There are two lighting circuits in my house, one upstairs and one downstairs. The upstairs circuit loops through a ceiling rose which supports a pendant light on the landing. There is a 1.0mm T&E cable from the rose to a 1-gang 2-way switch on the landing and then a 3&E cable from that switch to another one downstairs in the hall to provide 2-way switching. The downstairs switch uses 1-gang of a 3-gang switch, the other 2 gangs controlling lights on the downstairs circuit.
The upstairs switch was sited in an old metal back-box inserted into a partition wall. The plasterboard around the box was breaking up, so as I'm about to redecorate that area I removed the switch, removed the old box, repaired the plasterboard, fitted a new plastic 'drywall' backbox, and refitted the switch.
Prior to starting work I pulled the fuse for the upstairs circuit. However when I waved my detection 'pen' over the switch, much to my surprise it registered some voltage. So I also pulled the fuse for the downstairs circuit, after which no voltage detected, nor did I find any when I opened up behind the switch and probed with the multimeter. Once I had completed all the work I replaced the downstairs fuse first and retested with the pen - nothing detected. Then replaced the fuse for the upstairs circuit and did get a voltage detected, as I would have expected. All works fine now.
So, what might have caused this curious initial reading? My first thought was some sort of 'borrowed live' in the 3-gang downstairs between the upstairs and downstairs circuit, but I had replaced that 3-gang not long ago and I am certain there was nothing like that. [ In any case, I've worked on other fittings on either the up or down stairs circuits recently without any problem that would suggest an incorrect connection between the two. ] My next thought is that, as the conduit which feeds the 3-gang contains cables from both circuits, was there some sort of induced voltage caused by them being in such close proximity? I also checked the ends of both cables behind the upstairs switch and there was no obvious sign of split insulation that might have lead to a cross-connection.
Any thoughts please on what might have been the cause and/or what else to check for?
TAI
The upstairs switch was sited in an old metal back-box inserted into a partition wall. The plasterboard around the box was breaking up, so as I'm about to redecorate that area I removed the switch, removed the old box, repaired the plasterboard, fitted a new plastic 'drywall' backbox, and refitted the switch.
Prior to starting work I pulled the fuse for the upstairs circuit. However when I waved my detection 'pen' over the switch, much to my surprise it registered some voltage. So I also pulled the fuse for the downstairs circuit, after which no voltage detected, nor did I find any when I opened up behind the switch and probed with the multimeter. Once I had completed all the work I replaced the downstairs fuse first and retested with the pen - nothing detected. Then replaced the fuse for the upstairs circuit and did get a voltage detected, as I would have expected. All works fine now.
So, what might have caused this curious initial reading? My first thought was some sort of 'borrowed live' in the 3-gang downstairs between the upstairs and downstairs circuit, but I had replaced that 3-gang not long ago and I am certain there was nothing like that. [ In any case, I've worked on other fittings on either the up or down stairs circuits recently without any problem that would suggest an incorrect connection between the two. ] My next thought is that, as the conduit which feeds the 3-gang contains cables from both circuits, was there some sort of induced voltage caused by them being in such close proximity? I also checked the ends of both cables behind the upstairs switch and there was no obvious sign of split insulation that might have lead to a cross-connection.
Any thoughts please on what might have been the cause and/or what else to check for?
TAI
