2 pole AFCI protection

[Minuteman]

I thought of a question while reading the AFCI in 2008 thread and did not want to muddy the water, but...

Ever since AFCI's have hit the NEC, the idea has been "all 120 volt" circuits (single pole). I rewired a house where the furnace 6/3 NM was pierced by a A/C guy's self tapping screw. The fire started above the hall closet between 2 bedrooms. Could a 2 pole AFCI prevented the fire?

And if a 2 pole AFCI would work, why not a MAIN AFCI? (I'm not wanting to pee on 77401's cupcake, but will it work?)

 

[don_resqcapt19]

I rewired a house where the furnace 6/3 NM was pierced by a A/C guy's self tapping screw. The fire started above the hall closet between 2 bedrooms. Could a 2 pole AFCI prevented the fire?

In theory, yes the AFCI could prevent this.

And if a 2 pole AFCI would work, why not a MAIN AFCI?

The currently available 15 and 20 amp AFCIs do not look at the arc signature unless the fault current exceeds 75 amps. This would have to be scaled up for a main AFCI to the point where it would not protect the smaller circuits. Also the AFCI has 30-50mA ground fault protection, (in my opinion, the GFP part of the AFCI really provides most of the protection, not the fancy arc signature electronics) and the total leakage of all of the branch circuits and the electrical equipment in the house may exceed this amount.
Don

 

[dnem]

In theory, yes the AFCI could prevent this.

The currently available 15 and 20 amp AFCIs do not look at the arc signature unless the fault current exceeds 75 amps. This would have to be scaled up for a main AFCI to the point where it would not protect the smaller circuits. Also the AFCI has 30-50mA ground fault protection, (in my opinion, the GFP part of the AFCI really provides most of the protection, not the fancy arc signature electronics) and the total leakage of all of the branch circuits and the electrical equipment in the house may exceed this amount.
Don

A big advantage of all 120v circuits in the house going to AFCI that never gets mentions is that 3 wire HRs will always be landed properly. The multicircuit HRs will need 2poles. You'll never get the black and red on the same side of the phase with doubled current on the neutral.

I my opinion, doubled neutral current is one of the worst hazards still often present on brand new houses.

David

 

[charlie b]

In theory, yes the AFCI could prevent this.

Are you sure, Don? Based only on the original description, I am not so sure.

We don't know whether it was arcing that started the fire. We don't even know if there was any arcing at all. A screw inserted into a 6/3 NM might just cause a leakage current to ground. The current might not have been high enough to trip the breaker, but still be high enough to create a local hot spot.

I am not going on a campaign against whole-house AFCI coverage. But that's only because I am not a political animal. I'm against it, simply because I have seen no evidence that house fires would be prevented by it. This particular house fire, in my humble opinion (OK, maybe not so humble), would not have been prevented by AFCI devices. So I would really hate to see it used as an example of why we should use AFCI breakers everywhere.

 

[Minuteman]

No, the insurance adjuster said that the screw nicked the ungrounded conductors and OVER TIME caused a phase to phase short. The people were asleep and one made it out.