Ever wonder why your arc-fault breaker trips randomly? An electrician explains how arc-fault circuit interrupters work and what causes AFCI nuisance tripping.

Anyone who built or did major renovations to a home over the last decade is probably familiar with the term arc-fault breakers, or AFCI breakers.

You may even recognize these arc-fault circuit interrupter breakers, with their visible TEST button in your circuit breaker panel. After having been slowly phased in over the past 3 NEC Code cycles, these new arc-fault breakers are now required on almost every 120V circuit in the finished space of the house.

What does an AFCI breaker do?

While the amount of digital electronics packed into these AFCI breakers is amazing, the function of them is quite simple: detect the unique electronic signature of asparking or arcing connection and trip the breaker before it starts a fire.

The arcing can be caused by a nail through a wire, a frayed electrical cord, or a miss-wired or failing product. The goal is to reduce fires resulting from failed wiring.

What causes arc-fault breaker nuisance tripping?

The biggest challenges that we electricians have faced during this introduction of new technology has been nuisance tripping of any product that has a motor with brushes. For example, when you peer into a power drill while it’s running, you can see the sparks created by the brushes. Sparking and arcing are exactly what the AFCI breaker is looking for, and so it will interrupt the circuit power.

The engineers who have designed these AFCI Breakers found that motors, like the ones found in the power drill, vacuums and treadmills produce a rhythmic, mechanical arc pattern. So they program these patterns into the digital “brains” of the AFCI breaker so it won’t trip as often on these common household items.

But at times, older products with motors will cause the AFCI breaker to trip. This is because the brushes wear down on these older motors, which causes the arc pattern to become irregular and not as rhythmic as when the motor was new. This causes AFCI to detect this as an arc and trip the circuit for safety.

While this type of nuisance tripping can be frustrating, the good news is that the AFCI breakers are being continually improved.




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