Voltage on a CPC?

[spud1]

Hi everyone,

Hopefully someone can help me with this?!

I am in the throws of finishing a domestic rewire in an empty house this week and yesterday was re-sleeving some CPC's in pendant light fitting that my apprentice had sleeved poorly.

Rather naughtily I was doing this while the pendant was live, but as the house was empty and the CPCs would only be disconnected momentarily I saw no real danger.

Having said that... during the disconnection and 'de-sleeving' of the CPC's I noticed a rather slight shock sensation when I touched the switch cable CPC and either of mains loop CPC's simaeltaneously? (It should be noted that the lighting circuit is wired in the traditional mains loop and single switch cable method).

On closer inspection and measurement using my multi-meter I found 190 volts between the switch cable CPC !

I investigated further and found the same voltage between the switch cable CPC & neutral but only usual small residual voltage between both mains loop CPC's and neutral.

Out of curiosity I disconnected the switch cable live, switch live and CPC both ends, I.R tested it at 500V DC only to get clear results @ >200 Ohms across the board.

Hoping I had cleared the 'fault' in the process described above I put everything back to together again, re-energised the the circuit and re-tested my CPC's and as before I had 190 Volts on the switch cable CPC!?

It should be noted that the switch back-box in question, although a metal back-box, is mounted on a wooden stud in a wooden stud partition so I don't think their could be any parallel paths or differing earth potentials as a result generating this potential difference.

The only thing I can think is that the switch is a dimmer and maybe there is some kind of induced voltage at play from the inductivity of the dimmer interacting with the metal back-box, but that really is clutching at straws!

Help Please!

 

[i=p/u]

Must be taking rcd out ????

 

[spud1]

No not at all, that's the strange thing.
RCD has tested fine too, Ze and Zs for lighting circuit in question within expected perameters and rest of circuit has clear I.R.?

 

[i=p/u]

I'd say go to start of lighting circuit and measure voltage there and move along

 

[spud1]

See what your saying but the rest of the circuit is fine, it is literally this one switch cable CPC that shows, when disconnected from the rest of the energized circuit 190V to earth/neutral...

 

[monkeyelectric]

Have you tried isolating the other circuits in the house to see if it disappears?

 

[spud1]

No I haven't but I will, interesting suggestion thanks

 

[Marvo]

The earth is functional as well as protective. Switch mode power supplies and electronic ballasts to name but two items can introduce a small current into the CPC from their input circuitry and surge arrestors. Have you got electronic transformers, LED drivers or electronic ballasts connected to the circuit? Connecting everything back up and check the leakage current by clamping the L+N simultaneously with an earth leakage clamp meter.

 

[Lucien Nunes]

This subject keeps coming up! When disconnected from the supply CPC, the CPC in the switch cable is floating, i.e. free to take up any voltage. A switch cable is particularly relevant because its CPC lies between two conductors one or both of which are connected to line, so it may be coupled to line potential by a lower capacitive reactance that that coupling it to the rest of the environment at earth potential. Because these reactances form a voltage divider, one can expect the isolated CPC to sit somewhere above half of mains voltage, in your case 190V. A modern DMM has a high input impedance that will not heavily load even this high source impedance, and will therefore give nearly the full reading.

The maximum current that can flow into the CPC via the stray capacitance of typical sizes of T+E is very low, around 12µA per metre for 1.5mm², however a cable 20m long would give you 0.24mA and under some circumstances you can feel currents in this order of magnitude. If there are devices with interference suppression capacitors connected to the CPC, such as switched-mode power supplies, then the current can be very much higher and easily felt, without there being a fault. Also dimmers, which create a voltage waveform with higher dV/dt than the sine wave of ordinary mains, can pass a higher peak current through a given cable capacitance.

Of course you should check for faults, however your insulation test would have shown up anything major already. My suspicion is that you were feeling capacitive leakage that is normal and harmless, and that there is no fault. This is one of the reasons you should never disconnect a CPC when energised - even if no other fault exists things can rise to high potentials and on large systems deliver dangerous leakage currents. An industrial wiring system can leak 100mA into its CPC, enough to kill you, through capacitance alone even when the insulation is perfect.

Finally, as I mentioned in a previous reply on this subject, the title 'voltage on a CPC' is distracting. When the bare core of a T&E is disconnected, it's not really a CPC, just a floating piece of conductor. Unlike a CPC, it's quite normal for it to be at some arbitrary voltage to earth.

 

[spud1]

Sorry for the late reply Lucien but that was a perfect answer to my question thank-you!
Sorry to make you go over old ground and sorry for the mis-leading/distracting title.

 

[timo1]

Lucine, that has to be one of the most concise and helpful replies I have seen. Thanks.

 

[R1CH-G]

Lucine, that has to be one of the most concise and helpful replies I have seen. Thanks.

Completely agree - great answer - I feel a little unworthy right now...

 

[DPG]

Anybody with a mercury arc rectifier as an avatar is fine by me. Daz

 

[Lucien Nunes]

That's a self portrait... lost some weight since then but I can still do 350A. You should see my arms glow when I'm working hard