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.

 

[mainline]

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. 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) I have measured end to end resistance as follows:

Live to live 0.96 ohms
Neutral to neutral 0.75 ohms
CPC to CPC 1.8 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).

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

The socket outlets may not be double pole, so wouldn't disconnect the neutral, so if you have a N to E fault it could continue to trip even though the outlets are turned off.

The first thing would be to disconnect every appliance and try again.

Disconnect the ring from the CU and Insulation test the wiring.

Split the ring at a near mid-point at one of the sockets and insulation test again work your way around the failing side.

 

[AJB Temple]

Thank you. Not sure I understand. There is nothing plugged into any socket. Live, neutral and CPC are all disconnected currently from the CU on both sides of the ring. r1 is 0.96 ohms vs rn at 0.75ohms, which is more than suggested 0.5 ohm tolerance (as I understand it). As r1 is higher than rn, and as r2 gives an expected reading agaisnt rn, doesn't this suggest a defect on r1? Maybe a bad connection somewhere?

I expect I am bing dim here, but the only thing that has been touched since the circuit was working, was the one socket part of the ring (on wagos) being further extended from the wall. I have checked those wires, stripped them back a bit to give clean connection with wagos, and there is no visible fault.

 

[westward10]

I agree your end to ends seem askew but not the likely problem, as suggested you need to do an insulation resistance test.

 

[davesparks]

r1 is 0.96 ohms vs rn at 0.75ohms, which is more than suggested 0.5 ohm tolerance (as I understand it).

In the text book and the classroom that is indeed an issue, in the real world however it barely worth raising an eyebrow.

Maybe a bad connection somewhere?

Probably a connection with a fractionally higher resistance, which you could spend a bit of time chasing down and improving, but it won't be causing the tripping problem.

The first steps to finding this kind of fault are pretty simple.
1 unplug everything/isolate all permanently connected loads.
2 IR test the circuit to establish a baseline reading of the fault.
3 remove a socket approximately halfway around the ring and desperate all conductors, IR test each leg at the CU and establish which half of the ring has the fault on it.
4 identify all sockets on the faulty half of the ring and further split that into 2 halves again, test each of these sections.
5 continue narrowing it down until you have either found a socket with a fault or a single section of cable with a fault.

Once you've done this you can then work out how to repair it.

 

[DefyG]

You need to do an insulation resistance test (Do you have access to an insulation test meter) L-E, N-E (Mohms) rather than a continuity test.
As it is tripping without an appliance plugged in it would indicate most likely imo a N-E fault.

Splitting RFC as previously described by others would narrow it down.

 

[westward10]

I would try a continuity test between the cpcs and neutrals you never know.

 

[AJB Temple]

Thanks all. The real world vs "textbook" is something I have no experience of in this field. The meter I have is a Megger MFT1711. I also have some Kew sockets that I can fit to sockets and plug the Megger into. To a degree I am having to gain experience in how to use this meter as I go along, being super careful, but I am sure it will do resistance tests.

My initial thought was a N-E fault so I was surprised when the r1 was higher than rn. The wiring in this house is a real jumble. All the new parts are perfect, but the old part of the house has been thoroughly DIY'd by the previous owner and his son.

I will get back on it first thing in the morning.

 

[DefyG]

I would try a continuity test between the cpcs and neutrals you never know.

Agreed as that is what an IR test is doing, testing for any continuity between L-E, N-E.

Just trying to say End to End continuity tests don't help too much in this scenario.

 

[westward10]

The rn may be lower due to a fault to the cpc.

 

[westward10]

Agreed as that is what an IR test is doing, testing for any continuity between L-E, N-E.

Just trying to say End to End continuity tests don't help too much in this scenario.

Totally agree and when I get a "flat" IR I immediately switch to continuity.

 

[DefyG]

The rn may be lower due to a fault to the cpc.

It could but not as conclusive as an IR test.

 

[AJB Temple]

I will do an IR test. That is helpful.

My amateur logic suggested that CPC was unlikely to be at fault as the resistance r2 was as expected vs rn.

It now transpires that although the CU has upstairs sockets on a different RCD to downstairs sockets according to the labels, the reality is that some upstairs sockets at one end of the house are out of action as well. CU clearly not labelled correctly. I think that the socket which started all this off, goes directly up to that area and it seems very likely to me that I will have to excavate that wire from the wall and conduit

 

[littlespark]

Cables don’t generally break down for no reason… so the most likely issue is with a connection in the back of a socket, OR recent works have damaged a cable… any new things been screwed to the wall above a socket?

In the OP, it suggests that the circuit has been moved between mcbs and onto the other side rcd…
You may have left a neutral on the wrong N bar,
Each rcd supplies a bank of mcbs AND the corresponding neutral bar

You do say it’s a bit of a mess and you may have missed it.

 

[AJB Temple]

Thanks liitlespark. Yes, I did move the entire ring across the CU to a different RCD to help eliminate RCD and MCB issues. I moved all six ends. It just tripped that RCD so very unlikely to be an RCD or MCB issue I felt. I had a spare 32A MCB on that side that I knew worked. All six ends are now disconnected from the CU so that I can have everything else powered up. I am very clear about which wires are on each side of the defective ring inside the CU. I am not a domestic electrician so I spend a lot of time double checking everything, including proving dead every time I do anything (I am working on my own).

Yes, studwork was put up in the room where the fault first emerged. I have added 150mm to 200mm internal insulation and panelling to the whole core of the house. The stud was actually fixed to an adjacent wall but tomorrow I will remove the panelling in the area and check that there has been no screw or nail gun penetration into the conduit. It's unlikely (I did all the studwork) and it is 200mm away from where the conduit is buried in the wall. However, I will check it in daylight. Thank you.