Odd Light Switch Failure

[Wayne Mack]

This is my actual problem, it is not some sort of brain teaser. I have lights in the kitchen that are controlled by three switches. Something has failed and now, under certain combinations, flipping one of the switches either fails to turn the lights off or fails to turn the lights on. I am trying to figure out which switch is the bad one, so I only need to replace one of them. The Table switch does not seem to be the problem, but it is the most inconvenient one to access. Which switch should I replace first and what is the problem? My guess is that either the Stairs or the Hallway switch is leaving a connection floating. the circuit works as expected, but when it is power, then it either fails to turn the light on or off. I'm still at 50-50 choice between the two, but in the past the Stairs switch has occasionally made a popping noise, so I'll go with that one first.

 

[mosaix]

Three-switch wiring is not the easiest to understand. I suggest that you turn off the power and examine the wiring behind each faceplate. It may be a loose wire that just needs some attention. Alternatively, if a wire is broken or corroded and needs replacing then maybe an electrician should be your first port of call.

Edit: If you tinker with the wiring yourself then take a photograph before doing anything.

 

[Dave]

@mosaix beat me to it - Is it one of the switches themselves that are broken or a piece of the wiring between them?

 

[Wayne Mack]

I'm hoping that it isn't wiring. The lights in question are a bank of seven that are flush mounted to the ceiling. Wiring changes would require removing the ceiling and re-drywalling. The wiring is about 23 years old and is standard US wiring code for 110v. As it is not moving and and there are no indications of animals in the ceiling, I don't feel it is likely for it to be a wire failure.

Mechanical failures are far, far more common, so I feel it is safe to isolate it to the switches. If the switches are wired like the outlets I have changed out previously, they were wired using the push-in connections so that inspection of the wire to socket connection is impossible. My main goal in trying to isolate the switch logically is to avoid return trips to buy an additional switch if I am wrong (or buy multiple switches that go unused, if I guess right).

Rest assured, I do know where my breaker box is located and I have all of the breakers labeled. I have also done light electrical work, replacing outlets, installing switches, etc. I just found this to be a very puzzling failure mode and I am still trying to wrap my head around how it is working.

 

[Christine Wheelwright]

This is the best diagram I can find:

Flicking any switch leads to a change in bulb status (if it was on it will go off and vice versa). Each switch has two positions only.

It seems to me that the fourth line and the eighth line in your truth table are incorrect. These are the lines where both the hallway and stairs switches are up. So the table switch is fine and the problem exists in either the hallway or stairs switch.

 

[Wayne Mack]

This is the best diagram I can find:

View attachment 95098

Flicking any switch leads to a change in bulb status (if it was on it will go off and vice versa). Each switch has two positions only.

It seems to me that the fourth line and the eighth line in your truth table are incorrect. These are the lines where both the hallway and stairs switches are up. So the table switch is fine and the problem exists in either the hallway or stairs switch.

I popped the switch covers and confirmed that Hallway switch appears to be a Four-Way switch, it has four terminals. Stairs and Table both have three terminals. This is leading me to believe that the fault must be in the Four-Way Hallway switch as I do not see a way for either of the Three-Way switches to be able exhibit the failure mode of always on and always off. I'll probably put together some sort of table to sort this out for myself.

 

[Wayne Mack]

Here is what I diagrammed out as expected operation:

 

[Wayne Mack]

And this is what I diagrammed as a potential failure mode that explains the observed operation.

Spoiler: Failure Mode

I assumed that the four-way switch, Hallway, had a short in its Up position / cross-over configuration.

 

[Christine Wheelwright]

Nice! Now I'm wondering if it would have been easier to just change the switch without putting that much thought into it.

 

[Wayne Mack]

Nice! Now I'm wondering if it would have been easier to just change the switch without putting that much thought into it.

I would have likely tried changing the three-way stairs switch first. Then I would have gone back to buy another three-way switch and stared dumbfounded when I saw the actual four-way Hall switch.

Besides, the combinations where flipping a switch had no effect on the lights was driving me batty.

Thanks for the help, though. I hope to correct the problem in a single hardware store run.