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You can buy a $10 tester[0] that will detect open grounds, open neutrals, and any other wiring fault you'd care to think of. It doesn't take an experienced electrician to use; you plug it in, and see which lights turn on.

What I think is more likely in this case is the A/C turning off. All electric motors induce a current whose direction is opposed to the originally applied current (Lenz's Law). This is called back-EMF or counter-EMF. It's also proportional to the rate of change of the magnetic flux. You can see this when you turn on a motor like a vacuum cleaner, and observe the lights briefly dim. The same thing happens when a motor turns off. Additionally, things like vacuum cleaners or fans have relatively little resistance to them, so when they turn off they can coast down - small rate of change, small impact to voltage levels. An A/C or refrigerator compressor presents a relatively large resistance to its driving motor, so when it shuts off, it shuts off HARD. Big rate of change, big induced voltage spike. I don't know about a small motor like you'd see in a household, but large industrial motors can generate hundreds of volts when turned off. This is usually handled with a circuit called a snubber, but it doesn't generally matter in residential applications so it's rare to be included, as a cost savings measure.

[0]: https://www.kleintools.com/catalog/electrical-testers/recept...




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