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.

 

[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!

 

[pc1966]

L+N -CPC 29.1

Bemused. Will break the circuit at a socket now.

It looks like the pump you just disconnected. Test it separately and see if low IR to E, most likely water in somewhere!

 

[pc1966]

While it is possible to get a L-N fault that does not involve the CPC, it is hard to do with T&E cable due to its physical layout (CPC in the middle) and at accessories it is usually a fault to the back-box (cable nipped by screw sort of thing, missing grommet allowing cable-box cut, etc)

Do a continuity test L-N and see if it is low enough to trip the MCB side (so under 5-ish ohms) or it is tens of ohms or more like a small-medium motor waiting to start.

 

[AJB Temple]

This is so confusing! You guys are so helpful.

Right, I have connected CPCs back to the earth bar in CU. Put both live and both neutral from the ring into a 5 way wago, with a new wire out from it to test. Put green lead from Megger on the CU earth bar and red onto the lead from the wago connecting ring together. Insulation resistance test on that gives 35.8 M Ohms.

 

[pc1966]

Right, I have connected CPCs back to the earth bar in CU. Put both live and both neutral from the ring into a 5 way wago, with a new wire out from it to test. Put green lead from Megger on the CU earth bar and red onto the lead from the wago connecting ring together. Insulation resistance test on that gives 35.8 M Ohms.

That is good.

As a general rule, you always do IR testing with the CPCs connected to the MET (earth bar) so a fault to external metal / true Earth (e.g. screw through cable) is seen even if it is not touching the CPC conductors.

 

[pc1966]

Now check the pump!

 

[AJB Temple]

Right. Checking the pump is not so easy as the inspection hatch is a two man lift, as it is an inspection cover filled in with stone. However, I have reconnected the ring main back into the CU temporarily using Wagos and it has not tripped so far.

 

[pc1966]

Right. Checking the pump is not so easy as the inspection hatch is a two man lift, as it is an inspection cover filled in with stone.

Can't you IR test at its plug?

However, I have reconnected the ring main back into the CU temporarily using Wagos and it has not tripped so far.

That is encouraging!

 

[timhoward]

Right. Checking the pump is not so easy as the inspection hatch is a two man lift, as it is an inspection cover filled in with stone.

Do a quick L-E and N-E test using the clips on plug pins.

 

[davesparks]

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

They aren't, post 26 states line to neutral is 0.01Mohms

 

[pc1966]

They aren't, post 26 states line to neutral is 0.01Mohms

I think we all suspect that is not really the cable alone, and something is still connected. But the L/N to E are now good with the pump unplugged, so the culprit would now appear to be cornered awaiting the electron police...

 

[littlespark]

USB socket, or an actual fault within a socket itself.

I'm not really following everything that's happened today... just throwing another couple ideas in there

 

[AJB Temple]

OK. Thanks for all the help. I can't test the pump right now because the wire goes through an underground tube between the plug (to external socket) and the pump itself which is 4ft deep in a manhole inspection chamber. It's a set up I inherited with the house. It's chucking it down with rain and I feel unsafe doing outside electrical tests in the rain and dark.

With the pump unplugged, nothing is tripping. I will look at it properly tomorrow. But this big ring circuit is apparently now working OK. The fault appears to be entirely coincidental to my work on moving a socket in the room being refurbed. Sods law I suppose, as this led me to focus on the work I was doing, which it seems with nothing at all to do with the actual fault.

I am very grateful for all the help I have received from you all here on this thread. I know I've been slow to work through everything as I've had to

 

[pc1966]

I think you have been unlucky to have recent bad weather at the same time you were doing work, so a bit of a confusion. Sensible to wait until you can safely investigate the pump and see if it is recoverable/repairable or needs replacement.

Your r1, rN, and r2 values are a bit odd but 0.21 of a difference in L & N is not your biggest problem just now (though ideally less than 0.05 ohm). Most likely there is one poor joint/tarnished terminal behind it so if you have plans to refresh the sockets then you might find/fix it automatically doing that.

 

[DefyG]

I can't test the pump right now because the wire goes through an underground tube between the plug (to external socket) and the pump itself which is 4ft deep in a manhole inspection chamber.

You should be able to test it from the plug/socket. I doubt if a submersible that accessing will help unless it needs replacing.

 

[AJB Temple]

Thanks everyone. Was cut off in mid sentence as I suddenly felt ill and have been asleep. Anyway, thanks for the patience and help. I have learnt quite a lot. Mainly that I know less about domestic wiring issues than I thought! The guidance here has been very generous.

I am in the process of doing up the whole house, which is an old barn conversion, and replacing sockets as I have gone along. All new circuits have been either installed by a pro (the one currently in Eire) or by me and then checked by him.

I was unfortunate as said above that just as I was replacing sockets in a room (they all needed to be moved outwards due to panelling being installed) and so old brass sockets were switched to new SS Hagers, the fault occurred. At first it was a slow fault - in which I could reset the RCD and then it would trip after a few minutes. Then after a while would not reset at all. I just assumed for a long time that I had created the problem, even though I have replaced hundred of sockets in my time and am very meticulous.

The pump is a Draper unit and must be at least 10 years old. I fitted the outside socket a year ago, replacing an old one that was there before, and it was a quality unit and tested. The submersible pump is crucial and I have a spare at hand and will install that tomorrow.

The fault has done me a favour as it has made me trace the circuit round the house, and I now know it goes from the socket where I had been working, up through plastic conduit to a large bedroom where it feeds one single and two double sockets, branches off as a weird spur to a dressing room with two double sockets, then back down, through a hall and back to the CU. The bedroom is not yet refurbished and in the tracing process I have also discovered one loose socket that some dork has put a radiator across. You can't make it up. Next job will be stripping that lot out and replacing the wiring circuit entirely.

I have also made the decision to replace the 2018 CU with an entirely new Hager RCBO unit. The circuits are just confusing and illogical, and I would rather have the security of RCBOs, anti surge etc. My sparks chose Hager oriiginally as he said they are good quality sockets and CUs. The CUs elsewhere that he fitted (to new build parts and three outbuildings) are all Hager RCBOs.

This forum is a very knowledgeable resource. Super helpful.

If you are wondering why a DIYer is doing electrical stuff, it is simply that I am doing the refurb myself in my spare time, which includes a lot of new build, so it can take me ages to get some jobs done. The electrical work is therefore very piecemeal, so I found a friendly electrician who will guide and advise me, then check my work and do any Part Ps before we sign off on each section. That's why I very recently bought the Megger test unit etc, so I can do the tests he will do, make sure my work is OK before he checks it with his rig. He was going to guide me but it was Christmas /NY and so he is away with his family on holiday. Sods law.

 

[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.