Fault Finding: No end to End continuity of the Rn conductor

[nero44]

Hello Everyone,

further working down the list of my recent EICR findings...(last item actually)

This item reads: No end to End continuity of the Rn conductor CCT No:3 (Marked as FI)

CCT 3 is obviously a ring main, serving 3 rooms and two UFHs with FCU. The values from the EICR are
r1 = 0.43
rn = >999
r2 = 0.71
r1+r2 = 0.78

So apart from the rn all looks pretty normal. I have had a lot of work done in relation to this circuit, but I have a suspicion where this issue might come from. My question is, how do I go about
a) confirming the fault
b) getting as close as possible to the problem (I might need to remove a skirting board, so I want to be sure before I do this)

So far I've opened just one socket and checked continuity with my cheap multi meter. Measured at the socket, I have continuity from L2L, N2N but actually no continuity from CPC2CPC...Furthermore, I do have continuity between cpc and neutral, which I am guessing is due to the setup that neutral is grounded somewhere at the power supply. If I short neutral with one of the cpcs, I do have continuity from CPC2CPC, but if I short the other neutral with one of the cpcs there is no continuity.

What I don't understand is that there seems to be continuity on rn, but the EICR suggests otherwise? Is this because the sparky would have tested at the CU and fully isolated the ring there to ensure resistance would not bypass by going via CPC?

And also, if they have found a value for r2, why do I not get continuity at the socket when measuring from CPC2CPC?

 

[Murdochcat]

Assuming you know what you are doing then repeat the continuity tests for L, N & CPC for the circuit at the CU to confirm what was reported.

Then take a piece of paper and draw the rooms covered by the circuit and mark on all the sockets and fused spurs.

Then work your way around logically checked each and EVERY point on the circuit for loose or missing connections - could be the first thing you look at and could be the last.

Lack of continuity is very common on rings

So patience and time is required.

Where is the thread / post about the EICR?

 

[nero44]

Thanks for your response.

Assuming you know what you are doing then repeat the continuity tests for L, N & CPC for the circuit at the CU to confirm what was reported.

Never worked at the CU directly, so ideally I was looking at "indirect" methods of confirming for a start... hence my attempt to measure resistance at socket, but that just lead me to conclude that the EICR is wrong and it's actually CPC not N lacking continuity, but surely I am missing something here...

Where is the thread / post about the EICR?

There isn't a dedicated one, but I've asked so many questions here of over the last few months (and gotten very helpful replies!) and some of them were linked to it. In short, the company who installed my solar panels have also conducted an EICR raising a few C2 and FI, which I have been going through. So far I have resolved most of them (e.g. moving sockets out of skirting board) but this one is the last one to address.

What complicates everything is that I also had refurbishment works done (two separate projects with two different companies) at the same time in rooms being served by the ring final in question, AND I have also done a lot of DIY myself. Although on this particular circuit I have only swapped old plastic sockets with new screwless metal ones. And yes I will check those first, but that is not my main suspicion.

I can draw a diagram of how I think the circuit runs and how many sockets it serves etc, but if I cannot replicate the initial fault I can't fully confirm if I have fixed it in case I find a badly wired junction box.

 

[martygturner]

If it was me I would diconnect the middle socket of the ring and then use a plug in tester to work back down each leg.

 

[nero44]

you mean temporarily effectively making two radials out of it, putting power back on and then use a standard socket tester and see if all three have signals?

 

[EFLImpudence]

Be aware the socket testers cannot distinguish between Neutral and Earth.

If you have tested properly there isn't much point doing something else.


The electrician said Neutral no end to end; you have found CPC no end to end. It's hard to tell from the internet who is correct.

If you don't want to go into the CU then you will just have to disconnect - with the power OFF - a socket near the CU and a socket at the mid-point of the ring and with your meter find out which conductor is causing the problem and then where the problem is.

 

[Risteard]

He means the Neutral conductor. There isn't any such thing as an "Rn conductor". Rn relates to the resistance of the Neutral conductor, and is not a name for the conductor itself. Sloppy incorrect and meaningless use of terminology by your Electrician.

 

[nero44]

If you don't want to go into the CU then you will just have to disconnect - with the power OFF - a socket near the CU and a socket at the mid-point of the ring and with your meter find out which conductor is causing the problem and then where the problem is.

This sound doable, but I don't have any wander lead. For plain continuity I suppose I can just use some spare TE cable I have flying around to bring cables together from across the rooms?

