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]

You had me worried there so I just went and removed the panel in that section of wall (which luckily was not glued to the stud - 2nd sight saved me). However, there are no nail or screw penetrations into the wall where the conduit is buried. But I can't help thinking the problem lies somewhere in that section of cable.

 

[westward10]

Do an insulation resistance test.

 

[mainline]

You had me worried there so I just went and removed the panel in that section of wall (which luckily was not glued to the stud - 2nd sight saved me). However, there are no nail or screw penetrations into the wall where the conduit is buried. But I can't help thinking the problem lies somewhere in that section of cable.

Only testing will tell for certain.

Some like to think and some like to do, the ones that do both are the ones who fix it.

 

[AJB Temple]

I will definitely fix it. But please remember I am an amateur at DIY electrics and to a degree have to learn (courtesy of YT vids from GSH etc) as I go along. Hence it takes me ages to do what you guys would regard as obvious.

Naturally I want it all to be safe. Obviously I will get my electrician to check it when he gets back in February.

 

[AJB Temple]

OK. Here is what is happening to me. I have disconnected all six ends from the CU as I don't want to test the whole board, just this one circuit. Neon bulbed spur is switched off (this feeds an outside socket). Nothing is plugged into any socket as far as I can tell. (Do I need to worry about switch positions on the sockets?)

MFT1711 has green and red leads plugged into same colour sockets, leads checked for OK and zeroed. Left dial set to 500v in the red zone. Right dial is on 30 in the yellow (but doesn't matter?) "Air test" shows 999 as expected. Tested together and I get a 0 (dead short) as expected so test leads OK and meter working.

Dividing the ring circuit into Left and Right I get:

Left side
L-N 0.00 M ohms
L-CPC 0.15
N-CPC 0.15
L+N-CPC 0.14

Right side
L-N 0.00
L-CPC 0.17
N-CPC 0.15
L+N -CPC 0.15

This suggests to me that I have a dead short on the live side somewhere? Is that correct? Even though L-L continuity seems to be OK as testing at o.81 ohms today.

 

[Timbo]

Did you split the ring at the furthest point into 2 sections, or just separate at the CU?

 

[AJB Temple]

Doing this in breaks from work so a bit slow.
No - so far I have just split it at the CU. I don't know where the furthest part is really as the circuit seems to run through part of the upstairs. However, I will now remove a socket some way away from the CU and do the tests again. I presume I just leave the wires disconnected in free air at the socket point.

 

[pc1966]

Right side
L-N 0.00
L-CPC 0.17
N-CPC 0.15
L+N -CPC 0.15

This suggests to me that I have a dead short on the live side somewhere? Is that correct? Even though L-L continuity seems to be OK as testing at o.81 ohms today.

The L-N of near-zero on IR could be a fault, but it could be some load you have not found to disconnect (e.g. fridge with socket behind it).

The IR of below 1M on either L or N to E is very suspicious. With it is your fault, or you have some device with SPD included (such as a protected extension block), or there is an old RCD socket that has some internal path to trip it on L-N reversal.

First step is to search again and unplug / switch off everything on sockets or FCU.

Second step is to repeat IR at 250V, if now high you have SPD, if still low a fault (or odd RCD socket)

Third step is to find somewhere on the RFC and break L and N (you can leave CPC connected. Repeat the left/right test and see if only one "half" is now faulty. Rinse and repeat the socket break position until you pin it down. Probably in the region of recent work!

 

[AJB Temple]

Thank you. That is really helpful.

There is no chance of a fridge or similar being connected as all that stuff runs off a totally new RCBO CU with totally different feed from meter. But, as I don't know exactly where this circuit runs, it is possible that there is something connected that is hidden that I don't know about.

 

[AJB Temple]

Ah. Brainwave. Thanks for the prompt, I have just realised that the circuit must feed an outside socket that has a rainwater sewer over flow pump connected to it. Float operated industrial grade motor. Also neons in the switch. Will unplug that and test again.

 

[AJB Temple]

New results on full circuit

Left side
L-N 0.01 M ohms
L-CPC 42.0
N-CPC 15.1
L+N-CPC 29.0

Right side
L-N 0.01
L-CPC 13.7
N-CPC 29.8
L+N -CPC 29.1

Bemused. Will break the circuit at a socket now.

 

[westward10]

No need for line to neutral IR tests. As it stands those readings are above minimum limits.

 

[AJB Temple]

OK, so what am I looking for?

 

[westward10]

Connect the cpcs back to the earth bar then IR test with all four line and neutral conductors connected together to earth.

 

[timhoward]

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

This post showed you had acceptable end-to-end readings.
So actually there isn't very much to be gained by doing the same test at both ends of the same wires that have been proven to be acceptably continuous!

L+N-CPC 29.0 (presumably MOhms)

As already explained above, the L+N to E results are adequate right now and would not cause an RCD to trip.
(In general <1Mohm is inadequate, <2 Mohm should be investigated. )

So splitting the circuit apart at a socket isn't required and won't help you. (If you had got 0.xx then breaking the circuit apart and trying each half would be the start of tracking it down)

The reason the next step suggested above is to connect the CPC's to the CPC bar is that it might be a fault to a water/gas pipe or something else that is bonded or has a low resistance to earth.
So connect the CPC's back up, and test L+N (either end!) to the CPC bar and see what you get.

If that is also a reasonable reading then you have the joy of an intermittent fault, where it depends on weather, timing, movement, or simply some equipment not being connected right now.

Report back and let us know!