How to track down an intermittent electrical fault?

This is in my workshop which I built, and wired, about 30 years ago. There are 12 fluorescent light fittings in 4 banks of 3. These controlled by a 4 gang light switch. This afternoon the lighting mcb has tripped twice for no obvious reason. Never happened before. Power circuits unaffected. Only change I have made recently is to change a tube and starter, 2 days ago. All lighting cables are 1.5mm surface mounted and 2.5 - 3.00 M above ground. I have had a bit of a rodent problem recently but suspect that is not relevant. I can't track the problem down as I cannot make it happen. A bit worrying as I'm going away for a few days. The lights will not be left turned on.
Any thoughts please? Nick.
I do have an angle grinder and a multimeter.
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Trip it yourself before you go?
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On 30/04/2014 19:59, Nick wrote:

Test it by running with half the lights on for a while:
1111 = trips 1100 = ?A 0110 = ?B
The results of A and B tells you which circuit is defective.
Then run with just that circuit hot and see if it trips.

It might be relevant but I don't see how.

It might be - they enjoy nibbling grey or black PVC for some reason.

While you are away isolate the faulty lighting circuit at the fuse box or breaker just in case there is a charring rodent somewhere slowly approaching red heat. A live to earth fault will trip the MCB but if the critter is across live to neutral...
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Martin Brown
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When i first had pet rats, i found out they absolutely loved eating the scart cables linking the sky box to the tv in my bedroom, suddenly one colour would disappear and the little bugger had got to the juicy copper cores again,
I asked the vet if there was anything i could put on the cables to deter her from eating them, he said to try that bitter lemon stuff you use to stop dogs licking their wounds,
Big mistake, that stuff was like a marinade to the rat, tho it did keep the cables alive a little longer as she'd lick all the bitter lemon off before devouring the actual cable,
Someone then suggested washing up liquid, nope, rats like that too it seems, tho it did turn her turds green, and i am sure she farted bubbles for a while.
There is just something in the cable make up that is irresistible to rats and mice, some say it's the electric current in the cable that attracts them, but mine seem'd to prefer signal cables over mains cables.... Tho i will never forget when Nugget the rat chewed through the phone line to the adsl box, we heard a squeak and the internet died, i looked behind the sofa and watched him move a few inches up the cable he had just severed, and in one snip he severed it again.... with the accompanied squeak as the cable bit back, he sat and thought for a few moments, then moved double the distance up the cable and 'snip - squeak' again,
i'm sure if i left him to it, he'd reach the socket, then start burrowing to follow the cable back to the exchange.
Reminds me of this dilbert strip: http://dilbert.com/strips/comic/2007-05-30/
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On 30/04/2014 21:11, Gazz wrote:

Putting the item in a cat's bed/sleeping place does seem to repel the rat. There's an idea that fear of cats is an inherited/evolutionary response. May not make for a happy rat though.
Gareth
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On 30/04/2014 21:11, Gazz wrote:

We used to have a lot of bother with underfloor high grade coax cables getting eaten on low noise signal cables. They much prefered the cables to the rodent bait and traps set to prevent them living in the void.
I don't think it is the mains they are after but something in the PVC possibly the plasticiser. Usually when they bite though live they promptly stop working and after a while start to smell bad.
Parrots amazingly can get away with it albeit with part of their beak ablated by the flash as they cut clean through the "big worm".

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On 30/04/2014 20:17, Martin Brown wrote:

.. it'll be getting about 10v, which probably isn't enough to do anything (except trip the breaker)
Andy
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Well I would suspect that there is nothing to worry about if the lights are turned off. Probably a shagged ballast that will show up sooner or later.
Do the sensible thing and turn the power off to the lights at the CU whilst you are away.
--
Adam


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On 30/04/2014 19:59, Nick wrote:

Change the trip. They do wear out, and a new one will still protect the circuit. Its far worse when you have a fault and the trip doesn't work.
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On 30/04/14 19:59, Nick wrote:

IN my case changing a bulb didn't work Changing the 12v transformer didn't work but did reveal the fat dead mouse still gripping the mains cable...
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Ineptocracy

(in-ep-toc’-ra-cy) – a system of government where the least capable to
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faulty choke. It happens.
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From KT24

Using a RISC OS computer running v5.18
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On 30/04/2014 19:59, Nick wrote:

At switch on, or when running for a time?
Given the age of the installation, a plausible cause would be a ballast in one of the fittings is suffering insulation failure of some description...

Might have changed the load on the ballast slightly - could be enough if its on its last legs. You could see if you get a repeat trip with that tube removed.

Insulation resistance check from L+N to E might be a useful test.
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John.
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On Wednesday, April 30, 2014 7:59:15 PM UTC+1, Nick wrote:

You've most likely got an insulation problem from L to N or L to E somewhere. Things that could help narrow it down are: - insulation test L+N to E - insulation test L to N with all tubes removed - disconnecting half the fittings (or some) and see if it still trips or not, repeat to narrow it down further each time
NT
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