GFI Caused a Fire!

[Robert Green]

From my understanding of MWC's (which could easily be wrong) if the shared neutral fails for any reason, you can potentially end up with much more than

120VAC where you don't want it. If a neutral fails in normally wired pair of outlets, there's usually no risk of voltage escalation or imbalance. However a failer neutral in a MWC can play out like this:

There are a couple of other considerations that affect the cost analysis, too. I have always been able to buy 12/2 w/g Romex on sale for much less than 12/3 w/g. So much less, in fact, that there was no difference in the price between running two 12/2 cables and using a MWC using 2/3. I assume that's a function of how much more 12/2 is manufactured than 12/3. It took a little longer to install and staple, but I really prefer separate neutrals. I assume that the different in installation time is a much bigger concern to builders of large tracts of homes.

MWC's require (at least I am pretty sure NEC and CEC say so) a double pole breaker requiring de-energizing both circuits to do repairs. Not a big issue, but at times it's been nice to have a nearby live outlet to plug tools into when I am working on a different circuit in the room. Not sure if the DP breakers with a single handle for MWC's come in "dual skinny" format, either, which is another concern for folks wanting to add new circuits to an older box.

I do have one multiwire circuit in the panel to power my XTB X10 repeater/coupler/amplifier but nothing else is plugged into that outlet, which has about 6' of wire running to the nearby circuit panel. Even that outlet is wired with two separate runs of 12/2 w/g Romex.

I am sure there are labor cost savings for developers building a series of houses, but in my case MWC's didn't make much sense except for the special case of the X10 device that by design has to straddle both hots to communicate between the phases.

[Robert Green]

I tend to classify houses by technology. Really old is K&T wiring, pretty old is cloth covered wiring, newer is grounded circuits and newest is GFCI/AFCI protected.

[bud--]

I dug into my archive of downloads. Of particular interest is

why have AFCIs available fault current on a branch circuit AFCI detection - block diagram "bad" arcs and normal arcs 30mA ground fault detection It might answer some of your questions. Details of detection, as gfretwell wrote, are probably proprietary.

Likely of less interest is:

includes info on 5A trip Note that "branch/feeder AFCIs" were the original ones that did not detect series arcs and are no longer available.

Prices likely will decline like GFCI prices did. But AFCIs are more complicated.

You can also use mechanical pencil leads wired in series with a load that runs at higher than 5A. Pencil leads give a pretty smooth arc. Is it a "bad" arc or a "normal" arc? Check the cpsc.gov link for some info on good and normal arcs.

I would just assume that if the test button works the AFCI is OK. There is a page at UL that says about what TimR got from inspectapedia. I do not find it surprising that testers are not available.

[bud--]

AFCIs include ground fault protection. I believe it is required at 50mA or less and is provided at 30mA. The cpsc.gov link I posted has information on ground fault protection and why it is included.

GFCIs will trip with a ground fault of 5mA, so AFCIs don't replace GFCIs. Another difference is that GFCIs have a couple additional components that will trip the GFCI whenever there is a downstream N-G connection, load or no-load. AFCIs will trip with a N-G downstream connection (as gfretwell wrote) but there has to be a load to produce a voltage drop on the neutral wire.

I don't think the NEC requires both GFCI and AFCI protection at the same location. GFCI wire-through receptacles can be used downstream from AFCIs.

[bud--]

I have a post that covers receptacle-type AFCIs, including that you can install them adjacent to the panel and use them to protect the circuit downstream (about the same as an AFCI circuit breaker). For new circuits, the NEC is particular about the wiring method from panel to AFCI receptacle.

AFCIs will be most useful when the new circuits they are installed on become old circuits.

[clare]

Split circuit receptacles were required by code in kitchens before GFCIs were required - and the ONLY way to put GFCI protection on these split kitchen outlets is to use a 2 pole panel mounted device.

Current code prefers non-split 20 amp GFCI outlets within a 59 inch radius of the sink, but still allows split 15 circuits

[bud--]

AFCIs include ground fault protection, but it is at about30mA. GFCIs will trip with a ground fault of 5mA, so AFCIs don't replace GFCIs.

I don't think the NEC requires both GFCI and AFCI protection at the same location. I haven't seen AFCI+GFCI devices. GFCI wire-through receptacles can be used downstream from AFCIs.

[gfretwell]

It will in 2014 (kitchens for sure and maybe bath I don't have my ROP here). As usual in this AFCI boondoggle Cutler Hammer has the device and drove the change.

