Switch leg trips AFCI in off position, Why?

J

JoeNorm

Senior Member
Location
WA
I have noticed if I am making up a light fixture on an AFCI circuit with the switch OFF and I cut through the hot and neutral together, it will trip the breaker. Can someone explain why this happens? Thanks
 
In some online research, I found this:

A combination AFCI breaker provides protection against parallel arcing (line to neutral), series arcing (a loose, broken, or otherwise high resistance segment in a single line), ground arcing (from line or neutral to ground), overload, and short circuit.

Neutral to ground as you are cutting is the apparent culprit.
 
JoeNorm said:
making up a light fixture on an AFCI circuit with the switch OFF and I cut through the hot and neutral together, it will trip the breaker
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You are the 3rd person in 20 years to demonstrate how AFCI’s pass UL1699/B.

When not tripping over false positives, or arc impedance, they trip over simple resistance.

Most AFCI’s will trip by holding bare EGC & Neutral in same hand, without touching each other.
 
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If it is an AFCI that has a GFPE component to it, all of them once did, then it is tripping on neutral to ground fault. Though there likely needs to be at least 30mA of current flowing in the incident to make it trip, which there could be if there is other loads on the same circuit at the time.
 
But in this case the hot is switched OFF, so not connected to anything, right? Or is the slight connection through the switch enough?
 
JoeNorm said:
But in this case the hot is switched OFF, so not connected to anything, right? Or is the slight connection through the switch enough?
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I've done this multiple times. You're cutting not just through the hot and neutral, but also through the EGC. The strippers/cutters cause a short between neutral and EGC. With the GFP built into the AFCI, it sees that as a ground fault and trips. The switch doesn't have to be on.
Newer (some) AFCI don't have the GFP function and wouldn't trip under these circumstances.
 
kwired said:
If it is an AFCI that has a GFPE component to it, all of them once did, then it is tripping on neutral to ground fault. Though there likely needs to be at least 30mA of current flowing in the incident to make it trip, which there could be if there is other loads on the same circuit at the time.
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While end-of-line outlets have negligible Neutral-to-EGC potential when properly wired & maintained, MWBC junctions certainly do increase potential between N-G, or energize loose / open neutrals per 300.13(B).

I believe Square-D, Siemens, & Eaton still make AFCI models with 30mA GFPE.
 
ramsy said:
While end-of-line outlets have negligible Neutral-to-EGC potential when properly wired & maintained, MWBC junctions certainly do increase potential between N-G, or energize loose / open neutrals per 300.13(B).

I believe Square-D, Siemens, & Eaton still make AFCI models with 30mA GFPE.
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Siemens do not, and AFAIK, at least some of Eaton's do not.
 
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