Sparking Earth

[pcboffinuk]

Thought I'd run this one past you knowledgable lot...

A client that I do website and database stuff for asked me a question about his wiring that I didn't know the answer to.

He has extended an existing dimmer switch with a simple 2 core and earth to relocate the dimmer. All is wired correctly and works as it should. However, when connecting the earth from the extension cable to the original back box he noticed that he gets a tiny spark and was asking why.

I don't have my meter to test anything.

It is standard loop in wiring as far as I can tell, and the extension cable doesn't run near any others (my first thought was possible induction voltage).

It is a tiny spark, but it is there and repeatable, so clearly there is a small current. Is the dimmer switch (Brass effect Volex) likely to be introducing this as there is nothing else I can see that could cause it?

Gavin

 

[jj4091]

get your meter

 

[RF Lighting]

I guess the cable was live when he connected the earth?

If so the spark will have been the induced voltage on the floating earth being grounded.

 

[DESL]

Funny you should say that I got called to an opticians where they kept getting shocks from the sink!!

Inspected board and found earth had been removed from the clamp on sheath of incomer so I went to reconnect it and BANG lights went.
A nail through lighting cable was the culprit

 

[nozspark]

If so the spark will have been the induced voltage on the floating earth being grounded.

Sound to be the answer to me aswell, although which has the floating earth - the backbox or the CPC.

I had one once that when measured had over 100V on it if my memory serves me right, they were getting a shock between 2 chrome light switches. After a morning of hunting I found out that the CPC's feeding SELV lighting were disconnected at several points.

This needs confirming and sorting if necessary!!!

 

[pcboffinuk]

Ah, I hadn't considered that the induced voltage would work within the same cable onto the earth, but it makes sense.

It can only be the extension that is floating I think, because it runs to a plastic back box in a stud wall, so has no other earth available. The cable was live when he connected the earth yes, and yes I did advise him that this wasn't the best way to do it!

Many Thanks for the answer.. something else I have learnt on the best electrical forum out there

Gavin