Something Fishy, What To Look Out For?!

[Sam Roberts]

Hi everyone,

I'm no test guru these days due to way too much time doing simple generator field supply work. I recently went to a new customers house and I'll admit I was a bit hasty with my diagnosis. They had been there for around 6 months and the cellar sockets have started tripping, damp springs to mind, I look behind a few sockets and they are corroded and have a layer of moist dust on them. The L-E on this circuit was about 0.4 MOhms so I call it a day and return with new sockets and a mind to simply strip back to some fresh conductors as there is loads spare and loose behind the dry lining boxes.

I return, do the intended work and test and the reading was 2.3 MOhms, pass but I was obviously compelled to improve this. The back room with the corrosion is underground and much damper that the other two rooms on the same circuit which have the walls exterior in open air. Sloping ground, back room underground and front not if that makes sense. So I have a look behind a few sockets and find an earth from one leg of the ring behind two of the sockets intentionally removed as they both had a terminal block on them. (Has anyone seen this before?) This gets me worried and it's blatant foul play. I connect the earths and re-test and lo and behold the reading drops to 0.04 MOhms. 

I only had a few minutes left and carried out a few tests. With the earths of this cellar ring disconnected from the board the L-E(earth bar in cu) was 0.8MOhms . There was also a reading of 85v in the earth of a lighting circuit.

This is where I ran out of time and simply condemned the circuit until I return.

Has anyone seen this sort of problem before and if so what should I be looking out for on my return for a full test and insp?

Thanks in advance, Sam.

 

[NozSpark]

TBH it sounds like there might be a few problems with the socket circuit.... one being damp,,, but there might also be damage to a cable or a very poor joint somewhere,,,,, you'll need to split the circuit down and find the problem(s)

As for 85V on the lighting circuit CPC.. (wrt N I guess?),,, there'll be a break in that CPC somewhere and the bit that you're testing is floating due to induced voltages

 

[ProDave]

You need to test each circuit individually with the CPC disconnected from the bar in the CU.  Only then will you know which circuits are good and which are bad, and hence were to look for your problems.

With damp like that I would expect poor readings on several circuits, light switches would be the next to look at.