Fault on lighting circuit

[Welsh_sparky_21]

All CPCs on upstairs and downstairs lights were live due to the cpc of upstairs being disconnected from earth bar. I was getting 230v between cpc and neutral at the light, the 2 circuits are separate and the rcd never tripped, anyone had this before or know why this would happen

 

[DPG]

What results did you get from dead tests?

 

[timhoward]

Start at the beginning, does the RCD work? If old and never tested they do fail to operate sometimes.
Then dead IR tests L+N to cpc bar to figure out what’s going on

 

[SJD]

It would be interesting to know if the upstairs CPC was deliberately disconnected (e.g. because the circuit had been tripping) or if it looked more like the connection had come loose & fallen out of the terminal.

Regarding downstairs, quite possibly the CPCs are interconnected via a hall/landing two-way cable (assuming modern wiring that includes a CPC in all cables). And then the downstairs CPC has been broken / disconnected somewhere as well, e.g. at the first fitting. So the rest of the downstairs CPC floats to the same voltage as upstairs, or even is the cause of the upstairs CPC showing a voltage.

How about some (R1+R2) results for both circuits?

 

[Welsh_sparky_21]

Didn’t get round to testing, was basically the last job I had to do before I finished, was replacing the switches for metal and replacing light. Turned circuit off and noticed the cpc was live, 230 between cpc and neutral. Checked all the metal switches and they were live, checked earths in DB and didn’t notice at first as they all look connected was only after I gave them all a tug I noticed it had snapped, re connected this and the voltage on the cpc was now at zero. Tested Rcd and it worked fine, to go back next week and find out why the cpc was going live for both circuits when upstairs was disconnected. Thought I’d ask in here as I’m not experienced in fault finding, especially a fault like this one. Any advice is appreciated

 

[Avo Mk8]

go back next week and find out why the cpc was going live for both circuits when upstairs was disconnected.

If you had switched off lighting upstairs at CU, but downstairs lighting circuit still on, the open circuit cpc could appear 'live' from a capacitively coupled "ghost" voltage coming from where the cpc and live (still on) are running together downstairs.
Any load on it and the voltage will disappear (for this particular phenomenon)
(And assuming upstairs and downstairs cpc's are connected together, which I think you were suggesting)
Just a possible explanation - not saying that's what it actually was !

 

[pc1966]

How were you measuring the voltage?

• If it is with a multimeter or similar then as @Avo Mk8 says it could be largely down to capacitance as such meters typically have around 10M ohm impedance so only need a few uA to register a significant voltage.
• If you used a voltage tester then it is far more worrying as they need a couple of mA to operate and so would suggest a L-CPC fault somewhere in the system.

When you go back I would restore the CPC and then IR test the light circuit (L+N to E) before doing anything more. That should prove if there is some insulation fault.

However, that might not be the end of the trouble as it is depressingly common to see light circuits with CPCs not connected as some plonker assumes that most lights are class II so the circuit does not need an earthed CPC even though the wiring regs explicitly say otherwise.

 

[Welsh_sparky_21]

How were you measuring the voltage?

• If it is with a multimeter or similar then as @Avo Mk8 says it could be largely down to capacitance as such meters typically have around 10M ohm impedance so only need a few uA to register a significant voltage.
• If you used a voltage tester then it is far more worrying as they need a couple of mA to operate and so would suggest a L-CPC fault somewhere in the system.

When you go back I would restore the CPC and then IR test the light circuit (L+N to E) before doing anything more. That should prove if there is some insulation fault.

However, that might not be the end of the trouble as it is depressingly common to see light circuits with CPCs not connected as some plonker assumes that most lights are class II so the circuit does not need an earthed CPC even though the wiring regs explicitly say otherwise.

It was with my fluke voltage tester. I was getting 230V between the cpc at the light and the neutral. Then checked all metal switches and was getting a voltage, tested between one of the switches and a socket and was getting 230v. That’s when I checked the db and noticed that the cpc had snapped. Re connected and all the voltages on the cpcs disappeared. Strange one, like a said I do have some experience finding faults but not very often you get one like this

 

[brianmoooore]

Needs IR testing. Sounds like there may be leakage from live to earth, but not nearly enough to trip an RCD.

 

[LastManOnline]

Needs IR testing. Sounds like there may be leakage from live to earth, but not nearly enough to trip an RCD.

There is almost certainly leakage from live to earth. But what kind of leakage is it? Intentional or unintentional?
If it was a socket circuit I would regard your issue, s as nothing unusual. Remove the CPC from your average ring circuit and you will almost certainly measure a significant voltage between L and E as a result of the modern appliances plugged in.
What throws me a little with your case is that its a lighting circuit. You are less likely to either intentional or natural leakage there.
The second oddity is the downstairs light CPC was live as well. That could be explaned away though by the original installer using a "borrowed earth" at some point during his installation. Happens often with the upstairs hallway light.