Fault on lighting circuit - What am I missing?

[rolyberkin]

I had a 10 minute look at an existing fault yesterday as walking out of a customers house as it was way past beer o'clock, the circuit is from a dual RCD board. Serves lighting in a loft conversion with balcony (outside lights) en suit, downlights shaver socket, bathroom fan and central pendant.

Tested IR which was very low at 13.44MΩ between joined live/neutral and earth and no continuity on earth and neutral and live conductors.

Energised circuit and everything was fine, then decided to test RCD and then the circuit tripped the RCD, both RCD's are ramp testing okay. Now continuity shows resistance between live/neutral and earth around 66Ω.

Thinking a nail through a cable/neutral fault somewhere but trying to work out in my head why it all energised okay and then tripped and fault re appeared during the RCD ramp test? Any suggestions or previous experience of this from anyone? (Going back monday morning!)

 

[buzzlightyear]

dead short capping possible , the best way of finding it to start dropping off the lights and switches same old drill.

 

[Midwest]

Sorry roly, being a bit thick. Can't understand your post. Circuit energises, but when does the RCD trip?

 

[buzzlightyear]

MID ,he had his beer glasses on before he looked that the job.
and ten beers after thinking about it .

 

[rolyberkin]

Sorry roly, being a bit thick. Can't understand your post. Circuit energises, but when does the RCD trip?

Tested it as the customer stated it was tripping and although low readings tested out okay. I then energised and all was okay, lights came on etc. I had moved the lighting circuit to the second RCD as thought the first may be dodgy so then ramp tested both RCD's. After ramp testing the RCD's the fault re appeared, definitely a neutral fault, just trying to work out the link between the ramp test and fault or just coincidence?

 

[Pete999]

Sorry roly, being a bit thick. Can't understand your post. Circuit energises, but when does the RCD trip?

When he energises the circuit? sorry I shouldn't

 

[Lucien Nunes]

Could be an ordinary intermittent fault i.e. something just about touching, varying between the two extremes you got. 66Ω could be something just barely touching metal-metal, or could be a fault in series with a load. If it's unstable, it's probably an intermittent, if it's stable it's probably through a load. In that case the 13MΩ may have been with the load switched off and 66Ω with it on. I don't see a direct connection with the ramp test other than that you energised the circuit.

 

[Wilko]

Hi RB - I'm blaming outside lights for lowish IR but a proper intermittent fault somewhere for the trip. Kind of lucky it faulted while you were there .

 

[littlespark]

Water in the outside lights.

Only way to find it is to systematically disconnect sections until the IR holds high

 

[rolyberkin]

Water in the outside lights.

Only way to find it is to systematically disconnect sections until the IR holds high

If it hadn't have been a Friday night and the beer wasn't calling me the outside lights would have been my next move!;-)

 

[buzzlightyear]

roly-poly, let us know how you get on .

 

[rolyberkin]

Will do, am over there tomorrow morning.

 

[rolyberkin]

Found an untraceable core of a three core and earth which had been earth sleeved and put into the earth bar, as soon as I removed it all the IR results shot up!? Then immediately found a neutral core which had been shorting on a back box, could have been either but by that time had lost the will to live and decided not to try to replicate it!

 

[bigspark17]

I dont think 13Mohms is low IR reading personally.. glad youve sorted it tho.