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.

 

[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.