Here is a quick n dirty sketch of what I believe the circuit looks like. Bedroom2 was one of the projects where we converted it into a Bathroom. This is where I suspect the fault will be. The bathroom obviously now does not have the two sockets anymore so the bathroom fitter will have just closed the ring via JB. My DIY was only a replacement of sockets in Bedroom3 and I am quite confident and already checked for loose connections. Bedroom1 so far untouched. Where I drew the amber lines is where I can break the circuit and test both sides...and my guess is I will have full continuity over the "long" way via CU, but not between the rooms. Unfortunately most of that cable is under a new bathroom floor, but as both sockets were in the skirting board I might have access opening the skirting board from the other side... let's see

 

[Murdochcat]

Have you done as I suggested as in looking at the connections behind sockets and fused spurs ?

And a bathroom fitter made changes ?

 

[nero44]

Not yet, but will do this after work today or tomorrow. Was just hoping I could investigate w/out taking out all the sockets, but I guess it is not actually that much effort and a more secure way of excluding a few sources of faults.

I will check all accessible sockets and FCU and provided that the fault is not there, will temporarily split the ring and confirm that the fault is indeed in the bathroom section. Will report back.

Have you done as I suggested as in looking at the connections behind sockets and fused spurs ?

And a bathroom fitter made changes ?

ah.. let's not go down that alley.. we had so many problems with them on this project....goes beyond the scope of this thread.

 

[nero44]

So patience and time is required.

I cannot stress enough how true this is!

So I did follow the pen and paper method and while it took forever, it seems to have been the only way. For some reason the sparky put a large JB just above the CU, which only had the two largest ring finals passing though. But this actually was very useful, as I could fully disconnect the ring in question without opening the CU. I opened up all reachable sockets and both FCUs and ended up with quite a few changes to my initial assumption of how the circuit runs. I must've run up and down from the cellar to the first floor about 30 times, each time reconnecting leads and testing. And then there are these moments when you think you got it all figured out, only to find continuity on two ends which just can't be possible...

After many iterations, I came up with below circuit.

I've tested separately L/N/E continuity from CU to JB1 (and therefore Socket 1&2) and from CU going the other way to JB to Socket 3 to JB2 (which is somewhere under the floorboard), but could confirm continuity of L/N/E to socket 3&4. I also found out the FCU/UFH of Bathroom1 is on another circuit.

So the fault is definitely somewhere from JB2, going under bathroom 2 along where there used to be 2 sockets and then to Socket1. I am not sure how to proceed from here, as Bathroom 2 is fully tiled with underfloor heating. I might be able to get to the lower JB/Socket in Bathroom2 via the hallway but I would need to lift some carpet and cut into floorboards. Technically possible and can be easily repaired, but if it's the other JB/Socket then I probably will never have access. In absence if this, I assume my only way out is to make two radials? One going from CU to the right to JB1 serving just socket 1 and socket 2 and then the other going from CU to the left to JB (serving the UFH) and then to socket 3 and 4.

Along the way I also found this exceptional wiring at the FCU... unfortunately unrelated to my CPC continuity problem...

Anyhow, I've still not solved this but going away for half-term so will be taking a fresh look in a week. Thanks for everyone's input so far.

 

[Ragnar_AT]

The easiest way of dealing with this seems to be downgrading the MCB to a B20, treating the whole circuit like one radial. That should provide plenty of power for a few bedroom sockets and one UFH mat.

 

[Murdochcat]

The easiest way of dealing with this seems to be downgrading the MCB to a B20, treating the whole circuit like one radial. That should provide plenty of power for a few bedroom sockets and one UFH mat.

Last resort IMHO

 

[nero44]

In case anyone was wondering.... I finally managed to resolve this!

But FFS I was looking at the wrong area...

I actually went ahead and lifted the hallway carpet and opened a few floorboards to trace cable routes. Took an entire day, as I wanted to be careful to not break any of the carpet, carpet grippers, insulation mat, plywood underlay etc... In the end, most cables just went somewhere else through joists so there was no JB or anything in sight for me to inspect. I did however see one set of cables following into an an unexpected direction and... lo and behold, I did uncover another unused socket in bedroom 1 hidden in the skirtingboard behind a wardrobe wall...when I managed to get access to it I could already see it was missing a screw and was not even held in place properly. CPC indeed was loose and after re-fixing and testing, I did have full continuity on all three measured at the CU... Phew!

A healthy mix relief and frustration at the same time after spending so much time investigating. I did manage to fix some squeaky floorboards and found a small patch of wood worm damage, onto which I applied treatment straight away. So at least it was not a complete waste of time going though all the effort of opening the floorboards. And I now know a lot more about cable routes in my house...

Thanks for all the guidance.

 

[Murdochcat]

So what are the end to end readings for the live, neutral and cpc?