Consumer Unit Wiring Problem

[daddyfreddy]

Okay, it's not the biggest problem but I wondered if anyone knew a quick way to work it out.

I've got an old fuse wire consumer unit that was installed with the house (1970) and all original socket locations etc. It has three 30 amp fuses. One that works just the top floor and the other two seem to work the other two floors together. i.e. to isolate any one of the circuits I have to remove both fuses.

Now, unless someone can correct me I'm assuming there has been an error in the wiring somewhere so that two ring circuits have interconnected somewhere. I've looked inside the CU and there are two wires going to each fuse so I presume each has it's own ring circuit.

Anyone know the quickest way to deduce where the cross connection lies and separate them?

[peter]

Can't help with an answer but if it's any consolation my house (built in 1988) is like this too - I have to remove both the upstairs and downstairs MCBs to isolate either, kitchen ring is ok. I assume there's no reason why it would deliberately be like this?

[dennis]

Sounds like someone has swapped the return legs on two rings. Its easy to tell if you disconnect them and buzz it out.

It needs sorting as 2.5mm T&E can't take the current from two 30A breakers and its dangerous if you forget to remove the fuses.

[Dave Plowman (News)]

I'd say the likelihood is that one 'feed or return' has been swapped on both rings. To check this, *switch off*, pull the fuses for both rings, remove the fuse holders and check continuity between the two lines (red) of the two circuits with a DVM set to continuity. There shouldn't be any. If there is, simply remove the wires from both fuse 'outputs' and separate, and find out by the same test which one belongs to each ring. Out of the four wires, two should read continuity, so mark those. Same with the other two. If all four show continuity between them, come back for some more thoughts. ;-)

[Brian Sharrock]

I _had_ a similar problem when I acquired a house that had been re-wired in the seventies. To isolate 'leccy from the 'upstairs' ring /'downstairs' ring it was necessary to remove both fuses. Many, (my wife would say too many) years later I stumbled across an original 1935-era wire that connected between a 'lounge' power outlet and a "bedroom" outlet. Each outlet [surface mounted patress] was a 'flat-pin' conventional switched outlet and each correctly wired to a 1970~era T+E 'ring' - just someone hadn't bothered to remove the original wiring that was acting as a bridge. I only found it when {Sod's Law} I was replacing the _final_ outlet in the Lounge - what the **** is this **** wire doing? I'll cut it and see what happens .... :)

I can't think of any protocol that'll find where the 'bridge' is .... as in your case the cables and the wires going to each fuse , in the CDU, all seemed to be in order ... but there was a 'hidden' (vertical) wire bridging two outlets .

HTH

[daddyfreddy]

Cheers all.

Dave and Dennis, thanks for that. The crossing of the return legs didn't occur to me, I'll check that out and see. Hopefully it will be that simple and I won't have to find a bridge under the boards. I do have the place ripped apart and have access to some underfloor areas so if it comes to the worst I should be able to resolve it.

[John Rumm]

If it turns out to not be a simply swap of the return wires then there are still various tests that you can do that don't involve ripping things to shreads.

[Newshound]

I might be wrong, but won't the 16th regs "ring continuity" test find a bridge in this case too? See for example Whitfield's Guide to 16th Regs, page 187.

[meow2222]

Until youve got it solved, removing one of the 2 30A fuses will get you back down to a sensible 30A fusing.

NT

[daddyfreddy]

Just looked at it today and with the help of a multimeter it turned out to be just as you guys suggested, return legs the wrong way round. That's sorted now. :-)

Having the ceiling plaster near the CU down at the moment I have access to all the cables as they run out the CU and towards their various circuits. The electrician ran cables out from the CU up the wall and then all the cables are cut and joined at junction boxes again, all left in the ceiling void. This is all original wiring done by the housebuilder so I can only assume the builders wanted to render the cables from the CU over before the cables were run around the house.

For one of the ring circuits the electrician was either short of a junction box or just plain lazy and decided to use one junction box to connect up the feed and return legs. i.e 4 wires in one box. I've also resolved that now as well.