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why the AFCI kept tripping. New house, finishing up a few odd items today when the floor cleaners came to me and said "soon as we try to use our vacuum in the front bedroom the breaker trips" Huh, must be your vacuum, nobody else has tripped a breaker yet, so I reset the breaker and try my shop vac, trips almost immediately. This circuit has been up and running for months with no issues, lights working fine, plugs work fine except for the vacs. I swapped the breaker with another, problem stays with the circuit. Went on to do some other things so I could ponder on it a while.

Next job, owner wants the bath wall sconce flipped over in the bath that goes with this bedroom, go in there to do that when I noticed a faint humming coming from the 110 CFM exhaust fan which is COMPLETELY COVERED WITH BLUE TAPE AND CAN'T GET ANY AIR. Apparently the tilers covered it to keep dust out, or maybe it was the painters. Once the tape was removed all was well. So the fan must have been really over-amping and the vacs were enough to throw the circuit over the edge, of course first thoughts are always a wiring error or bad AFCI now days.

Just goes to show, I have learned over the years when troubleshooting not to just rush in and tear everything apart right away. It usually pays to think about it for a while and reason things out.

20 years ago we would have grabbed the amp clamp first thing, what I'm trying to say is the AFCIs have changed the way we think about troubleshooting, we are much more conditioned to believe we have a wiring error than an overload which can lead to wasted time.

It would be nice if all AFCIs came with diagnostics, an LED that comes on for arc faults and another for ground faults.

BigB,
That's quite interesting, I would have suspected something like bad brushes/ commutator in the vacuum cleaner motor.

That sounds very foul to me.
I looked up the first 110 CFM bathroom fan datasheed i could find and it has 0.2 amps nominal current. This would only increase a little bit covered up, if not decrease (depending on the fan design).
Where should it dispass all the heat. otherwise, it would bake the windings and hopefully trip the thermal protection.
I'm very sure there is another fault hidden somehwere in the house that tripped the AFCI.

All I can tell you is that as soon as I uncovered the fan everything went back to normal and the owners have been in there for a month now with no issues. Maybe it wasn't an overload but some other motor related phenomena that caused it it simulate an arc fault.

Trouble shooting tip:ALWAYS LOOK AT THE SIMPLE THINGS 1ST!!!
A shorted CFL almost cost me a reinspection fee

Had a service call once where the Bathroom Heater/Fan was tripping the GFCI Breaker.

After taking everything apart and reassembling twice and swapping out Breaker I found out there was a direct short in the Heat Bulb.

Craziest service call I had was 'No lights in stockroom' at about 7PM for a national sport apparel/shoe store.

Checked the panel, employee said lights are on controller; that checked out OK. Started following conduit, saw box by entry to stockroom wall, hmmmmm......

Moved a few sneaker boxes............low and behold a toggle switch in 'off' position. Flipped it 'on'! Manager was in shock! Apparently, a box hit the switch that no one knew was there???

To save face, I removed the switch and spliced thru, as the controller was operational.

It's surprising how many times I solved a non-working outlet problem by turning on a switch that the Homeowner said didn't do anything.