Electrical – Why did the circuit blow after replacing a switch

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I replaced an older switch yesterday on an existing line of recessed lights. There are five lights on the one switch. On the same breaker; but different switch, there are 7 more recessed lights (all in a basement, in the process of being renovated, and easily accessible; also both lines are missing one bulb, so they are not pulling a full load). On the same breaker there is also an outlet that runs a dehumidifier. This has been the setup for over a year when the project started.

If it is an overload on the breaker it has never been an issue before. Yesterday I switched out an older switch that was very noisy when flipped, for a newer, quieter switch (not new, but pulled from another spot in the house). The major difference is that the newer switch had a ground and and the older switch did not.

When I came home tonight and flipped the switch I heard a noise and the breaker tripped (not the noise of the breaker tripping, an additional electrical noise, like a surge). The dehumidifier was also on when I turned the switch on. I reset the breaker and did it again, tripped again. I had this on for hours yesterday (though I think the dehumidifier went on after the lights were on). When I turn on the line with the 7 lights nothing trips. It only happens on the new switch. Can the ground be messing it up (or rather be pointing to a problem that I didn't know was there)?

Best Answer

There are only a few possible causes:

  • The newish switch was not wired correctly. I think this is the most probable cause: anything recently changed and not working should be reviewed. Maybe there is a stray strand of wire between the switched terminal and ground, or some wiring fault in the lights. If any of these are the problem, turning on the switch will trip the breaker even if the dehumidifier and other devices are off. That is, turning on only this switch is enough to blow the breaker.
  • The breaker has derated itself—that is, it is marginally defective. This happens sometimes, especially with heavily loaded circuits. Breakers get hot, or if they are tripped many times they can experience something similar to heat fatigue.
  • The load has increased: more bulbs or more wattage, or the humidifier is using more power. Many electrical systems do not provide steady voltage. If it has increased recently, from say 114 to 122 volts, that might be just enough to increase the power to where the breaker should trip.

If it were me, I would

  1. Shut off the circuit
  2. Pull the "new" switch and examine it for physical problems.
  3. Use a DMM to check for continuity between the black wire and ground: there should be none—that is, the ohms should be infinite.
  4. Disconnect the white wires (should be a wire nut in the switchbox) and check for continuity between white going to the lights and ground: should be none. The ohms between the white wire going to the breaker panel and ground should be close to 0.0.
  5. Check the resistance of the light circuit with the DMM between black and white. For 120 watts incandescent @ 120V, should be 8–10 ohms (cold tungsten has its resistance of about one fifteenth its hot temperature).

If the problem had not been found after that, I would reconnect the white wires (undo step 4), turn on the breaker, and put a clamp-on A.C. ammeter (actually an analog meter would be better for this test) to measure the current on a black wire, and turn the switch on. It can either spike strongly or not at all. Strongly is bad and means there is a fault in the wiring or fixtures. Not at all means more tracing of wire, perhaps from the circuit breaker panel is needed.

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