Wiring Ring mains to MCBs in consumer units

[jamie T]

In my flat I've discovered that each end of the two socket ring mains go back to separate MCBs. I've tested this by using a multimeter, turning off an MCB and still getting 240V on the live terminal (i.e I've got to switch off 2 MCBs to turn off the circuit). To clarify I have 2 32A MCBs per ring main, (4 in total). I would have thought that both ends of the ring main should go back to the same MCB (i.e. two wires into the MCB) The way it's been wired, each individual live wire goes back to an individual MCB.

Have I got 2 MCBs too many in my fusebox. I think its wired incorrectly am I right

Thanks.

Jamie

[rrh]

Yes you are right!

[Owain]

"jamie T" wrote | In my flat I've discovered that each end of the two socket ring mains | go back to separate MCBs. I've tested this by using a multimeter, | turning off an MCB and still getting 240V on the live terminal (i.e | I've got to switch off 2 MCBs to turn off the circuit). To clarify I | have 2 32A MCBs per ring main, (4 in total). I would have thought | that both ends of the ring main should go back to the same MCB (i.e. | two wires into the MCB) The way it's been wired, each individual live | wire goes back to an individual MCB. | Have I got 2 MCBs too many in my fusebox. I think its wired | incorrectly am I right

It is wired incorrectly. Each end of the ring should go back into the same MCB.

Owain

[Christian McArdle]

Yes. The current wiring is extremely dangerous and could easily lead to a fire. You should fix the problem immediately. I think the whole installation should be thoroughly tested. With one absolute shocker like that, there could be many other disturbing irregularities.

Christian.

[RichardS]

installation

does this mean that the circuits are effectively protected at (a chilling)

64A at the moment, or is the effect not quite that simple and liable to be indeterminate?

-- Richard Sampson

email me at richard at olifant d-ot co do-t uk

[Bob Eager]

No, they're protected at 32A. Which is fine, as that is well within the rating of the ring as a whole.

But, think what happens if ONE of those breakers trips. You are left with what is effectively a radial circuit, protected at 32A by the remaining breaker. But with a single cable feeding each outlet, the cable (if 2.5mmT+E) rated at as little as 20A.

[Christian McArdle]

It will be dependent on where the point loads are applied. It is protected at between 32A and 64A. I suspect it would be much closer to 64A on an average distribution of point loads.

Christian.

[Bob Eager]

(slaps head). Of course.... I was concentrating too much on the overload if one MCB tripped. Wther one is tripped or not, it'll still overload!

[Jim Ingram]

message

liable to be

I think that would only be correct if the load on the circuit was balenced e.g. at the mid point of the ring. If you put the 32A load within a few mm of one MCB I don't think you could draw much above

32A (at least not for long) before the 32A MCB trips but this is still above what the cables rated at since most of the current would be travelling down the shorter (lower resistance) leg. In summary your protecting the cable at 32A but the circuit between 32&64 A depending on where the load is.

HTH

Jim

Jim

[aj]

The overload tripping current will be very close to 64 amps (probably over), the impedance for an overload current will have negligable difference anywhere on the ring main cabling - (Kirchov's current law if you want to look it up and work it out exactly) If it was a short circuit current then then the difference in the impedance would have a measurable effect but then the short circuit current would be only for a very short period of time and into the thousands of amps range. It is a very serious fire hazard in an overload situation as this could be applied for a long perion melting the cable. It is also a bad situation for anyone working on it.

[Matt Beard]

Well, at the best this setup is non-standard and dangerously confusing (unless very clearly marked). However I would very much doubt that it is anything other than either a bad example of DIY or some typical cowboy work. I am not sure it the regs specifically forbid wiring multiple paths (they probably don't as it is not really a rational thing to do anyway) but at the very least you would need the wiring to be able to safely carry the 64A that you are effectively protected by (assuming a single load or fault at the mid-point of this "ring"). In this 32A+32A case you would need 6mm^2 conductors (rather large for a ring). Anyone got the time to do the calculations for a non mid-placd max load?

Anyway - whatever the calculations end up giving you for a "safe" layout (if it ever can be) I can't imagine you ever needing that much power in a flat. It's not a vast penthouse by any chance is it?

Matt

P.S. Short answer... theres something very wrong with your wiring!!

[Lurch]

On 28 May 2004 10:27:34 -0700, [e-mail] (Matt Beard) strung together this:

Nope, just dangerous.

They do in this way.

Nope, it's still dangerous and wrong.

[Alan James]

Well spotted, but you should confirm ring continuity of live, neutral and earth conductors by checking impedance across the open ends of the cables you think are the ring. Then they both go to a single MCB. There are other tests as well. Think there may be guidance on the TLC website.

Alan