Arc Fault Interrupter (AFI) breaker question (somewhat long)

I am replacing all the circuitry for outlets in the bedrooms of my house, as well as living room, family room and office. I am going to put all the new circuits which supply outlets in these rooms on Arc Fault Interrupter breakers. My house was built in 1910.

The house has two buses of 95 year old pipe and post wiring, one bus runs the length of the attic (the 'attic bus') and the other runs the length of the house within the second floor joists (the 'second floor bus').

The attic bus and second floor bus are wired in parallel and have for many years been connected to a 15 amp breaker in the load center. The attic bus supplied second floor ceiling lights and second floor outlets; the second floor bus supplies three ceiling lights on the first floor.

All of the loads on the attic bus are disconnected. The second floor bus has three loads - all are switched ceiling lights, two have relatively modern, albeit over 30 year-old silent toggle switches and the third has an antique push-button device from years gone by.

I decided that as an interim I might put this pipe and post wiring on an AFI breaker, to help deal with any possible loose connections. I was baffled to discover that when I came upstairs after making that change in the load center that upon turning on one of the first-floor ceiling lights (powered by the second floor bus) that the AFI breaker cut out after perhaps 250 to 500 milliseconds. By the way I have tried this with both of the toggle-type wall switches but not with the push-button antique switch. I also did not try turning on the light before turning on the breaker.

So I went back to the standard 15 amp breaker for this pipe and post wiring, because as far as I can remember (having lived here 31 years) the joint hasn't burned down even once yet!

Is it possible that some arcing in a wall switch might trigger an AFI breaker to drop out? This does not seem physically plausable - arcing should occur mostly on the breaking of a connection rather than on the making of a connection, I think.

I wonder if the nature of pipe and post wiring might interfere with the function of an AFI breaker - because the two conductors are separated by about 12 inches as they go around the house, transients might behave differently on this kind of wiring as compared to romex cable. But this explanation also seems like something of a stretch to me, a grasping at straws, a bit far fetched!

I would like to know if this is something that any others have experienced. By the way I have tried two different, brand new AFI breakers and both behaved exactly the same - there is definitely something that they do not like about that circuit or its wiring. Also by the way the breakers are QO type but have the green test buttons and so are not affected by the recent recall of Square-D AFI breakers.

Baffled, but keeping crossed fingers crossed, in Portland, Oregon - Jim Howe


Reply to
Jim Howe
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experienced. By the

Arc fault problems can be very frustrating to trace. I have found the following; loose bulbs, faulty bulbs (that did still illuminate), ground faults, worn fan motors, and the list goes on. My first guess in your situation is that the problem is in the light fixture you attempted to turn on. Try this: with the AFCI installed, turn each light on one at a time. That will help narrow things down.

Happy hunting,


Reply to
John Ray

Those breakers also respond to ground faults. If your circuit has a shared neutral, or excessive leakage current, then the ground fault function is tripping the breaker.

Ben Miller

Reply to
Ben Miller

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