No access to loft Lighting Circuit tripping RCD

[mcummins7]

Hi folks,

I've got a job where an RCD is tripping out on the upstairs lighting circuit where the cable runs in the loft are not accessible (unless of course I take the roof off or the ceilings down - neither of which are really feasible or customer friendly)!

I've checked all the fittings and can't see a problem. I've removed all lamps but still trips out. I'll do an Insulation test on it tomorrow - if this fails, will that prove the cable has been compromised somewhere in the loft?

Any ideas on easier fault finding?

 

[Spudnik]

Not a lot you can do if there is a faulty cable, other than make holes in the ceiling to trace the fault.

 

[SirKit Breaker]

You could disconnect the circuit completely, identify the switch cables, IR the switch cables if all OK then do a temp lash up with new cables on the loop from room to room and see if it all works as it should, if so then start reconnecting the old cables one by one to see where the fault is. Good luck, and lets us know the outcome.

Cheers......Howard

 

[SDELEC]

If the insulation test fails it only proves there is a fault somewhere on the whole circuit. if you are testing from the cu downstairs then the fault could be anywhere between there and the loft.
i don't want to state the obvious but before you make too many holes try to isolate the fault down to as small an area / length of cable as possible, break down at a fitting mid point test each way , then breakdown faulty section mid point/ @ a fitting again if possible and test each way etc etc.The cables may be accessible /jb'd under the floor upstairs?
good luck, hope its a quick one!!

 

[BlueToBits]

Follow SDELEC's advice and you should be able to at least narrow it down, if not find it without any trouble.

You might like to look out for or try a few other things:
Pictures hanging nails above light switches
Pendant rose or fitting fixing screws that have peirced the cables above the ceiling
Do a low ohms (continuity) test L-E and L-N at various points along the circuit. If the fault is a direct short the lowest reading will be at the point closest to the fault.
The main supply cable from the consumer unit up to the loft is often out of zone as it passes behind the wall paster of the room above.
Use all you senses. Fault finding by smell is very underated!
There are other somewhat risky methods of finding such faults, I couldn't recommend them but they involve listening and looking for signs or arcing.
Good luck

 

[pennywise]

Persuade the client a new loft hatch would be cheaper in the long run rather than wrecking a ceiling if the fault is found to be in the loft space.

 

[bugsy]

Disconnect all the cables test each one seperately , that should identify the faulty leg , if its in the loft you might be lucky enough to pull a new one in by lashing to the old one . if not looks like a loft hatch/!

 

[mcummins7]

Thanks for your input guys. I'll check it out and let you know the outcome.

Thinking more about it now, "Bluetobits" comment regarding hanging hooks etc., slammed into walls may be the one, as they've hung an old five lamp WALL lantern by a big screw (time unknown) which is right above the wires coming out for the light fitting.

HOWEVER - these lights have been there for 15 years or so and the trip appeared to happen at 4am (known by the clock stopping on the cooker)!!

We'll see. I'll let you know......................

 

[mcummins7]

Well I found the fault chaps. Luckily the circuit cables had been run up to the light switches (don't know why?), and so testing each way was relatively easy. Found the faulty leg, lifted a couple of floor boards and found the fault in a cable running from a JB to a wall light which was wired into the circuit as it had a pull switch.

Job done.......