three times the cable rating will still provide fault protection to a conductor (even though it's way over for overload protection) - provided the loop impedance is suitable of course. Think of those OSG tables of Zs for various rated fuses and conductor sizes - typically towards the top/right there's a number of "NP" where the fuse has no chance of protecting that sized conductor against faults.

So, making a few assumptions and generalizations like if the one fuse is something like the circuit rating and the parallel conductors add up to something similar (e.g 63A circuit and two 32A conductors in parallel), the more conductors in parallel you have, the smaller each of them is likely to be compared with the fuse - e.g. 4x 16A or 8x 8A.  The problem with faults if of course they can occur anywhere along a circuit, so where parallel conductors are insulated from each other (which they will be otherwise they'd be one stranded conductor, not parallel conductors), we could quite easily have a situation where just one of the parallel conductors has to carry (and withstand) pretty much the entire fault current. With 2 conductors in parallel, they're each likely to be at least half the fuse rating - so fault protection is pretty much guaranteed from a single device. With 3 conductors, 1/3rd of the fuse rating, it's getting a bit uncomfortable, 4 or 5 (with each conductor 1/4 or 1/5 the fuse rating) starts to look really dodgy, and beyond that lie dragons. Hence the idea of supplying a fault protection device for each conductor - which of course is sized match the conductor's withstand (or even two per conductor - one each end - since faults can be fed from both ends, as all the conductors are connected together at both ends and so opening the one at the supply end doesn't disconnect the fault...)

MCBs are slightly different in their fault handling characteristics, there's a less direct relationship between rating and energy let-through, but still there's a minimum size conductor a give rating/type/make of MCB can protect - and if you keep dividing the conductor into more thinner ones, at some point you'll still hit that limit.

    - Andy.

AJJewsbury said:

With 2 conductors in parallel, they're each likely to be at least half the fuse rating

I was thinking that if you want two circuit protective devices both legs would match the fuse/MCB rating and be doubled to add integrity, circuits for essential services, so each leg can work independently. 

Would not they also be full size if doubling up to half the volt drop?