Why AFCI?

[bthielen]

I did a quick search but didn't find an answer. Can someone explain what an AFCI is actually doing? In other words, what is it protecting against? My second question is, why did the writers of the code only require AFCI protection in the bedrooms? Seems that whatever we are being protected against would be good throughout the entire residence.

Just curious.

Bob

 

[charlie b]

Re: Why AFCI?

I think this has been addressed in the forum before. But here?s a couple of quick answers.

(1) Suppose a lamp cord (as an example) has been pinched, so that part of the hot conductor is cut. It starts arcing between the strands upstream of the cut and the strands downstream of the cut. The current associated with the arcs flow along the wire, so the current on the hot wire always equals the current on the neutral. Therefore, a GFCI would not detect the event. The arcs are of high currents, but of low durations. Therefore, the branch circuit breaker would not detect the event. Continued arcing could lead to overheating, and to a fire. The AFCI device is designed to detect currents that display the signature of this type of arcing, and to trip before a fire could start.

(2) The code says to put AFCIs in bedrooms primarily because the industry would never accept an immediate change that required them everywhere. As a starting point, it was an easy enough sell. If time shows them to be effective, and if mass-production makes them cheap, you might see them as the one and only type of breaker in any new dwelling unit?s power panel.

Or am I just getting cynical in my old age? Perhaps the code-committee members might give a better answer.

 

[don_resqcapt19]

Re: Why AFCI?

Bob,
Look at this thread on another forum. Also search the archives for this forum. There have been many many threads on this subject.
Don

 

[jimwalker]

Re: Why AFCI?

BOB
You will be seeing afci required in all lighting circuits within a few years,the limit to bedrooms was to prevent a major hike in cost to new homes.Same thing happened with gfci.
As to what they do, there often are situations that create small arcs but not enough to kick a breaker.Things like bad lamp cords,romex NM that is just barely touches a drywall screw,worn out receptacles,switches goin bad.We have all heard the thhn in emt shake when there is a short,well the same thing happens to romex and if near a screw might arc.Takes years of this before it might carbonize and catch fire.So the afci is doing a worthwhile job.Modern electronics have come a long ways.

 

[roger]

Re: Why AFCI?

Charlie B, correct me if I'm wrong, but the series arcing is only detected by the GFP protection incorporated into the AFCI.

The original version (before the GFP incorporation) would not detect series arcs, and would only see parallel arcs which seem to me to be a very rare occurance.

Roger

 

[hurk27]

Re: Why AFCI?

We just had a CH rep. at our IEAI meeting last month, and the way he described it is the GFP in a AFCI will detect any series arc that migrates to the grounding conductor or anything that is bonded to ground. The IC. chip in the AFCI is suppose to detect the signature of the arc wave form in a hot to neutral arc (parallel arc). but as of now there is no detection circuit that can detect the series arc and he said that it would be virtual imposable to have this as any switched load will produce this same signature. and this would cause neusent tripping that they want to avoid. The GFP in a AFCI uses the same principle that the GFCI uses.

[ December 12, 2003, 07:04 PM: Message edited by: hurk27 ]

 

[roger]

Re: Why AFCI?

Wayne, don't you mean series arcs?

Roger

 

[Ed MacLaren]

Re: Why AFCI?

My second question is, why did the writers of the code only require AFCI protection in the bedrooms?

Originally - Electric blankets! (Look up "electric blanket fire" in Google)

but the series arcing is only detected by the GFP protection incorporated into the AFCI.

I've noticed a number of references to series arcing being detected by the GFCI of the AFCI breaker.

GFP and GFCI sensing devices are basically zero sequence CTs, and can only detect current leakage to ground or to an equipment grounding conductor.

My idea of a series arc is a break in either of the circuit conductors in such a way that the arc resistance is in series with the load resistance.

Ed

 

[don_resqcapt19]

Re: Why AFCI?

Ed,
The AFCI cannot actually detect a series arc. The GFP part of the AFCI breaker opens the circuit after the heat produced by the series fault melts enough insulation to cause a ground fault or a parallel arcing fault that the AFCI itself will open.
Don

 

[hurk27]

Re: Why AFCI?

Yes Roger I did. Thanks I guess I need a proof reader LOL