Any ideas on this one?

[Tunni]

Hi Folks, have just finished replacing an old fuseboard with a new rcbo jobby. To say there have been a few surprises would be putting it mildly. The property is a late 60's bungalow and has had all the usual mods. Loft conversion, new kitchen etc. First surprise was the 3, 32A fuses with 8 conductors, variously paired, turned out not to be 3 rings with a couple of spurs but 1 ring and 6 completely independent radials. Oh well, easy enough to change a couple of rcbos and derate for the individual circuits. However...
Next up the lighting, 3 circuits, apparently to 2 separate 6A fuses, now 3 rcbos, yep you guessed, the dreaded shared neutral and pop, the kitchen light trips the hall light and all manner of dumness. Identify the 2 connected circuits and put them both on the same breaker, all good then? Sadly not. One of the lighting circuits, the one not interconnected by shared neutral, tests through fine, readings at all points, normal. The "twinned circuits", now on the same breaker are a different matter though. No tripping and the rcbo test button working as ought, however I can't get Ze and I can't measure R1+R2 as there is 22V running to earth with a proportionate decline in L to N voltage.
If I try to do a 3 point no trip Zs the meter says I have reverse polarity, swap the leads and I get exactly the same. I've pulled down the ceiling roses on the affected lights and can't see any loose or ballsed up connections but I have spotted the dreaded single wires of doom and that a couple of the lights have no cpc. Those that do I can't check to earth continuity as the voltage is too high. So good people where next? Split the circuits down and IR test? Is there anything less destructive to try first?
Suggestions very welcome!

 

[littlespark]

Afraid not.
Just got to split the circuits down to identify where the fault is.
My money will be on a joint box that’s no longer accessible.

any electronics that could be affecting it? Smokes, LEDs, PIRs?

 

[Tunni]

Hi littlespark, sadly that's my thinking however the property being a much bashed around bungalow doesn't lend itself to easy access at any point. The ceiling roses are all below junction boxes, in so much as only the switched live and the neutral plus occasional cpc are present. The smokes are all battery.

 

[ipf]

Did you do any pre testing and explain to the client about possible/probable problems prior to installation to cover you.
Seems it may take a while, not just the one fault, I'd wager.
60/70's bungalows are a particular nightmare. Good luck.

 

[Tunni]

Ipf, normally I will run a rule over the odd bit and bob to make sure I'm not going in totally blind. This time though...Access has been a real issue not just with this darn covid business but also to the board. A lot of circuits in a small space, you can guess I'm sure. The client is understanding thankfully but how far that will go if things start getting a bit more intrusive is anyone's guess.
I know that the work I have done at this property has made the installation fundamentally safer and that the problem predates any intrusion by myself. The client I am sure accepts this also. I don't want to leave a potential time bomb though. If the client says they don't want me lifting carpets and floor boards to sort this out. It's their choice I know. I want to sleep at night though, I don't like the idea of leaving this one unresolved.
That voltage has to come from somewhere so I'm guessing one of the many radials. I'm going back with renewed vigor to the IR testing. I guess if I measure the voltage between the Neutral/cpc of the lighting circuit and knock off the others I might find a point where the voltage drops off. That will show me any interconnection intended or not and give me a starting point perhaps.
Any tricks learnt over the years for such situations happily accepted!

 

[Welchyboy1]

When your trying to carry out your r1 and r2 test but getting 22v floating voltage, switch one rcbo off at a time till that voltage goes to identify what circuit your picking it up from
my guess would be another borrowed L/N conductor from Another circuit, I’ve seen it a lot, from maybe a boiler needing an extra permanent live, or a light that’s been wired off a switch point with no neutral etc and they have pinched a neutral from the socket circuit etc, but may not be creating sufficient current to open your Rcbo ?

loft conversion jobs are the WORST for problems/access you have my sympathies!

 

[littlespark]

It’s not usual for a jb above every light.... unless wired in conduit. Only SW and neutral at light suggests central jb or looped neutrals at switches.

it’s hard trying to explain to customer that any fault has always been there, and it’s your new sensitive rcd that’s brought it up.... They’re just thinking “we’ll it was working fine until you touched it!”

 

[Tunni]

Hi Folks, yes my guess is the central jb on the lighting, I've not found any neutrals in the switches. I'll concentrate on finding the source of the floating voltage and take it from there I guess. There is a boiler in the loft so I'll give that a good looking over but my results to date show that on a radial with a couple of bedroom sockets, although one of these has been connected via an fcu to a pir external flood. I hadn't found any problems on that circuit although I hadn't found the floating voltage at that point either. Why is it always the last one of the day?!

 

[buzzlightyear]

I guess there was on testing done berore hand then. What puzzles me when peaple don't do a EICR, you has well take blind man in and say sort that out will you.

 

[Tunni]

In a perfect world and all that Buzz. Never is though eh? Particularly now with this bleedin virus buggerin up access

 

[static zap]

Divide and conquer ,
any signs of leaks , damp , recent plaster ..etc Worsening the IR.
Wall lights , clock circuits .. left dangling !

 

[Tunni]

Well that was a day, TFI! Anyways to cut a long story short I tracked the issue down to 2 downstairs lights wired from a junction box in the loft. I'm not exactly sure to this moment what is going on there. Unfortunately said junction box is underneath the loft conversion and completely boxed in along with all access down to the cu and anything but the final switched wire to the lights. No obvious way of adding new circuitry, no obvious way of getting to the assumed position of said box. Situation explained patiently to householders who have adopted the attitude that as it must have been there for 30 years it can stay there for a few more.
I now have that miserable feeling of a job not done right and the mystery of what exactly to put on the certificate. Not a happy bunny at the moment. I'm no perfectionist, the world doesn't work that way and there is no joy in trying to force it. I don't like this situation though but what can I do?

 

[Wilko]

Hi - You mentioned you couldn’t get Ze - assume you meant Zs? If I may ask, what is the installation means of earthing and Ze?

You must be able to attest to Zs etc for the EIC, not really an option. Relaxing here, I’m thinking - turn all light switches to on, isolate and dis the circuit from the board totally and remove lamps. Using Long Lead start measuring the conductor continuity individually from the light fittings back to the board end.