Two way+intermediate domestic light circuit extraneous voltage head scratching

Good to hear that has cracked it. In the old days of filament lamps and moving coil meters that actually took some current to move the needle, we never saw these effects, as the addition of the slightest real load caused the voltage to just  collapse. However the effect was always there to a greater or lesser extent. Now with electronics that starts to do things at lower currents, with fractions of a mA in some cases, the effect of wiring capacitance is more obvious. One of the funniest  I had was a misfiring Worcester Bosch boiler that would come on when it was supposed to be off, but not very often.  In that case a cap between the 'fire now' line from the thermostats and so on, pulling down to neutral, calmed it down so it was no longer 'haunted' !

A capacitor 'load' is a lot cooler and more convenient to fit in a back box or whatever than a resistor - of course just watch the voltage rating, and do not fit too much capacitance - lots of uF lead to a risk of storing a lethal charge if while you are testing things the capacitor gets isolated at the crest of a wave, and unnecessarily large currents circulating when the system is in operation. If you go really mad with tens of uF the in rush of charging the capacitor can shorten the life of light switches etc .

In my experience 0.1uF is a good size for any wiring of less than and upto perhaps 100m long. If you  ever need to do a massive warehouse or something you may need more than one.... (there are also 'snubbers' made to take the edge off arcing motors that are a cap and a small series resistor of hundred ohms or so in an encapsulated blob, and  these can also be pressed into service in the 'anti-flicker' role as well as motor suppression (examples ) if you are ever up a creek without paddle again.)

One to salt away in the book of useful tricks and dodges.

Mike

My Schneider Volt stick sometimes gives false positives where supposedly dead cables run parallel to live cables where the live cable(s) induces a current in the supposedly dead cables.

A test lamp is a good way to double check.

Drummond MTL1000 Spare Bulb - Martindale Electric (martindale-electric.co.uk)

Z.

My Schneider Volt stick sometimes gives false positives where supposedly dead cables run parallel to live cables where the live cable(s) induces a current in the supposedly dead cables.

A test lamp is a good way to double check.

Drummond MTL1000 Spare Bulb - Martindale Electric (martindale-electric.co.uk)

Z.

Of course a cheapskate like me puts a dummy load on it and uses the modern voltage testers (great care needed though)