1960's cable has 'shocking' 20Vac to ground, when disconnected

[RickDastardly]

I recently found that our side building is partly wired in 1960's VIR cable (which has no integrated earth continuity), embedded into the cement ceiling and render. The scum who owned the house previously did the trick of replacing the old cable with new where it could be seen, but then hiding junction boxes onto the old cable where they thought it wouldn't be found. There's a light fitting and a socket, on two VIR circuits back to junctions with the appropriate new T&E circuits.

I disconnected the VIR at the junction box that pulling fuses proved to be the correct light circuit, and the light in the side building went off as expected. A voltage detector showed no mains on either the disconnected end or the light fitting end of the VIR. Then I started to disconnect the VIR in the light fitting to replace with a new run of cable... and got a shock when my hand bridged the cable and the earthed metal bulkhead fitting. Checking with a multimeter shows about 1Vac between the red and black, but each has 20Vac to ground. Enough to cause a good buzz.

WTF?

Can anyone offer any insight into this before I proceed? Where on earth (pun intended) is that 20Vac with enough current available to cause a shock coming from?

Edit: I might have a cause... I pulled the fuse on the socket circuit whitch is also junctioned into a (different) run of VIR. The 20Vac went away leaving just 10 to 100 mV as expected on a long cable run near others. Could it be that the light VIR butts up against the socket VIR somewhere (they don't visibly but are embedded into cement so who knows), and the VIR is conductive enough to leak electrons from socket cable to light cable? I can tell you that where I can get to the VIR, it literally crumbles to powder if I wiggle it.

 

[fourtytwo]

Induction from adjacent cables hopefully.
I recently bought a house where the same trick had been played in the entire house!
Bastards, I had to have it re-wired at no small cost.
Some people are just ****'s and **** on anybody without a care in the world, like the spiv in a BMW who pulled into the mother & baby parking space in the local supermarket, on his own, no passengers just because he was to lazy to walk a bit further......

 

[Harry Bloomfield]

Checking with a multimeter shows about 1Vac between the red and black, but each has 20Vac to ground. Enough to cause a good buzz.

I'm surprised you could even feel a voltage as low as 20v, never mind get a 'good buzz' from it. To cause a shock, only needs a few milliamps. It's the volts that shocks, but the amps that kills.

The 1v between the L and N, could just be induced voltage from other cables, or even the surrounding environment. There doesn't need to be any actual electrical contact, between conductors, or leakage. VRI (or VIR) tends to dry out and crumble close to terminals and accessories, where there is more exposure to air, allowing the oils to evaporate. An emergency fix, is to simply sleeve the exposed ends.

Another quick fix, to ensure your safety, would be to add an RCD, to cover all the circuits. That would have hopefully tripped, when you received your shock, and avoided worse.

The 20v was likely the differential voltage between neutral and earth, there is almost always some voltage difference, nothing to worry about.

 

[RickDastardly]

Some people are just ****'s and **** on anybody without a care in the world

Yep - if I ever get my hands on the people who DIY wrecked this house I'll strangle them with that cable. This issue is, believe it or not, completely trivial compared to some of the stuff I've found out they've done, and hidden. Two days after moving into the house (which we paid for the most expensive survey for first) I discovered a mains water supply leak under the building. They knew about it (a neighbour knew).

I'm surprised you could even feel a voltage as low as 20v

Same. While I'm not en electrician as such, I have an MEng in microelectronics and I'm well familiar with shocks, lol. It was a definite good buzz though so obviously can supply enough current for that even at 20V. At first I thought I had sliced my hand on the fitting until I realized it was a shock, and checked again by touching it. My hands were a bit sweaty.

Anyway, the 20V vanishes as soon as I disconnect the other VRI cable, and that is being ripped out too in half an hour once I got the floorboards up to where I found it with an inspection camera.

 

[ericmark]

I tried to disconnect cables in my parents house to stop the RCD tripping, I gave up as it seemed each time I disconnected what I thought was the cause, found something else wrong, so got whole house rewired.

There was a mixture of VIR and PVC but not due to any attempt to hide the VIR, it was simply due to my dad adding sockets.

He was in 1954 when he bought the house new, proud of it having 8 scokets my grand dad only had two, one hall one landing. In the main the lights were used to power irons etc.

I at the time before the rewire I only had an insulation tester, since then I have bought a new clamp on meter to measure down to 0.001 amp. In this house whole leakage is around 22 mA which spread between 14 circuits is nothing.

But volt wise using the non contact volt range on the clamp on, seems to show some odd results. It has 4 bars so does show how much voltage but although better than simple volt stick, it is still a little hit and miss.

But volts tell one nothing really, it is how much leakage ohms or milliamps, this shows how bad it really is, and of course does it trip the RCD?

 

[RickDastardly]

Yeah, the joys of old (and randomly 'improved') wiring.

When I was 14 (early 80's) I part rewired my parents' huge kitchen. I was just fitting them an extra socket, but discovered by accident that turning on/off the fluorescent lights in the kitchen caused them to flicker on briefly every time their switch was toggled... when the main switch in the fuse box was turned off and the kitchen ring circuit (which it turned out the lights were connected to) fuse was pulled for good measure. Nothing at all should have been live!

 

[Harry Bloomfield]

But volts tell one nothing really, it is how much leakage ohms or milliamps, this shows how bad it really is, and of course does it trip the RCD?

A typical high input impedance meter, most people use, tells you not a lot, when it comes to lengthy wires, especially when they are close to other wires.