Insulation Reistance problems

pencilpusher · Oct 17, 2011

I have been to a job today for a CU change. Quick test between N and E gave the following results:

2 RFC on RCD - downstairs 4.5M ohms, upstairs 2.2M ohms. No RCD tripping problems.

Non protected side - downstairs lighting 13M ohms, the remaining circuits 300M ohms.

Installation is 10 years old. No apparent alterations.

Any ideas where to start looking re RFC's and down lights please. I know they are within regs (just) but I would expect results to be higher than these three.

Thanks

PP

malcolmsanford · Oct 17, 2011

Check for dampness anywhere perhaps there is an outside socket or light on the circuits. Perhaps there is a spur off the ring that goes to a garage/shed with again dampness.

pencilpusher · Oct 17, 2011

Thanks. Only one outisde light on lighting circuit. No outside sockets or spurs going outside.

malcolmsanford · Oct 17, 2011

I would still go with dampness somewhere,

Split the ring roughly in the middle and IR test both legs and see what that gives you. Did you completely remove the cables from the Earth/Neutral Bars?

pencilpusher · Oct 17, 2011

I tested the whole board initially but got low results so removed neutrals one at a time. On the non RCD side, when downstairs lights removed from neutral bar, I got 300M ohms for the rest in that bar and 13M ohms for the lighting (neutral removed and tested to whole earth bar) On RFC's I removed the neutrals and tested them seperately to whole earth bar. Should I test removed neutrals to removed CPC's?

malcolmsanford · Oct 17, 2011

Yes I would disconnect all 6 cables to each RFC and test, remember that Earths and Neutrals are commoned at the CU and you will never get the bottom of this if you have all sorts of variables in the mix.

Though I admire your dedication, as you point out these are acceptable, if low results, so how far are you going to go along with this?

pencilpusher · Oct 17, 2011

Thanks Malcolm. I will do that. I am thorough and I will do the job but I like to just delve a bit to see if there is a serious underlying problem here. Although I am not doing a PIR here as such, I do wonder what this IR may be in 1 or 2 years time.

I am also hoping that with the new board, the RFC's won't start tripping out the RCD. The RFC's are not at the moment but with my luck they will start to!

I take your advice on dampness but is it possible it could be an issue at a socket for example?

Thanks

PP

malcolmsanford · Oct 17, 2011

If you got these results on the L-N test then I would say that 95% you had a load still connected, but as it is on the N-E test then it's not a load, nor really can it be a dead short.

As you say the installation is 10 yrs old, so unless you have had some really heavy loads, or severely undersized cables unlikely that there is damage to the cables to allow the insulation to break down, so it is likely to be dampness somewhere, from either an outside accessory or damp from the walls.

Just go about it methodically, disconnect all the cables for the circuit to make sure they are completely separate from the others and test, then break the circuit down until you find it.