Electrical Test screwdriver mystery

[daverk]

Here's an odd one for you older and wiser electricians out there:

Was in the loft having pulled a disused supply to the garage back into the attic. Used a test screwdriver to test it for live & not live before moving it, and again not live & live once it had been brought into the loft to terminate it in an Ashley junction box for possible future alternative use.

Before moving the cable, the test screwdriver found the normal one live (red) at its extremity. After moving the cable, the test screwdriver found 3 lives, i.e, red, black and protective earth, again at its extremity!

That circuit is on a 30mA RCD (which did not trip) with 32A MCB (which did not 'blow' either).

As a precaution, I then removed most of the cable which had been pulled-in, because it had been on a broken catenary and it was conceivably but remotely possible that the neutral and earth had both been stretched and perhaps broken and in contact with the live. I even stripped off the outer PVC sheath to try to find evidence of stretching or cross-connection. Nothing found, and the inner sheaths and wires (2.5mm2) were in very sound condition, and had continuity (tested). So I think that we can rule out a faulty cable.

After removing most of the old cable and terminating it in the Ashley junction box, all was normal according to the test screwdriver, with just the red showing live, and everything is back on and working normally.

So it's a bit of a mystery.

It was very hot and humid up in the loft, could that have affected the operation of the test screwdriver to give a result which showed 3 apparent lives on that one occasion but did not trip the RCD?? (The screwdriver is the sort where you press your thumb against a metal button to light the neon if there is a live contact.)

If not, what else?

The test screwdriver, RCD, and MCB all appear to be functioning normally, and the other items on that particular circuit are also working normally, and properly connected as far as I can establish.

 

[Risteard]

A phase tester won't tell you much of use. Throw it in the bin.

 

[ColJack]

those things are extremely innacurate and very dangerous.. how do you know that the neon hasn't gone and that "dead" wire is in fact quite live..

why are you taking chances anyway..? the main switch should have been off removing ALL power from your house..

throw it in the bin and go and buy a decent VDE terminal driver and a multimeter.. both shouldn't set you back more than about £15..

 

[wingcoax]

Who's got the picture?

 

[Taylortwocities]

Untitled

• Taylortwocities
• 11 Feb 2009
• 20

 

[daverk]

those things are extremely innacurate and very dangerous.. how do you know that the neon hasn't gone and that "dead" wire is in fact quite live..

why are you taking chances anyway..? the main switch should have been off removing ALL power from your house..

throw it in the bin and go and buy a decent VDE terminal driver and a multimeter.. both shouldn't set you back more than about £15..

The main DP switch was off, of course. And the DP isolator which I've just had fitted to be doubly sure. And the RCD for good measure. I used the neon screwdriver to check that power was there again after switching on again, but on that occasion I got 3 apparent lives.

I have been reading the other threads and appreciate that there are rather polarised views about the use of neon screwdrivers. All I'm looking for is a likely explanation for why I had the 3 apparent lives on reconnection.

I will though look at your suggestion of the VDE terminal driver. I've got a multimeter, but it needs too many hands in the confined space of the loft...

 

[ColJack]

this disused feed, was it disconnected from whatever it used to feed?

when you tested it again was it connected to the new whatever you were feeding with it?

 

[maguire]

I got one of those as a freebie from christ knows where, but wait this not only tests for live it also wait for it detects microwave leakage well that us covered init, do I use it ? of course I do its got a pocket clip on it keeps all diesel receipts together in the cubby
Dev

 

[daverk]

this disused feed, was it disconnected from whatever it used to feed?

when you tested it again was it connected to the new whatever you were feeding with it?

Yes it was disconnected from what it used to feed.
After it was moved, that end was not connected to anything but was terminated in a 3-terminal Ashley box (with a sleeved earth); and when power was restored, it was live once more (supplied from a RCD-protected socket circuit, and nothing changed at that end) - all three then live according to the neon as described in the OP, then only the red was live once most of the cable had been removed.

 

[electronicsuk]

Yes it was disconnected from what it used to feed.
After it was moved, that end was not connected to anything but was terminated in a 3-terminal Ashley box (with a sleeved earth); and when power was restored, it was live once more (supplied from a RCD-protected socket circuit, and nothing changed at that end) - all three then live according to the neon as described in the OP, then only the red was live once most of the cable had been removed.

From what you describe there it almost sounds a bit like capacitive coupling between live and neutral/earth, but I can't see how this would happen unless you had disconnected N/E at the supply end (presumably in the consumer unit?) but with the circuit energised. I suppose there could even be some coupling from other cables running in close proximity, but we're stretching things a bit now.

It would seem unlikely that those two conductors truly were 'live', as it would require either a combination of faults leading to live and N/E coming into contact with each other and N/E being open circuit, otherwise the RCD and/or MCB protecting the circuit would have tripped.

 

[LiamPope]

supplied from a RCD-protected socket circuit

The feed to a garage was a spur from a socket ring? Well done for disconnecting it. Personally I'd remove it completely from where it spurs and not make it available for future use.

 

[daverk]

From what you describe there it almost sounds a bit like capacitive coupling between live and neutral/earth, but I can't see how this would happen unless you had disconnected N/E at the supply end (presumably in the consumer unit?) but with the circuit energised. I suppose there could even be some coupling from other cables running in close proximity, but we're stretching things a bit now.

It would seem unlikely that those two conductors truly were 'live', as it would require either a combination of faults leading to live and N/E coming into contact with each other and N/E being open circuit, otherwise the RCD and/or MCB protecting the circuit would have tripped.

Yes it's weird. The earthing by the way is TNCS. The DPs in the consumer unit and on the board were "off". No other adjacent cables. All I could think of was that the general humidity and sweatiness in a confined space was causing leakage to the N and E which the screwdriver then sensed, but that is just a guess.

 

[daverk]

supplied from a RCD-protected socket circuit

The feed to a garage was a spur from a socket ring? Well done for disconnecting it. Personally I'd remove it completely from where it spurs and not make it available for future use.

It's an old place and although most circuitry is not too mad, there are some strange things. When I can find exactly where it spurs from (probably a connection box in another roof area - I think that circuit is a radial rather than a ring) I'll whip that final section of cabling out, because it could be an intermittent fault in the remaining cable which caused the problem. But it's strange that when reconnected, that the RCD did not trip.

 

[Monkeh]

Put the screwdriver away and get a multimeter. Neon screwdrivers cannot be relied upon for anything except misleading you.

 

[Risteard]

Put the screwdriver away and get a multimeter. Neon screwdrivers cannot be relied upon for anything except misleading you.

Don't use a multimeter either. Use an approved voltage indicator/test lamp.