Wiring regs re Consumer units?

[tarquinlinbin]

Hi all, I have an old Wylex metal clad consumer unit,the one which has the circuit breaker incomer over on the right and a number of rewireable cartridge fuses on the left,in this case.6 way i think. Sometime in the past the rewireable carts have been replaced with MCB;s,which are also a few years old now,they are the ones with pop out buttons instead of flipper switches. Anyway for years we hav had occasional probs with the main CB flipping off. It might go a year without doing it then again it might go twice in one month and then not agian for another year!. Its totally random and attempts to isolate any fault have drawn a big zero.

I did have plans to whip it out and fit a CU which had an isolator switch incomer and then 6 MCBs. The house is only about 25 years old and all the wiring is fine.

I happened to chat to an electrician who i bumped into the other day and he said that you must now fit a split load CU and that my plan was not permissable.

He did say it might be possible to trace a fault and also test the existing CB incomer. Quoted £250 for replacing existing with an MK split load unit..

Any comments please?

jo

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[Lurch]

On or around Sun, 30 Oct 2005 18:35:50 +0000, tarquinlinbin mused:

There is a fault somewhere in the wiring, removing pretective devices is not the way to remedy such problems. You need to test the wiring to find out what the fault is, or have someone do it for you as I'm assuming that the reason you haven't done so is because you can't.

The guy is an idiot. You have gone about the job the wrong way and he seems to have gone about explaining the situation in an equally incorrect, but entirely different way. The most common CU to be installed is a soplit load but it doesn't *have* to be a split load CU.

That's where I would start. Get a test\faultfinding thing going before deciding what to do, although I would be inclined to replace the CU anyway by the sounds of it. The chances are though there will be other issues that require attention, such as earthing etc...

I wouldn't let him near the place myself, but that price isn't too bad. I wouldn't turn up and just fit a CU though as there is a known fault with your wiring.

See above!

[Andy Wade]

I take it the "main CB" means a whole-house RCD (or older ELCB) between the mater and the Wylex CU? Can you clarify which sort it is, or provide a photo? ISTR that you're in Manchester, so I guess you're not on an overhead supply.

Well there's no evidence that there's 'owt wrong with the CU, but there is evidence that there might be an intermittent wiring fault, or appliance fault. The RCD must be tripping for a reason. If it's an old ELCB then the trips could be due to external factors outside your installation.

He's right, in effect. The whole-house RCD/trip is no longer permitted, and you must have RCD protection for sockets likely to be used to feed portable equipment outside. A split-load CU, with the sockets on the RCD side is the usual solution, but not necessarily the only one.

I'd attempt some fault finding first - starting with insulation resistance tests on the fixed wiring, then moving on to appliances, particularly fixed appliances such as an immersion heater.

£250 for a CU replacement is cheap, but probably won't of itself cure the underlying problem, although it could mask it if the fault is then on the non-RCD side of the new CU. I'd concentrate on trying to isolate the fault first.

[tarquinlinbin]

Yes its the older ELCB unit,built in to the right hand side of a gun metal grey metal wylex cu. I live in an urban area,

This problem has occured for a number of years.As i say,it isnt a regular thing it can flip 3 times a year or not at all in any given year!. I have done various tests to try and isolate it. On one occasion i shut off all the individual circuits on the mcbs except for the sockets (to keep the fridge going!),and went away for the weekend. It had tripped when i got back. At that stage there was nothing else plugged in as i made a point of unplugging everything so the only thing drawing power in the whole house was the fridge. That is fairly new. It replaced an older one. When we had the old fridge it also tripped occasionally!.

It seems then that by a process of elimination, Assuming there IS a fault it must be on the ring main circuitry.

I remember in a previous house i had,it had exactly the same model CU. I had just moved into the house and it was a new house at the time. I had my new electric cooker connected and i found that when i turned too many rings on the cooker on,the ELCB flipped off!. A quick call to the builder and an electrician came round and swapped the ELCB unit only,and it worked fine from then on.

I suppose my only option left is to unscrew every wall box and check the wiring. The ELCB unit is probably no longer avaialble for replacement on its own.

Unfortunately i am only equipped with a good electrical knowledge and a high qulity mutlimeter. I dont have such things as ELCB testers/meggers etc.

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[Dave Plowman (News)]

Trouble is that 'isolating' a circuit via an MCB doesn't actually isolate it as regards an ELCB.

[Mike Harrison]

RCDs can also trip due to external causes - a couple of times the underground cable in my road has gone up in smoke (literally) and has tripped my RCD in the process.

[meow2222]

If there is a wiring or appliance fault, which is the most likely, replacing the CU will mean replacing the ELCB with an RCD, and these are far more sensitive, so your occasianl trip will probably turn into a cant get the power to stay on at all situation. And no he isnt allowed to put the old ELCB back. So he's setting you up for a bunch of extra spend basically.

An ELCB is not hard to test. They must trip when the voltage between their 2 earth terminals has reached 50v.

3 things can cause ELCBs to trip:

- fairly heavy leakage fault in wiring or appliance, easy to find with a multimeter, and normally the cause.

- placement of earth electrode too close to another installation with a major earth fault.

- faulty ELCB.

You've got both ELCB and MCBs, so there is no reason at all to replace the CU. Other than profit.

NT