120V earth??

[autolycus]

Hi,

I have now finished converting all my unearthed metal light switches to plastic as talked about at //www.diynot.com/forums/viewtopic.php?p=827868

Interestingly, it turned out that for two of the rooms (the bathroom and downstairs WC) we actually did have a T&E cable serving the switch.

They also had metal back boxes as opposed to the plastic ones used elsewhere in the house. (When I say plastic, I am talking about some kind of old-fashioned plastic ones for use in brick walls rather than modern dry-lining ones).

At both the bathroom and the WC, the cable was fairly modern looking with a single piece of copper for each conductor, rather than the three-stranded silver-coloured conductors used in the rest of the rooms. At the WC switch the earth was bare and at the bathroom it was sheathed in plain green (so presumably not that modern)

On each switch, I measured the voltage between the live and the earth. At the bathroom it was somewhere between 230 and 240V, so I reckon the earth there is ok.

At the WC, the voltage came out at 120V (which probably explains why I used to get a slight tingling when I ran my finger over the old metal switch!).

I can only conclude from that that, for some reason, the so-called earth wire is actually at 120V, thus giving a PD between live and "earth" of 120V.

Is there any other possible explanation? Where could the 120V for the earth be coming from?

Is the safest bet just to disconnect the "earth" in the switch and put a choc block on it?

Dave.

 

[Adam_151]

Sounds like the cpc has become disconnected at some point, it would appear to be 'hovering' halfway between phase and neutral becuase its capacitivly coupled to both due to being in the same cable

The only correct solution is to find the open circuit in it, and repair it

 

[autolycus]

I'd actually be surprised if the CPC is connected anywhere.

The house is from the early part of the 1960s and so the original lighting circuits are all wired with PVC covered T-without-E cable.

My suspicion is that at some point, someone has replaced the cable from the WC light ceiling rose to the switch and used T+E as that was all that was available by that time.

 

[Adam_151]

In that case then, you effectivly don't have a CPC, and shouldn't be using metal switch plates, actually thinking about it, connecting your cpc that isn' actually connected down to earth anywhere is worse than not connecting it at all!

You can either tape up the cpc and use plastic switches for now as a short term measure, or draw some new cables....

Oh, sorry, just noticed, you're going from metal to plastic, not vice versa? ... oh well, I'm sure the core points are still clear enough

 

[autolycus]

I guess before I do that I need to get the light fitting down and see what the wiring looks like in the ceiling too. It may be that the CPC is actually meant to be there after all and is just connected to the wrong thing in the ceiling.....