What could explain this odd electrical behavior with a light switching off...

bake_betty

In our 1960s house, a lot of circuits are by room and not split between overhead lighting and wall receptacles. One small bedroom has a switch at the door which powers the overhead ceiling fan light fixture and one wall receptacle, then there are three other wall receptacles not switched. We have it setup so the switched wall receptacle has a lamp plugged in and on, so you flip the switch and it turns on that light. Sometimes we also turn on the...

Originally asked by cr0 on Stack Exchange (diy) (CC BY-SA 4.0).

uphilltech

Great question — answering as the UphillTech team:

Does the light that's momentarily turning off have a soft-start feature? Check this by using the wall switch and observing carefully as the light comes on. Does it come on instantly with the noise of the switch or is there a very brief delay? If it's a silent switch, give it a slap to turn it on while watching the light and compare the timing of the slap sound with the light coming on. If there is a slight delay, suggesting a soft-start feature, I think that feature is being activated by the momentary voltage spike on the neutral wire that comes from disconnecting the inductive load (the motor) from the hot wire. Electronics could sense this as no voltage between hot and neutral. If the...

Full answer by MTA: https://diy.stackexchange.com/a/317822 (CC BY-SA 4.0).

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