[gfretwell]

I just looked at John's post on ECN The GFCI and AFCI will be kitchen and laundry,

[Guv Bob]

Girl Friend Interruptus will definitely cause a fire.

[Stormin Mormon]

[Nate Nagel]

In my case, not any more than doing it any other way - I didn't have a good path to the basement and the house was wired the old school way with the primary wiring paths going from ceiling box to ceiling box so ripping everything out and starting over was easy. I reused the homerun to the panel because it was BX and would have been hard to repull. I did have to pull one run from a wall recep to a ceiling box behind a plaster (exterior!) wall which if I'd missed it would have required me to open the wall... but fortunately I didn't mess that one up.

nate

[Nate Nagel]

To be fair, I would be hard pressed to describe the difference between an undesirable arc and one caused by a plug being withdrawn slowly under load, so it is hard to fault the breaker for not being able to determine it.

nate

[abdul saleem]

[Nate Nagel]

yes they are but not at the same level as a GFCI. A GFCI intended for personnel protection will trip at 5 mA of fault current, the ones in an AFCI breaker are a higher threshold, I think 30 mA? so while they do pretty much the same thing you cannot rely on the AFCI to provide GFCI protection where required.

nate

[Nate Nagel]

OK, I see that the question was already answered... sorry...

I actually had a situation where I was required to use AFCIs and GFCIs together. In my last house while I was in the process of rewiring the upstairs, I had half of the floor properly grounded and half still using the original cloth covered ungrounded NM cable. Unfortunately the PO of the place had installed grounding type receptacles anyway throughout even though there was only originally one box with a proper ground. Strictly reading the code at that point I could not install ungrounded receps (I did order some but never installed them - my eventual goal was to rewire the rest of the floor including the original wiring that was still reasonably acceptable, but we sold the place first) alternately I could provide GFCI protection. So that is what I did, I installed a GFCI recep at the first box on the circuit and an AFCI breaker in the panel (since it was a floor with three bedrooms.)

It seemed like a Mickey Mouse setup, but I never had any trouble with it.

One concern I did have though was what if someone were living in a house that needed some kind of life support equipment? Could not a nuisance trim be potentially fatal? I'm guessing people more knowledgeable than I have already run the numbers on this and have determined that AFCIs are less of a net risk than not having them.

nate

[Nate Nagel]

If there's a mixed neutral on a non-Edison circuit, that's a code violation, potentially a hazard, and should be fixed!

nate

[Nate Nagel]

It's basically a 120/240 two-phase circuit with two hots and a neutral. The advantage of using it is that it requires two less wires to be pulled than two separate 120VAC circuits, because only one neutral and one ground is required rather than two of each. The reason that you can get away with it is that if you are only using 120VAC loads, but they are evenly split between the two hot legs, the current in the neutral will actually be zero! This is because the two 120VAC hots are 180 degrees out of phase (they add to a potential of 240VAC) it's very similar to the way a three phase system works with 480VAC/277VAC if you are familiar with that but with only two "phases."

Apparently this was devised by Edison as a means to provide two DC circuits with only three wires (I guess the two "phases" in that case would have been one wire at the positive nominal voltage relative to ground and the other would be at negative nominal voltage relative to ground) and someone appropriated the terminology to include the analogous AC configuration... which makes sense as we all know that Edison was a proponent of DC not AC. (Tesla and Westinghouse were the AC guys...)

The problem with using an AFCI on an Edison circuit is that you would need a two pole breaker, which do exist but are unsurprisingly more expensive than a single pole unit (or even two single pole units!) An alternative would be two single pole breakers, but the manufacturer would have to list them as being acceptable to use with a handle tie and I haven't researched it enough to determine if there are any like that or not.

I'm assuming what he means by a "mixed neutral" is one where neutrals from one circuit are cross-connected to another circuit. That would be a code violation and has been for ages - when neutrals from different circuits are connected together you can no longer assure that the currents in a cable sum to zero which has been a requirement for a very long time.

So... you shouldn't have that situation, and if you do, might as well find out about it now and fix it (should not require opening any walls unless you have hidden splices which would be another violation...)

nate

[Nate Nagel]

That's a Canadian thing... it is not required in the US and appears not to be common practice although it would still make sense to do it that way in many situations.

nate

[Nate Nagel]

Can you use pigtails in Canada? (e.g. purple wire nuts with copper pigtails) that could be another option if for whatever reason it is inconvenient to use the breakers)

nate