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g/electrical • Posted by u/panel_pete •

AFCI nuisance tripping on a treadmill circuit — is it the treadmill or the breaker? Troubleshooting

Homeowner has a NordicTrack on a dedicated 20A AFCI in the basement. Breaker trips maybe once a week, always while running, never at startup.

Swapped the breaker for a new Square D HOM120CAFI. Still trips. Put a plain non-AFCI breaker on it temporarily to test and it has not tripped in three weeks.

I know where this is going — the customer wants me to leave the standard breaker in. I am not doing that. But I would like to actually understand the mechanism before I tell them the treadmill is the problem.

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u/admin_jake ADMIN •
✓ Accepted Solution

Both of the above are true and there is a diagnostic that separates them, which is what you actually asked for.

A real series arc gets worse with load and gets worse over time. Controller noise is constant. So: run the treadmill at a steady 3mph for twenty minutes and log trips, then run it at 7mph for twenty minutes and log trips. If the trip rate scales with motor current, you have a developing fault in the machine — brushes, a chafed cord, a loose terminal in the motor housing. If the trip rate is the same at both speeds, it is controller emissions.

Either way the breaker stays. But if it is the machine, you can tell the customer to get it serviced instead of blaming your work, and that conversation goes differently.

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✓ Solution
u/panel_pete OP •

Ran that yesterday. Two trips at 7mph in twenty minutes, zero at 3mph. It is the machine. Cord has a flat spot where it has been under a leg of the frame for who knows how long.

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u/breaker_bob •

And there it is. That cord was going to arc through eventually with or without the AFCI, the only difference is whether it happened with a breaker watching. Show the homeowner the flat spot. Nothing convinces a customer like being able to point at the actual damage.

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u/nec_nerd •

Treadmill motor controllers are SCR or PWM speed drives and they produce exactly the high-frequency current signature the arc detection algorithm is looking for. It is not a false positive in the sense that the breaker is broken — the breaker is correctly detecting something that looks identical to a series arc and is not one.

210.12 has no exception for this. The breaker stays.

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u/milwaukee_mark •

Had this exact fight with a customer over a treadmill. Ended up being resolved by the treadmill, which failed six weeks later. Motor brushes were arcing. The AFCI was right the whole time and I felt pretty good about having held the line.

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