Fault on Ring final circuit

[AJB Temple]

Please advise what I should test / check next.

My usual qualified electrician who did all of the work here is in Ireland for 4 weeks and not contactable. I need to fix this asap. Ring final circuit tripping. I am not clueless and I have some test gear. Can fully isolate the supply with an isolation switch adjacent to the meter.

Circuit has worked fine until 2 days ago when temp wagos replacing an old socket were removed in a room being refurbed. This was purely to extend the wire by about a foot to get past new insulation. Not sure but I think the wire may have been pulled slightly during fixing new stud. The wire has since been stripped back about 20cm into the plastic conduit buried in the wall and no insulation damage can be seen. The ring has been made continuous at this point with Wagos (red to red, black to black and CPC to CPC) and carefully checked. Also tried it with a new socket. This is the only alteration to the circuit that has occurred.
Everything has been unplugged on the whole ring as far as I can tell.

This ring circuit serves about 25 double Hager sockets in 3 rooms of ground floor of house including the living rooms. Mainly used for plug in LCD lights and one LCD TV, so load is low. However, I don’t exactly know where the circuit goes as for example it also has at least one run to the attic (powers the internet router) and I expect it has at least 2 spurs. I have switched off the known fused spurs.

Circuit connects to 32A MCB on RH side of dual RCD Hager 16 way board installed and certified 2018 and checked 2022. However, wires to CU on this particular circuit are red/black and probably date back to circa 1998 when house was converted.

RCD trips if the ring circuit is connected to the MCB, or to another MCB, or to an MCB and different RCD on the other side of the board. MCB in off position. Still trips as soon as circuit energised. Cable is T&E 2.5mm with what looks like 1.5mm CPC but might be 1mm (see test results). CU wiring is not beautiful.

If live and neutral from this ring final are disconnected from the CU, (ie disconnect from neutral bar and MCB) nothing trips. We have other circuits in the house with RCBO CU’s and these are unaffected. These power the main load circuits luckily (eg kitchen and heating).

Using Megger MFT1711 (not in calibration), but leads checked and zeroed, full battery in tester, I have measured end to end resistance as follows:

r1 Live to live 0.96 ohms
rn Neutral to neutral 0.75 ohms
r2 CPC to CPC 1.9 ohms

I don’t know what these tested at when the CU was installed. These readings suggest to me that there is not a break in the circuit but I expected live and neutral readings to be closer. (I am not a pro obviously, but I have an engineering background in aeronautical electronics). rn being lower than r1 suggests a fault on the live circuit somewhere?

If CPC is 1mm then r2 is a ratio of 2.5 and a value of 1.875 (against rn) is close to the 1.9 ohms I measured for r2. If I do it against r1 then I get an expected very high r2 value of 2.4 ohms, which again suggests I have a problem with my r1 reading and fault on the live circuit?

Please advise what I should check next. Any advice and hints gratefully received. Thanks.

 

[AJB Temple]

CONCLUSION

(Couldn't see how to edit title)

It was not belting it down with rain today, so lifted the manhole cover. The pump is about 2 metres away from the socket and down a brick built chamber around 4 or 5 feet deep. It's fed from various storm drains and then pumped out to a large pond which then drains into a stream. Clean water only. But lots of it in a storm and it's a sunken terrace that can flood. The cable to the pump runs through a metal conduit (you can see where I am going here) underground just below the paving level but above the normal activation point for the float switch. The cable is terminated in a new 13amp rubber plug which goes into a new and good quality outdoor socket fitted by me. Put the megger on the socket and it tests fine. No water ingress either.

Took the plug off and pulled the wire through with a feeder twine. Cable from pump has been nibbled by something, probably voles (we have lots) about 1 meter into the pipe. They must have been determined as pipe is only about an inch diameter at most and had a thick rubber cable in it. Degraded insulation was shorting out on the pipe I expect. Cable replaced with new rubberised cable and pump tested. Runs fine, no trips. Plugged both ends of the cable pipe with silicone.

The lesson for me is that I assumed I was at fault and spent ages going over my work trying to find a non existent error. It was only when one of you guys (thanks very much PC1966) mentioned a fridge being connected, that I remembered the outdoor pump which I had forgotten was on that ring circuit (daft design - but I didn't do it). Pure coincidence that the pump feed shorting occurred just when I was working on something on the same circuit.