[paulgarett]

I installed a new sub panel for a customer and AFCI breakers for all the branch circuits. 3 of the AFCI breakers tripped. I discovered the problem for two of these breakers. They were both sharing a neutrals with other circuits. The previous homeowner had a handyman do a bunch of electrical work and he made a lot of mistakes; won't go into all them now.
The third AFCI breaker, however, I have not fixed yet. I had to put a standard breaker in place, temporarily, until I have time to finish the troubleshoot.
So what I found on this circuit was: if I disconnected a hot and neutral power out, located at a switch box, which provided power to the living room outlets, the AFCI did not trip.
I checked the resistance between this hot and neutral, power out, and the resistance reading was about 175 ohms. The living room circuit contains no light fixtures. I though that maybe the resistance reading was from a lamp, but there are no lamps on that circuit. My other guess is improper wiring at one of the receptacles. I did not have time to fix the problem yet.

What are some reasons for getting 175 + ohms between a hot and neutral, if no resistors (none aware of anyway) are connected to this line?

 

[chicken steve]

What are some reasons for getting 175 + ohms between a hot and neutral, if no resistors (none aware of anyway) are connected to this line?

smoke detector?

~CS~

 

[paulgarett]

I don't think so. It would be good to install a few in this house though.

 

[Dennis Alwon]

There must be something else on the circuit or you have a short. Go to the middle of the run and split it there and see which way the problem is.

 

[brian john]

Smoke detector
Door bell

Divide and conquer

Do you have a megger? If so start at 50 VDC and work up.

 

[paulgarett]

I have a megger. Why use a meager on residential branch circuit, not for a motor.
An ohm meter on the DMM would be sufficient. I only read 175 ohms or so, why use a meager? Fill me in. I'm interested.

 

[8V71]

Did you by chance disconnect one, the other, then both, to see if the breaker tripped? (3 tests) Amp clamp both wires to see current draw?

 

[macmikeman]

Exterior Gfi on that circuit?

 

[paulgarett]

No, but that is a great idea. I may have to try that.
Do you know of any tools that may make this troubleshoot easier and faster?

 

[paulgarett]

Yes, but the GFI was disconnected from the section of the circuit under test.

 

[Service Call]

Also a neutral pressed against a ground will cause a AFCI to trip.

 

[8V71]

No, but that is a great idea. I may have to try that.
Do you know of any tools that may make this troubleshoot easier and faster?

With an unknown problem like, where do the wires really go, I wouldn't trust an ohm reading unless I disconnected power to the entire house because a little bit of stray voltage can give you bad readings. The tests that I mentioned will let you know if you're on the right track or not. It will tell you if you have another shared neutral or if you really have a 175 ohm load because the current will be the same on both conductors and match the math.

Occupancy sensors, lighted switches, just some other things to think about.