0

I'm setting up a workshop and have several tools planed which I've heard don't get along with Arc Fault Interrupters. I don't know if this is true or just something people tell themselves to get out of using the new AFCI breakers... Specifically my 120V circuits will include serval devices with motor speed controllers, PWM heaters, stepping motors, etc. All will cause expensive material rework if breaker/circuits are erroneously tripped. Specific equipment include: a Small CNC MIll, Laser Cutter/Etcher, Industrial Sewing Machine, 3 D plastic extrusion printer, and vacuum pumps. Does anyone have experience with AFCI breakers and these types of equipment? Also, the 2017 NEC requires AFCIs for Home 120V loads, but what is the reading for a dedicated workshop? (I understand that 2020 code will bring them to 240V circuits as well)

3
  • 1
    None of what you've mentioned (possible exception being the sewing machine) is normally an AC-Line connected brush-type motor, which is the usual thing that has problems with AFCI, because there are arcs in normal operation. Once it's gone through a DC power supply the AFCI doesn't get to see anything to excite it. At least two of the items you mention do call for excellent fire/smoke/heat detection (and kill-switch ability if unattended) since they are devices known to catch fire when things go wrong (laser cutter and plastic extrusion printer.) But an AFCI will know nothing about those
    – Ecnerwal
    Sep 21 '20 at 22:25
  • There's no 240V requirement for AFCI in 2020 NEC. Are you sure you are not confusing AFCI and GFCI? Sep 22 '20 at 2:41
  • Copy, your right its gfci requirements that expand to 240v circuits in 2020. Our local codes are years from adopting those fortunately.
    – mark f
    Sep 22 '20 at 4:04
1

The kinds of loads that are commonly associated with nuisance tripping of AFCIs are those with brush-type AC motors, called "Universal" motors, because you have AC power going across brushes to a commutator like a DC motor would, so the AC coming from the AFCI breaker is seeing the arcing taking place in that commutator, which has a similar profile to an arcing fault. So small kitchen appliances, vacuum cleaners, portable power tools and things like that which have little high speed motors will have these types of motors.

In a true DC motor with brushes, the AC supply system is buffered from the brushes via the rectifier that converts AC to DC, so the AC that is coming from the AFCI breaker is not seeing the arc directly. So things like Servos (which are often brushless now anyway), PWM controllers and drives, power supplies etc. all have that rectifier as a buffer and generally don't aggravate AFCI breakers.

But also, this issue of Universal motors causing nuisance tripping has been over blown to some extent. Over time, the AFCI breaker mfrs have been continuously improving the response algorithms of the little microprocessors inside of the breakers to reduce the nuisance tripping.

1
  • 2
    TL;DR is "But also, this issue of Universal motors causing nuisance tripping has been over blown to some extent. Over time, the AFCI breaker mfrs have been continuously improving the response algorithms of the little microprocessors inside of the breakers to reduce the nuisance tripping." Sep 21 '20 at 22:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.