Home Wiring Question - tripping CAFCI breaker

Alright, this one is stumping me. My wife was vacuuming earlier the master bedroom, and a CAFCI breaker tripped. I went to reset it, and it immediately tripped again. Vacuum is unplugged. I swapped the breaker with a guest room, and the master bedroom wiring still trips the new breaker, so I don't think it is the original breaker. All switches are turned off for that circuit, and no devices are plugged in unless there's a weird branch off to some other room I don't know about. 

According to the internet, if it trips immediately while holding down the 'test' button, that indicates a ground fault of some sort. I have measured 100k Ohm between the power and ground at several different outlets and at the breaker itself when I unplug the wires from the breaker. I'd expect this to need to be higher, but AC circuits are weird. Thoughts on what to check next?

When you reset the breaker, did you actually turn it all the way off and then back on, or did you just try to turn it on again? 

In reply to 90BuickCentury :

All the way - I've learned that lesson before, haha. It also tripped the known-good breaker when I swapped breakers, so that also rules out an incomplete reset

Ok. Did you pull the cover plate and inspect the outlet that the vacuum was plugged into? Could have shorted out there. Check the plug and wires for smoke residue.

In reply to 90BuickCentury :

Yep. Fully removed it, and tripping remained. No smells anywhere along the circuit, and 100k ohm between hot and ground at that outlet as well as others in the room and at the breaker

She broke it, tell her to figure it out :)

In reply to Steve_Jones :

That logic doesn't really hold once the bank accounts are linked, haha

It sounds like the neutral and ground are touching at some point.  Those breakers measure current flow going out and coming back, if current is going to ground instead of coming back on the neutral it will trip.  You could try replacing the combination breaker with a standard breaker and seeing if that holds.  

rustyvw said:

It sounds like the neutral and ground are touching at some point.  Those breakers measure current flow going out and coming back, if current is going to ground instead of coming back on the neutral it will trip.  You could try replacing the combination breaker with a standard breaker and seeing if that holds.  

100k ohm tells me I don't have a hard short, but it seems to be a gray area for a soft short, especially since I am measuring with an old, cheap DMM. The breaker I did swap with was also a combination breaker, so I did not try a standard one. I have a spare standard one to throw in, so that'll be easy. The question then is what to do next - if it does not trip, then that just confirms what I already think I know, and I still need to find where the soft short is.  

Maybe it's your vacuum  (we call em sweepers around here).

In reply to gsettle :

Vacuum (and every other device I could find) is unplugged. All switches are off. There should be no load in the circuit

That style of breaker is designed to detect arc faults and is very sensitive. You may have a loose connection somewhere. Find every device on that circuit is and check each connection for tightness. If they installed the cheap receptacles with the push in connection style replace them with the screw clamp type. 

Wayslow said:

That style of breaker is designed to detect arc faults and is very sensitive. You may have a loose connection somewhere. Find every device on that circuit is and check each connection for tightness. If they installed the cheap receptacles with the push in connection style replace them with the screw clamp type. 

Yeah, some of the circuits in the house won't even let you run a DC motor since it arcs at the commutator, and that can trip them. They also did use the push style everywhere. Would it be more conclusive to just disconnect each outlet entirely? Can't be loose if it is disconnected, but what I think is tight might theoretically not be. Or would having no outlet at all on the circuit freak out the arc fault detection?

I'd start with a bad breaker. Chang it. 

The carbon brush motor in the vacuum probably caused the initial fault and now the breaker has failed. 

Toyman! said:

I'd start with a bad breaker. Chang it. 

The carbon brush motor in the vacuum probably caused the initial fault and now the breaker has failed. 

 

No, the breaker was ruled out when I swapped it with a known-good breaker, and the known-good breaker also tripped. 

I missed that. Reading comprehension fail on my part.

Then you are down to pulling every receptacle in the circuit and finding the problem. I'd check the connections and test. If it fails, I'd pull the receptacles and test. If it fails again then you are down to the wiring. In that case, I wish you luck. BTDT to track down a finishing nail that had been driven through a piece of Romex in an apartment. It took 15 years for that nail to short out. It took forever to find and was a nightmare to repair.