Standard 15A circuit breaker trips randomly under no load

Question

A few months ago I came home and turned on a light switch with no result. Checked the breaker panel and found the a lighting circuit breaker tripped. It reset with no issue.

A few days/weeks go by and the same thing happens again. This time the breaker can't be reset. Waiting a few hours or until next morning allows the breaker to be reset. At the present time the breaker was able to be reset but after turning on lights on the circuit it popped again (can't definitively tie it to a particular light or switch). I'm leaving it off.

Each time this has tripped no lights have been turned on and nothing has been plugged into the lone receptacle on the circuit.

I tightened the screws on the breaker at the panel, no joy. I swapped the wire to a different 15A breaker and that tripped also, so I think the breaker is ok.

I checked the wiring and the connections at the only receptacle and that seem good.

I can check all the switches and see if the connections are tight and look for shorts there.

Items on the circuit: Fluorescent ceiling and closet lights, one receptacle, one exterior flood light with a motion detector.

Question: Does having a light switch off isolate the associated light fixture from the circuit for trouble shooting purposes or do I physically have to disconnect the wires at the light fixture to see if that fixture is causing a short?

Answer 1

Even though you call them CBs, I'm trusting that you know the difference between a plain breaker and a GFCI/AFCI. If your breaker has a TEST button, this advice is no good.

The usual reason for this, particularly in a lighting circuit, is a ground hitting a hot wire/screw, typically on a switch - switches can shift when you operate them. It's either hitting the bare ground wire in the box, or it's hitting the side of a metal box. *

So if you remember a flash or sudden failure when you operated a switch, that's the first place to look.

One thing we often see is people using backstab connections. Those are a lazy shortcut used by builders in a hurry, but they provide generally poor connections. Those cause opens not shorts, so they aren't directly responsible for this. But it using backstabs means they didn't use the screws, and often the screws are left in the full, all-the-way-out** position. These are much more likely to snag a ground wire or hit the side of the box.

Once the screws are down (preferably with the wires under them and tightened to torque), you can avert further trouble by wrapping electrical tape around the perimeter of the switch (or receptacle) so the screws are covered up. I do that to make them safer to handle.

Another way hot wires trip is when they are wearing their "Daisy Dukes", that is, when the wire is stripped too much for the backstab, and leaving exposed conductor for a ground wire to find. That is bad workmanship, but then, so are backstabs generally :)

* Metal boxes are very much your friend. They provide a better safety shield for the wiring, they conduct heat away from hotspots so they stay too cool to start a fire, and (annoying though this may be right now) they assure trips in cases like this instead of leaving a problem to smolder. We've had people tear them out and go plastic in cases like this, but that's just hiding the symptom.

** screws have a detent/stop to keep them captive. It damages the threads to unscrew them any further. For screw-to-clamp types, it will make the anvil fall off, and the receptacle is scrap at that point!

Answer 2

Coming back to this after more than a year. I wanted to give feedback about the solution to my problem and can't remember if I did or not.

I found that the electrician that wired the house had used the back stab method to attach the wires to the light switches. The plastic of the switch body surrounding the wire entry hole had fatigued after 20+ years of resisting the pressure of the bent wire inside the switch box and had cracked. This made a loose connection which shutting doors and even bumping the wall near the switch caused a short. Moral of the story, for me, never backstab.

Answer 3

I had this once, checked the circuits, bulbs, fittings then finally thought "ok, replace the breaker", put a new breaker in and it was cured...

Another one, was a downstairs lighting circuit that was ok when going to bed and not working every morning - breaker off... Checked everything for weeks... Finally was redoing the upstairs wiring and took up a floor board and found 1 long nail that had punctured one wire... I stepped on that board every morning getting out of bed : every time I stepped on it > blown breaker...

Some faults take a huge amount of detective work...