Need Help With Lighting Circuit Puzzle.

[alba2]

I am in a 5-year-old property, so all circuits are new. There are 4 x 240V 7W downlights in the bedroom. I decided to install a bedroom ceiling spotlight by means of a piggyback off one of the downlights.

In preparation for installation, I switched off at the wall switch and the annotated bedroom lighting circuit in the consumer unit. Before making the wire connection to the existing downlight I took the further precaution of running a Schneider voltage detector over the loop in and out lighting circuit. To my surprise, the S.V.D. lit up.

I then switched off all the lighting circuits in the C.U. but the bedroom downlight wire was still showing live on the S.V.D. It was only when I then switched off the 13 amp socket circuit in the bedroom that the lighting circuit went truly dead, as indicated by the V.D.

This has got me puzzled. What is the explanation? If this is a problem, how do I check it out.

Thanks

 

[bernardgreen]

Non contact voltage sensing pens are not the best way to check a circuit is dead and safe to work on. They can give fatally incorrect indication of no voltage

They detect the difference between the potential around the wire being tested and the potential of the person holding the pen.

In your case the wire being tested could have been dead and your body was picking up a potential from the 13 amp socket cables.

Or maybe the lights have been supplied from the 13 amp socket circuit for some reason.

The only safe way to verify a circuit is dead and safe to work on is by testing with a two probe device. One probe to a known good Earth and the the other probe on the wire to be tested.

 

[alba2]

Thanks, Bernard
I should not and will not again, put my faith in voltage sensing pens. I watched the electrician using one during installation and wrongly assumed that they were kosher.
I tested with a handheld meter, and all was well, ie no voltage.

I had immediately jumped to the wrong conclusion that there must have been some sort of miss-wiring. This was further reinforced when I switched off the 13 amp wall socket circuit and the pen no longer registered at the ceiling. Incidentally, I was working on the ceiling and there was no 13 amp circuit anywhere nearby.