Thoughts on linked 16A MCB for ring final?

[ericmark]

If instead of a 32A MCB one fitted a double 16A one feeding each end of ring, then if too much plugged in at one end it would trip rather than over load, and if ring is broken then neither leg can be overloaded.

There is still a problem if only neutral broken, but seems risk is reduced, so why is it not done?

 

[JohnW2]

If instead of a 32A MCB one fitted a double 16A one feeding each end of ring, then if too much plugged in at one end it would trip rather than over load, and if ring is broken then neither leg can be overloaded. There is still a problem if only neutral broken, but seems risk is reduced, so why is it not done?

Well, for a start, and particularly if the CCC of the cable (as installed) is 27A, then that could easily result in one of the MCBs tripping 'totally unnecessarily' ... and one of the other problems with such a system would be that, if that happened, one would be unaware of the trip, and the entire load on the circuit would then be going through one 16A MCB (which therefore would probably trip at some point!).

Another issue is that of safety. Although, of course, proper 'testing for dead' should always be done, that's not necessarily always done - and people are not used to having to operate two MCBs to render a circuit 'safe' to work on.

One might also question what would be the point of what you suggest. One might just as well cut the ring in the middle and then 'upgrade' your two 16A MCBs to 20A or 25A ones as appropriate!

Kind Regards, John

 

[ericmark]

I was referring to a twin B16 amp MCB so one tripping would always trip both as they are linked.

 

[JohnW2]

I was referring to a twin B16 amp MCB so one tripping would always trip both as they are linked.

Sorry, I missed that.

However, it remains the case that if the CCC is 27A (and possibly even if it were only 20A) there would be an appreciable risk that loads fairly close to one end of the ring could result in 'unnecessary' trips of a 16A device at currents well below the CCC of the cable being protected. A twin B20 or B25 (as appropriate) would avoid that, but would presumably be non-compliant with regs.

The reality, of course, is that it is extremely unlikely that, in practice the current in any part of a ring with Method C 2.5mm² cable (protected by a B32) would ever get overloaded - it would require more than 27A of load (in itself pretty rare for any ring) to be drawn very close to one end of the ring - and, even then, the degree of 'overload' would be small.

I still don't really see what advantage your proposal would have over two 25A radials (or 20A ones if, unusually, the cable installation method required it).

Kind Regards, John

 

[donrkebab]

So when one of them sees 16A+ it trips and you lose power to the whole circuit, and then you replace the 2x16A by a 32A.

You're trying to fix something that aint broken.

 

[SimonH2]

Given the CCC of the cable, a linked 2 pole 20 or 25A would work on the same basis - making a 40 or 50A ring. From memory, CBA going to get the book, the RFC is only allowed with a 30/32A device which would preclude that. Might it be allowed as a case of parallel conductors rather than considering it as a ring final ?
It would cause issues with spurs though - since (in theory) a fault on a spur could require considerably more current to trip the supply.

 

[flameport]

The solution is simple.
Don't install ring final circuits.

 

[JohnW2]

Given the CCC of the cable, a linked 2 pole 20 or 25A would work on the same basis - making a 40 or 50A ring.

Indeed - as I wrote ....

A twin B20 or B25 (as appropriate) would avoid that ....

From memory, CBA going to get the book, the RFC is only allowed with a 30/32A device which would preclude that.

Indeed, again - which is why I went on to write ...

.... but would presumably be non-compliant with regs.

Might it be allowed as a case of parallel conductors rather than considering it as a ring final ?

Well, as above, it certainly could not be justified in terms of the 'ring final dispensation' in the regs.

It presumably would be 'safe', since there's no way that the current anywhere in the ring could be greater than it was at the end of the ring which was carrying the greater current - so, if the current anywhere exceeded the CCC of the cable, the currents at the ends of the ring would also exceed the In of one or other of the linked MCBs (assuming they had an In appropriate to the CCC). However, I doubt that many electricians would want to have to justify doing that, since it's presumably not something for which they could simply quote a relevant part of BS7671.

However, as I have asked eric, would there be any real point?

Kind Regards, John