How to test integrity of a mains cable and MCB ratings

[Richard]

Hi, I have an outhouse powered from its own MCB on my consumer unit via a 6mm2 3 core cable that has been laid throughout a new extension etc to an exterior grade fusebox in the outhouse. The outhouse has one double socket running off a 32A MCB from which I run a washing machine and a tumble dryer off. The lights in the outhouse run from the other MCB (6A) in the fusebox. Recently I have been experiencing occasional unpredictable trips off the outhouse fusebox. I presume this means the fault is downstream of this fusebox (i.e. in the outhouse) and is nothing to do with the long supply cable. However, I would still like to test the integrity of the cable asap - just in case.

How do I go about doing this easily? With a mutimeter? If so how?

Also, regarding MCBs - what should the one in the main consumer unit be for the set up described above? Do you simply need to add the two together - i.e. get a >38A one or is there a different recommendation for calculating such figures?

If I wanted to extend the 32A circuit into the adjacent shed, and I wanted to run a few power tools off it, could I do this from the outhouse circuit (as if adding more sockets to an ordinary radial circuit).

If I ran a cable to another exterior grade fusebox in the shed (from the outhouse 32A circuit - for isolation purposes) with a 32A MCB, what would the MCB in the outhouse fusebox and the consumer unit now need to be?

Many thanks for any ideas and your time.

[Andy Wade]

What is tripping? Does the outhouse consumer unit have an RCD as its main switch and is it that that's tripping? Or is an MCB tripping, or the MCB at the house end?

If it's an RCD tripping I'd be suspecting appliance faults or excessive earth leakage in the outhouse first.

You can check for dead shorts or opens with a multimeter, but for proper testing you'll need an insulation resistance tester, low-ohms meter and a loop tester. Test procedures are in the IEE On-Site Guide (OSG) and in GN3.

There are several considerations here - maximum demand on the distribution circuit, fault and overload protection of the cable and discriminative operation of the protective devices. With your set-up the max. demand is likely to be no more than 21 A - 20 A being the usual assumption for a double socket plus, say, and amp for lighting. It sounds as if the 32 A circuit and 6 mm^2 cable are adequate (subject to voltage drop), but I'd want to reduce the 32 A MCB in the outhouse to a

20 A one. (Even then you won't necessarily get discrimination if there's a dead s/c fault on the socket circuit.)

How far away is the shed from the outhouse and by "run a few power tools" do you just mean hand held power tools of up to a few hundred watts used one at a time? - or bigger machinery?

One option would be to add another 16 or 20 A socket circuit on the outhouse CU to feed sockets in the shed (any lighting could be on a fused spur from this, if you want to keep things simple). Obviously all sockets here should have 30 mA RCD protection.

Diversity is allowed and 32 A may well still be enough, but we don't really have enough info at this stage. Keep the MCB protecting the shed sockets to the smallest value consistent with the intended loading - i.e. 16 or 20 A, not 32.

I feel you may be somewhat out of your depth here and should consider using a qualified electrician. And we haven't go on to earthing arrangements yet...

Also see

[Dave Plowman (News)]

To test all the parameters of a cable requires expensive test equipment - which any decent electrician should have. But beware that if it is a long run the earth might fail those tests due to too high a resistance.

Simple addition. But there are different types of MCBs - the time taken for an overload to trip them as well as the current rating.

[Richard]

Thanks for your help...inline answers...

Correction to what I said earlier.. There is a 40A RCCB, one 32A MCB and one 6A MCB in the outhouse. The RCCB is what is tripping (only occassionally as I said). House end doesnt trip at all.

I think that is the problem personally, but wanted to check the difficult to replace item first!

Sounds difficult! May need a spark ...

6m away - I laid 10mm2 TW&E just in case previously, hoped overspeccing would help. Just hand tools - no big stuff (and lighting)

Thanks again!

[John Rumm]

The 40A bit of the rating on your RCCB (aka RCD) will just be it maximum current rating. There should also be a trip current specified on it. Frequently this is 30mA (or 0.03A).

You may find some information here to help work out what is going wrong:

> How far away is the shed from the outhouse and by "run a few power

That ought to be adequate ;-)

[Richard]

Thanks :-)

Looked at the website - could well be the damp outside. If I wanted to reduce the effect of damp without too much expense, what would be a reasonable swap for the existing socket. Also what is the best conduit to run the cable in - i've seen metal and plastic, round and square and cant decide! Metal boxes and conduit looks the most common used for workshops/outhouses in my experience - any thoughts?? Thanks.

[Andy Wade]

What makes you think the socket is causing the RCD-trip problem?

6 and 10 mm^2 T&E cables will go in 25 mm PVC conduit OK, for short straight-ish runs, with one or two bends at most. (Use bends, not elbows.) 20 mm conduit is too small, and I doubt you'd want to use 32, 38 or 50 mm to cope with longer or bendier runs. It's OK to use PVC conduit outdoors, but drill drain hole(s) at the lowest point(s) so that any water that enters can escape. In my view T&E cable isn't suitable for underground work unless in a continuous watertight duct - something which PVC conduit is not. Stick to SWA for the underground sections, as others have advised. It isn't difficult to use, nor is it particularly expensive.

20 mm PVC conduit would be fine for internal wiring of a typical DIY workshop, using either T&E cable for simple stuff, or singles for more elaborate wring.

Steel conduit is obviously tougher, but is much harder to work and you'd need to hire a bender and threading tools.