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Report of electrical/lighting anomaly at grandma's house. Obviously, it will be a while before I can check things in person. Would appreciate diagnosis of most likely problem. (A relevant search-result was not found using the particular terms tried.)

From memory (and forgive any misapplied terminology): 2-gang duplex wall plate (i.e. four cutouts). Duplex outlet on left. Double toggle switch on right. One switch controls a "chrome" circline fluorescent ceiling fixture (12 inch + 8 inch bulbs). The other a simple 24" T12 fluorescent over-sink vanity light. Outlets working fine.

Reported that one switch has become "dominant". If ceiling light is on by itself, toggling the vanity on closes the ceiling light off (vanity light comes on). If vanity light is on by itself, flipping the ceiling-light switch does nothing. Each fixture by itself is toggled on/off by its respective switch when used alone.

Is it almost certainly a bad switch, or could a fixture (bulb or ballast) or wire somehow have gone bad to produce this effect?

If it's relevant: all wiring would be 60-85 years old, and insulation could even be cloth-of-any-color. (And it probably wouldn't be impossible that a light switch somewhere in this house may have been wired to break neutral rather than hot, or not have a ground wire connected.)

Thank you.

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    Were any changes made recently to the wiring at this switch? Apr 9 '20 at 11:48
  • Could be that they share a common neutral wire going from the switch box, then at the first fixture, the neutral breaks out and carries on to the second fixture. But the neutral has come loose somewhere, so when the non-"dominant" switch is turned on, the circuit is incomplete until the other one is turned on, because it is back feeding through that fixture and giving the fluorescent ballast enough voltage to work.Open up the junction boxes behind the fixtures and check all of the connections for corrosion. It's common in bathroom fixtures.
    – JRaef
    Apr 9 '20 at 18:44
  • >TPE- Spontaneous change in behavior. No electrical work done for years.
    – revans19
    Apr 10 '20 at 7:13
  • >JRaef- Very interesting. ANY wiring disorder is possible in this house. (If the problem is behind the fixtures, we'd get a pro. Swapping out a bad wall-switch would be no big deal, but years ago I saw what was behind the ceiling fixture, and the old wiring was not only brittle, there was so much stuffed up there, it looked like a full bowl of spaghetti.)
    – revans19
    Apr 10 '20 at 7:33

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