Short Circuit Troubleshooting

[FS7]

We have an upstairs "loft" consisting of a bedroom and bathroom. It's the only thing up there as it is a ranch-style home with just a little bit of a second floor. We don't go up there much at all as it's only for guests (or for me when my wife was pregnant and thrashing around).

I noticed the last time I went up that there was no power. The breaker (an AFCI) was tripped, so my immediate assumption was that something was wrong with the breaker. Turns out there's a short (no resistance between hot and neutral on the breaker). If I disconnect all of the receptacles, the breaker is fine, so the wiring to the room is OK. I haven't traced the circuit yet.

I have replaced (not just rewired) every switch and receptacle using either back-wired or side-wired terminals, so there are no more backstabs. The short persists. I disconnected all electronic fan controls (two for the ceiling fan, one for the bathroom fan) and the short persists. I guess it could be in a light fixture or the ceiling fan, but this seems odd given the mechanical switches are off (there should be an open circuit, so even if there were a short in the fixture box it wouldn't trip the breaker). I don't see anything obvious in the receptacle or switch boxes.

What are the possibilities here? We haven't hung anything or drilled into the wall, inside or out. Am I looking at rodent damage inside the wall? I know that's unlikely but I'm confused as to what else it could be at this point. It's not a loose connection since I have fixed all of those.

[Mr_Mike]

When I have this type of problem, I pull all devices on the circuit and disconnect the wiring. Figure out the first item on the circuit by turning on the breaker and finding voltage. Connect/wire nut those wires to the next leg and find the next. Eventually, you either have all devices bypassed without shorts, or you find a short. If you get that far, its one of the devices, so go back and add them back in until the breaker trips.

Also...

Is it tripping on an arc fault or on current? Temporarily install a normal breaker and troubleshoot.

[FS7]

When I have this type of problem, I pull all devices on the circuit and disconnect the wiring.  Figure out the first item on the circuit by turning on the breaker and finding voltage.  Connect/wire nut those wires to the next leg and find the next.  Eventually, you either have all devices bypassed without shorts, or you find a short.  If you get that far, its one of the devices, so go back and add them back in until the breaker trips.  

Also...

Is it tripping on an arc fault or on current?  Temporarily install a normal breaker and troubleshoot.

That was going to be my next course of action. I started to do this but what I thought was the point of entry into the room wasn't. I took pictures of the wiring before drywall went up, but that doesn't help find where it comes in (I am 0/1 on that).

The only things still wired are a ceiling fan (with both hot leads disconnected), a three-way recessed light with switches off, a recessed light with the switch off, and a closet light fixture with the switch off. Off the top of my head it doesn't seem like a device is likely but I guess it's possible.

I have tried swapping the breaker and it still trips, but that was with another AFCI. The resistance between the hot and neutral on the breaker is 0, so it's a short and not an arc fault. The other AFCI breakers have about 100k resistance.

[Mr_Mike]

You keep saying zero resistance on the breaker. Is that pulled or live? Lift the wires off the breaker and test the circuit, not the breaker.

I have no experience with AF breakers and have no clue what a breaker would measure if you measured just the breaker. Just concentrate on the wires.

There is also the possibility that you have missed a device in another room or on the exterior of the house.

[FS7]

I measure zero resistance with the breaker mounted but not live (it won't stay live). I can try the wires themselves.

I tested everything outside and it's all still active, so there doesn't seem to be any outdoor receptacles or fixtures on this circuit. I was hoping that would be the case. There may yet be something hidden. I still think that's more likely than a mouse chewing through NM-B.

[TDKPE]

There may be continuity through the load side of the AFCI breaker, even when off.  Best to try the wiring alone.

As to reasons a short could exist in the wiring inside a wall, it's possible a staple was driven too hard, and eventually, through heating/cooling cycles, worked through to contact the copper.  It's not common, but it's not unheard-of either.  And without isolating connected loads like the fan and other fixtures and luminaires, there's no way to know what's going on in there without testing from box to box, as suggested earlier.

But do start with lifting the conductors from the breaker.

[Herb G]

When arc fault breakers first hit the market, there was an enormous recall due to the breakers being faulty.
My money says you have a bad breaker.

[Mr_Mike]

I measure zero resistance with the breaker mounted but not live (it won't stay live). I can try the wires themselves.

I tested everything outside and it's all still active, so there doesn't seem to be any outdoor receptacles or fixtures on this circuit. I was hoping that would be the case. There may yet be something hidden. I still think that's more likely than a mouse chewing through NM-B.

You still need to find the offending run of wire, so the method above is still the best starting point.

[FS7]

When arc fault breakers first hit the market, there was an enormous recall due to the breakers being faulty.
My money says you have a bad breaker.

I already tried swapping with a new breaker...no luck. The short condition tells me a regular breaker wouldn't help either.

[gMike]

Having gone through this on my Father's house after he passed, check the wires at each box where they enter the box and look for frayed/broken insulation.  Depending on the age of the house, they may have added devices after the initial wiring that are in completely different locations from anything else on the circuit making it very difficult to locate a fault.  Let us know how the mystery turns out!