Induced voltage or dodgy wiring?

It does sound like it's just picking up a harmless stray voltage from an adjacent conductor - more likely capacitive coupling rather than induction - possibly between L and SL in the T&E switch drop. (Presumably the single to the wall lights happens to be spaced further away from the L so is affected far less)


The same effect has been known to make low energy lamps flicker on when switched off - it's not that unusual.


Modern high impedance voltmeters are notorious for spotting such things - older "analogue" meters tended to have a lower impedance so just connecting the meter would usually allow the voltage to collapse to next to nothing. A kludge would be to lower the impedance of your meter by shunting it with a moderate resistance (a few k Ohms) - if the voltage then disappears, then it's almost certainly just capacitive coupling and nothing to worry about. (I gather some meters have such a facility built-in these days). Probably worth doing a check just in case it is something nasty giving similar symptoms, but I would have thought the chances of that were low.


   - Andy.

I agree with Andy.


I first noticed this when testing a 2-way lighting circuit. As expected, I got 230 V for the Zs test, but when I went to change over both switches, I noticed that the voltage did not drop to nil. IIRC, it was about 20 - 30 V.


That had me puzzled, so I tried it at home. Similar result. Then I put a multimeter in the circuit and with the switch off, I got about 1 mA.


So off does not mean 100% off. ?

I thought for a moment you were going to say the neutral for the wall lights comes back via the sockets.  I presume for now it does not, but if it did, that would be bad, but may not show on your tests.


If this voltage is serious or not rather depends on the input impedance of your volt meter -- if you have a megger, you could measure the input resistance of your MFT  on the volts range, and the voltage of the megger at the same time.. If not then the data sheet page of the handbook may reveal.

I suspect it is very high (some millions of ohms) and you are not reading 23V from some odd faulty connection, so much as the effect of a very small capacitive coupling between parallel cables.

If the line going out to the lights before and after the switch run close by one-another for a few metres, then a (very poor) parallel plate capacitor is formed , with the two 'plates' being the live and switched core respectively. This may be a few hundred picofarads or so per metre of adjacent cable (small parasitic capacitors are formed in this way between lots of other things too, but they do not normally trouble us or show up on the meter. This sort of thing only really matters when you need to consider operation at higher voltages or higher frequencies)

The clue is that makes me thing this is that the 23V falls to near zero when any current is  drawn.

If you want to be sure, and are feeling brave, (and at 23V you can be, but please do not be brave with significantly higher voltages) even a wet finger across the same place as meter will cause a voltage from such capacitive pick-up to dip quite noticeably. A better and safer  test uses  a few K of resistance or a small capacitor of 100nF or so on croc clips.

If the voltage does not droop, it is not a capacitance effect.

I suspect in this case  it is capacitance and if so all it is telling you is something about the cable layout in the wall, and nothing is wrong at all.


EDIT, OK, I'm too slow, the others beat me to it while I was composing. But we agree.

Thanks for the help gents, much appreciated.


Regards


F

I measured something similar recently with three different instruments, Megger MFT  230V, Fluke DMM  170V, old analogue AVO (Mk7 IIRC) 7V.  The last was surprise given its relatively low impedance (circa 10k ohms on the range selected).  Stray capacitance to deliver all this was order of a few nF over about 100m aged T&E cable.

It's funny that this topic crops up now as recently I saw a video on YouTube  by David Savoury look it up as its a very informative video.  Word to the wise don't watch it whilst eating seriously don't you'll be sick

"If you want to be sure, and are feeling brave, (and at 23V you can be, but please do not be brave with significantly higher voltages) even a wet finger across the same place as meter will cause a voltage from such capacitive pick-up to dip quite noticeably. "


Don`t we still do this with mains voltage these days then? ?

The really good engineers from in earlier era keep both hands behind their backs when inspecting  live wiring, or looking into machines with whirling sharp bits, and look rather like Prince Charles does on a Royal visit.  For testing at least one hand stays behind the back, that way you do not become the load resistor that completes a circuit, and if the worst happens you have a spare one behind you.

Folk raised in the era of interlocked safety guards and so on may be caught out when the power stays on even with the covers off.