"Interesting" neon indication.

[Jon Read]

Hi,

I've recently had my flat re-wired (by a NICIEC registered electrician, and have the certificate to prove it :-)). Part of the job was to install new wall lights in the lounge. After the re-wiring I removed the wall lights to have the room re-plastered and have just finished painting it.

When I was replacing the wall lights, I was using a neon indicator screwdriver to fix the hanging bracket onto the wall and was quite surprised to see it light up. The cables run down through the wall from the ceiling, underneath the bracket (between the two fixing screws) and out of the wall about 0.5" below where the bracket sits. There was no connection between the bracket and the cable.

I then tested it with my (analogue) multimeter, which registered no current and a very low (< 1v) pd between the earth wire and the bracket.

Any ideas as to why this should be? The (RCD protected) lighting circuit was isolated by switching it off at the CU. The cables do run alongside mains cables for most of their length (I had all the cables routed along the tops of the walls to be hidden by coving as I have concrete floors and ceilings).

It's the fact that it was the fixing bracket, not the wire itself, that lit the indicator is what's bothering me.

Thanks for any suggestions as to what might cause this!

Cheers, Jon

[Andy Hall]

Presumably the bracket is not earthed. In which case this is capacitive pickup from the cable and it's unlikely that there is a fault. The bracket should have an earth terminal somewhere. If you run a wire from that (cover in green and yellow sleeve) to the cable earth terminal, the indication should disappear.

Some neon screwdrivers are very sensitive.....

.andy

To email, substitute .nospam with .gl

[Jon Read]

Thanks Andy. You're correct, the bracket wasn't earthed but it doesn't have an earth connection. It's designed to be completely covered by the front part of the light fitting, which is itself made of (painted) metal and earthed. I would assume that the connection between the front of the fitting and the bracket earths it and the fixing screws.

I'm probably being pretty ignorant here, but as the lighting circuit was switched off at the CU how can there be capacitive pickup in the bracket?

The light works fine and doesn't trip the RCD.

The screwdriver is this one, I've no idea how sensitive it is.

Jon

[Andy Hall]

Probably from cable to cable.

Try attaching the earthed front part without the fitting on the wall. I expect that the problem will vanish. If it doesn't, call the electrician back......

.andy

To email, substitute .nospam with .gl

[Colin Wilson]

Could be an induced current - when a "switched off" cable runs alongside another, it can become live (even though its off) because of the proximity of the other cable.

There was a linesman killed in Scotland last week while working on a

275kv pylon carrying two circuits - one was "off", the other was still live. The "dead" one got him.

[Chris Oates]

Do you have any dimmers - that'll do it

[stefek.zaba]

Eeek. I expect the HSE will be all over this one. I have nothing beyond marginal and theoretical knowledge of high-voltage working, but I'd'a thunk Standard Operating Proceedcake would be to actively earth conductors you're doing maintenance on, not just isolate and leave them floating. After all, it's Standard Operating Procedure to wham gert big conducting spikes through buried supply cables when working on them, to discharge capacitavely-stored energy, innit? So it's not like the ideas of capacitive and inductive coupling are foreign to the engineering practice of electricity distribution...

Do you have any pointers to details of the Scottish incident, Colin?

Stefek

[PoP]

Never having worked at that sort of potential I'd have thought page 1 of the service manual would have been to strap the line to earth!

PoP

Replying to the email address given by my news reader will result in your own email address being instantly added to my anti-spam database! If you really want to contact me try changing the prefix in the given email address to my newsgroup posting name.....

[John Stumbles]

Reminds me of a hair-raising tale (one of many such) our Electrical Power lecturer at college told us from his days in Ye Olde CEGB: linemen working on a HV pylon, one side dead, the other still live, so they chuck an earthing strap down over the dead circuit to discharge induced EMF from the live side .... only [you're waaay ahead of me, aren't you? :-)] they got the wrong side!

-- John Stumbles

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

-+ Experiments to demonstrate the existence of Sod's Law by dropping slices of buttered toast all failed. That's Sod's Law.

[Colin Wilson]

Not yet, although it was released in a press article (forgot which paper, I might be able to check tomorrow)

I think (at first glance) it was in The Herald. Glasgow (UK): Dec 18,

2003. pg. 8 - the tight ass b*stards want you to pay to retrieve the article though - "A MAN has died while carrying out routine work on overhead power lines, police said yesterday"

[Colin Wilson]

Evening Times. Glasgow (UK): Dec 18, 2003. pg. 11

Worker dies in cable tragedy

The ScottishPower engineer had been carrying out routine tests on overhead high-voltage lines at Damhead Farm, Craigend Road, Airdrie, Lanarkshire.

A spokeswoman for ScottishPower said: "We have started a full investigation and we are deeply saddened by the death of a colleague.

[Dave Liquorice]

I think they will as well, some one fouled up big time, or more likely several people in small ways.

When the guys have come round here to work on the 11kv stuff they:

a) Drive in a earth stake connected to 25mm^2 cables with clamps. b) Use wooden ladders. c) Check that the relevant breaker is open. d) Check it really is open with a yard long "volt stick". e) Connect the earthed cables to the incoming feeder wires with a long handled pole that operates the cable clamp devices. f) Check again with the volt stick. g) Start the work.

[Huge]

Never seen one of the old (1970s?) daytime TV safety films...?

I still recall the EHT safety acronym; SIDE

Switch off Isolate Dump Earth

[Colin Wilson]

Oh, and from The Scotsman

Worker killed

A MAN has died while carrying out routine work on overhead power lines.

The 44-year-old was involved in the accident in Airdrie at about 12:30pm on Tuesday. The man, who was employed by Scottish Power, was testing overhead lines at Damhead Farm, Craigend Road.

[Colin Wilson]

*SPLASH*

Arc welding for beginners :-)

[Colin Wilson]

Worker killed

A MAN has died while carrying out routine work on overhead power lines.

The 44-year-old was involved in the accident in Airdrie at about 12:30pm on Tuesday. The man, who was employed by Scottish Power, was testing overhead lines at Damhead Farm, Craigend Road.

[Colin Wilson]

You missed a step - for EACH SPOUT they have to prove its dead before you poke your hand up there, unless its covered under a zone of work waiver permit

[Chris Oates]

How quaint - wood In the US it is mandatory to use fibreglass ladders

[Dave Liquorice]

I was only watching from the ground. What does "EACH SPOUT" mean?

The times they have come here was once to change the tapping on the transformer so we get a tad under 240v not 250 peaking 260... The second to replace one of the 11kv insulators that was breaking down and tripping the line protection.

[Chris Oates]

Obviously to do with the analogy of electricity behaving like water