Lighting circuit not working

[PB1]

Hi all
I went to a customer this afternoon to look at downstairs lighting circuit which was not working. It's a 4 way wylex board with a resettable 5amp breaker for the lighting circuit. This was off and would not reset. Apparently the fault occurred a few days ago when owner switched on light and nothing happened, no bang, just not working.
They had a new boiler fitted two weeks ago. Most work was done in former airing cupboard upstairs, and in loft area accessed from bathroom, which is adjacent to boiler cupboard. It was just a new boiler with wireless controller.
I removed all loads and did IR readings and got 0.00MOhms between line-neutral, line-earth,neutral-earth. Did continuity test between line-neutral and got just under 4 ohms - presumably the resistance of the circuit that was short circuited at some point.
I've checked all pendants and other light fittings including some dodgy looking wall lights, and disconnected them one by one but IR readings stayed the same.
I put my head up in the loft area where some new pipes have been run to see if i could see any damage to cables and saw lots of fairly recent looking droppings - I think they were too big for mice, so probably rats. the house owner said they had had rats a couple of years ago, but had got rid of them and hadn't heard any scurrying of feet recently, whereas that was what alerted them before. as their house is now nice and warm with the new boiler they wondered if that might have attracted rats back.
I've left it with them for now and they are calling in a rodent control company before I do any more work.

I've not come across rodent damage to cables before although I've read about it on here. From your experience do you think this is a likely cause of the problem? When I go back is it a case of just visually inspecting the cables to look for damage?

Thanks for any help.

 

[Snapester]

Sounds to me as though its been disconnected or similar fault, i have seen rodent damage and you don't usually loose the whole feed, i mean it would trip but on testing you would get continuity at the pendants if you get what i mean. As the rat would fry and circuit would trip!
What readings did you get doing R1+R2, was there any continuity throughout the lighting circuit? From what i'm reading it sounds like there is no feed to the circuit at all? You need to trace which leg has the fault on it.

 

[hasel5]

Wtf is this aboutv

 

[Richard Burns]

There could be a whole number of causes for this fault, but rodent damage could be one of them. Yes look for signs of wear on the cable normally a series of indentations penetrating to varying depths, the easiest sign is a dead rat with the cable clamped in its jaws!
Rats tend to gnaw at PVC to wear down their teeth and can easily cut through to the cores over time.
Do not handle wet cables if you have a cut on your hands just in case of the remote possibility of weils disease and always wash your hands afterwards.

If none of this works then start to break down the circuit into sections and test, it is slightly surprising that all the cores are shorted so could just be an over twisted/heated cable somewhere.