Circuit protection for distribution circuit

well if was 3 fuses they may not all go at once, and that is common arrangement. It rather depends on what is at the other end and if the load will care. Some types of rotating machinery do not appreciate 'single phasing' and need a dropout configured so that if one phase drops the thing is cut off, but that is normally in the control gear

Mike.

There will be no 3 phase loads.

then you have no problems

That sounds like 3 SP circuits with a shared neutral even if they are all in the same cable. See the definition in Part 2 on page 31. The danger is that if one (or two) of the MCBs is opened, the neutral may still be carrying a current.

It would effectively be 3 single phase circuits sharing a neutral. Which brings to mind neutral currents. If for example, three single phase 40A circuits sharing a neutral of a 10mm SWA would result in 120A neutral currents.. too much

But how is it ok for meter tails to be the same size on 3 phase supplies?

Yes if they were all on the same phase; no if they are different phases.

Ah yes of course.
Need to brush up on my 3 phase theory!

So long as by the time the distribution circuit splits up in one place to supply the individual single phase loads , and from then on each of them has a separate neutral, you should be OK - so for example a 3 phase fed to one 3 phase board with lots of single phase outbound circuits should be   fine - as the board  itslef is a 3 phase item, and might just as well be fed by 3 fuses. However, if the load and of the cable somehow splits to supply 3 separate singe phase boards, and did so in such a way that the outbound neutrals are shared, then that is not right.

It gets more complex with things like 3 phases directly supplying a bank of light switches, where really that is the final circuit, and you should be careful to avoid the shared neutral , as an example then, someone may well pull the fuse to kill the lights on one room and then open the neutral for a different zone and get a belt off it from the other phase still energised.

But the question was not originally about neutrals - yes it needs watching, but the ganged MCB at the origin is not the thing that makes a circuit a 3 phase circuit  one or 3 separated single phase ones - it is rather what is at the far end.

Mike.

mapj1 said:

and might just as well be fed by 3 fuses

I am not sure that I agree. It is much easier to open an MCB (just like a light switch) than to remove a fuse. The arrangement would provide fault protection and overload protection perfectly well, but that is not quite the point.

but on a distribution circuit quite often we do not really want folk casually flicking things  off - if they want to work on the final circuits then they ought to be turning off at the last DB - in this example the one where things split out from 3 phases to lots of singles but they could just as easily be a mix of 1p and 3P loads. 

Also given how badly MCBs co-ordinate in the face of faults  of varying currents,  there is a lot to be said for fuses further back  and MCBs nearer the load - and with high PSSC they can act as an energy limiter, reducing the size and expense of the downstream switchgear.
I agree that at a pop festival or similar plug and play you will have MCBs and ganged ones, all the way back to the genset, but in a factory setting that freedom may be more of a hindrance than a help.

Not saying always, just that it can be . And, in cases where we accept 3 fuses, and the failure of one is not a big deal, then we ought to be OK with non-ganged breakers, though that would be more unusual.

Mike