AFCIs and GFCIs 1

[tarzan56007]

Just wondering if anyone has any experience with using both a ground fault recepticle in line with a arc fault recepticle, is it better to have one in front of the other? will one nuisence trip the other?

 

[RonShap]

Most AFCI breakers have a GFCI component to its protection characteristic. There should be no difference whether GFCI or AFCI comes first in the circuit.

 

[busbar]

Near-term availability of AFCI receptacles is unlikely with current codes specifying "entire circuit protection"—effectively achievable (only) by AFCIs at the circuit origination.

 

[NEMA6P]

If you want to avoid interoperability problems, I'd do what RonShap suggested;get a combo unit. Probably a breaker rather than a recept.

If you don't use a combo unit, put the AFCI closest to the load. The GFCI uses a differential current transformer to look for current imbalance between hot and neutral, so it will work equally well no matter where it is placed in the circuit. The AFCI uses a digital signal processor to listen for the high frequency signature of a sputtering arc. And that sound changes with distance from the fault due to cable impedence. So the AFCI should be closer to the load. i.e. a recept.

Right now I think the major application of the AFCI is in dwelling bedrooms. There was an investigation by NEMA (or maybe the CPSC) that said that a significant percentage of electrical-related housefire fatalities were caused by smoldering extension cords running either portable AC units or portable heaters. The failure mode was a "sputtering arc" that was hot enough to eventually ignite adjacent combustibles, but not a solid-enough short to trip a breaker or fuse.

AFCIs are still pretty new technology. There is a major war going on right now between the codeheads who are trying to write them into the NEC in the name of safety and the building trade associations who are fighting them. The building trade associations are fighting approval of ACFIs on a jurisdiction-by-jurisdiction basis because of units' cost and the cost of electrician call-backs to replace poorly performing (nuisance tripping) units. All of this has left the AFCI manufacturers in a awkward position and dampened the market, and hence development of these items.

 

[mc5w]

All of the manufacturers of AFCI circuit breakers say the a GFCI receptacle can be used downstream.

An AFCI does have a GFCI built into it but the trip level of the internal GFCI is 30 to 50 milliamperes, which is no good for people protection. For people protection you do need a downstream GFCI that does protect people.

The 30 to 50 mA internal ground fault trip in an AFCI is what usually initiates a nuisance trip. In some cases the neutral has an accidental reground. The manufacturers now recommend doing a 500 volt or 1,000 volt megger test before connecting wiring to an AFCI. Unfortunately, this has to be done for free because most jobs are price jobs and if your bid is too high you do not get the job.

My buddies have also had instances where when a load is turned off or the starting switch of a motor cuts off circuit inductance will force a phantom ground fault to flow through the circuit capacitance to ground. If only 0.02% of the starting current of a 1/2 Horsepower code M 120 volt split phase motor diverts into the motor frame when the starting switch cuts off an AFCI will read that as a ground fault and trip.

Some of the problems with AFCIs also are the result of doing compatibility testing ONLY with brand new equipment with the result that when cheap 120 volt motors are 10 or 20 years old they will trip an AFCI evne though there is no fire hazard.

Mike Cole, mc5w@earthlink.net