is this capacitive coupling?

Spent the evening searching the forum & am pretty sure that my prob is cap. coupling, but wanted to run the details past you men of wisdom.

I'm stringing up my "lights down", radial, using T&E. Goes like this: CU--kitchen--bathrm--office--dining--. That's as far as I've got. I have rigged two-way switching in kitchen and dining.

Filament bulbs everywhere save kitch which has an old tube.

On checking the dining lumi with a fluke digital meter, I found a reliable 235V when "On" but 25V when "off" one way and 88V when switched off the other way (2-way-switched).

These odd voltages gradually (over a few secs) decayed away to <1V when I experimentally shorted blue-brown at the lumi. when I did this (stupid maybe but I have total confidence in my superfast new mcbs) there was not even the slightest spark.

I'm using 3&E for two-way switching; some of this is run inside old steel conduit but this is properly earthed.

Thing is, I dont have particularly long cableruns yet - certainly the 3&E is under 15m.

The filaments and tube are all behaving well (on is on and off is off)

Yes, probably. If you want to be sure, without blowing something up, measure the voltages again but with something connected in parallel with the meter to give the capacitance something to work against as a voltage divider.
Either a real capcaitor (for test a few uF taken from a scrap motor or strip light fitting will do) or a small test lamp, such as a 15W filament type. In either case the shunt comopnent should be desiged to take the mains voltages, just in case it isn't just capcitive coupling after all.
If it is, the voltage will be much less with the shunt in place than without.
(actually even adding a second meter in paralell with the first is often enough to change the reading.)

These induced voltages are actually the reason that insulation tests have to be done with a DC - AC readings would be too confusing, as the capcitive effects on a long run can lead to quite significant displacement currents.. confusion for the unwary, and often those who should know better.

Please don't deliberatly short to trip MCBs in future - the contact pads really don't apprecaiate it, and will eventually ablate away. If you must remote trip, avoid surges of a few hundred amps with a resistance - even a few ohms of metal clad wire wound resistance or so will be so much nicer than a dead short.
cheers

mapj1 said:

Yes, probably. If you want to be sure, without blowing something up, measure the voltages again but with something connected in parallel with the meter to give the capacitance something to work against as a voltage divider.

Click to expand...

Or alternatively use an analogue voltmeter (one with a proper meter I mean, not an analogue display on a DVMs LCD panel....)