Problem with domestic ring ,continuity ok IR tests not

[Roger]

Advice please

Ok I have RCD protected ring main on a 32 amp MCB repeating tripping so first of all established no continuity on CPC repaired that easy to find with wander lead.Thought I had found the issue as well as the conductor had broken and was touching the live.Well it looked like that when I pulled the socket off.
Reconnect at CU and still tripping.Split ring and figure out what's on it and route it takes.Started doing IR tests on splits and found 1.05Mohms between live and CPC.NE and LN fine,full range.
I then took off every socket fascia and found 2 with the earth mechanism broken.Carried on doing IR tests on all split legs and now have 2 legs with poor readings always between live and CPC.One leg between a socket and another leg from last socket in ring to CU.This now kills 3 sockets !

Its a lovely finished house ,fully tiled floor ,fitted kitchen with marble everywhere,start pulling this lot out will be a nightmare and not practical.

Why would this happen all of a sudden ?

I can get a majority of the sockets to work but its a 20amp Radial now !

Brain is frazzled,too much work and not enough time.

 

[Richard Burns]

You do not say what is tripping,is it the RCD or the MCB?
So far from what you have said there is no result that would cause either to trip.
Therefore what do you think is causing the tripping?
If this is repeated tripping rather than a continuous trip it is more likely a connected appliance, check for FCUs from the ring to outside lighting for instance.

 

[Roger]

Its the RCD that is tripping,I have checked everything that is on the circuit.Pulled the kitchen apart ,cupboards bare and still it tripped.My search was pretty exhaustive so sure i had everything known as to what was on that circuit.They have some built in cupboards which might have something behind them i guess.
I asked whether the had had any recent work carried out - no,any picture hanging (the old favourite) been to a number of these-no.

When I connect one leg between sockets that caused it to trip and there is nothing in between that I can see.
With the low IR reading I thought that there was a short between live and earth on that particular leg.I have one other leg that also shows a terrible reading.
All other legs between sockets all go to over range on my megger.
Install about 7 years old ,2 mk 10 way boards pretty well spread.RCD that trips covers 4 circuits and it always the same circuit that makes it go.
Funnily enough went to a summer house today with a tripping RCD as well,quick look nothing obvious.loads of GU10 lamps 8 in a 10 ft by 8ft room ! as its a posh shed no fire rated enclosures and cheap crappy light fitting all had earth tail/lead not connected just left loose.
Ex show model so it was cheap !

Appreciate your help.

 

[Richard Burns]

Very difficult to fault find over the internet and so I could be wrong but these are my thoughts.
For the RCD to trip there must be a difference in current from L to N, normally a leakage to earth.
The maximum resistance to earth for the RCD to trip would be 0.008 Mohms and you are very far above that so I would not be thinking that this was the problem. If you were to test the continuity between L and cpc with the circuit fully assembled (but not on!) you would be looking for a <8000ohm resistance for there to be a shorting problem that trips the RCD. You might even find it if you were to do the figure 8 testing on the ring.

The most likely reason (with the IR readings you are getting) is that the circuit has been cross connected to another circuit somewhere, try a continuity test between the ring neutral and neutrals of the other circuits.

Alternatively have a look carefully at the cables where they come into accessories and ensure that there is not any scraped insulation that will contact a back box / cpc once it is in place. boxes with no grommets, etc.

1Mohm is a bit low on the cables as well so is there a possibility of rodent attack behind the wall damaging the cables or are they all plastered in place?
Again it may be just damaged insulation at terminations.

Good luck!