In household circuit breaker panels, you'll find two different types of breakers: Arc Fault and Ground Fault Circuit Interrupters.

Ground Fault (GFCI) breakers are typically used in places where water might come in contact with an outlet/wire, such as bathrooms and garages. They have circuitry to sense when the current out is different from the current in, which usually means the circuit is shorted to ground. This is to prevent electrical shock to humans.

Arc Fault (AFCI) breakers sensors to check for arcing, in order to prevent electrical fires.

(And then there's just regular circuit breakers, which trip if the current drawn through them is over their limit, such as 20 amps.)

Why are these two not combined into a single breaker which trips if it detects either condition? Or, alternately, why not have one of each breaker in series for each circuit, for the same result?

EDIT - Looks like AFCI and GFCI are sometimes combined by using an AFCI breaker combined with a GFCI outlet (the kind with the Test and Reset buttons, often seen in bathrooms). But that still leaves most outlets with only current-limiting protection, why?

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    RE "Arc Fault (AFCI) breakers are used in the majority of household circuits," this is not true where I live (USA). Check the prices of AFCI compared to regular breakers, and you'll see why. – The Photon Nov 18 '16 at 16:45
  • I have seen a good deal of breaker panels in various countries, I don't think I have ever seen an AFCI in a private installation and doubt the "majority" part, I would even label them rare – PlasmaHH Nov 18 '16 at 18:14
  • What type of breaker is commonly used in houses, then? – Martin Carney Nov 18 '16 at 18:15
  • I would call them overcurrent breaker if you need a special name, but around here they are just called breakers. – PlasmaHH Nov 18 '16 at 18:18
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    GFCI IS combined with AFCI in breakers, already. And there is the older AFCI type and the newer dual-function AFCI (called "combination"), as well. All of which are combined in single breakers. – jonk Nov 18 '16 at 21:13

There's more going on here than you think.

In the bad old days, all we had was the plain old thermal-magnetic circuit breaker (or worse yet, thermal-only fuses for folks with old stuff). Folks got zapped by hairdryers dropped into bathtubs and watched as poor connections and damaged cords turned their house into a bonfire for the local FD.

In the 70s, as microcircuit technology developed, the Ground Fault Circuit Interrupter was introduced. These devices use electronics along with a current transformer to measure the difference between hot and neutral, and open the circuit if it is excessive -- in the power distribution world, the equivalent function is called a "differential trip". UL set two trip thresholds for these devices -- 6mA for protection of personnel (the GFCI we find in our bathrooms), and 30 or 100mA for protection of equipment (a so-called Ground Fault Protector for Equipment, or GFPE, device). The GFCI was made available in both a receptacle form factor, originally intended for "quick fix" retrofits, and a circuit breaker form factor, originally intended for new construction or GFCI applications outside the scope of 15 and 20A receptacles. GFPEs, however, were only made available in breaker form as they are used in a limited set of applications, mainly to protect long heating cable/tape runs or high-powered feeders where ground faults can cause serious fires.

Fast-forward 20-odd years now, into the late 90s. Microcircuit and microcomputer technology has advanced significantly, and GFCIs have become a well-known part of house wiring, deployed in a variety of wet and damp location applications. Dropping a toaster into the bathtub becomes futile as an assassination method. However, houses are still burning down from electrical faults, aggravated by postwar copper shortages causing AA-1000 wiring and steel screws to be pressed into dwelling unit service for a time in the 60s (aka the aluminum wiring debacle). However, the use of GFPEs on high powered feeders has proven to be a successful fire protection measure in that arena, and some testing performed by CPSC and UL revealed that damaged cords and wires were a major problem for house fires.

Enter the first generation Arc Fault Circuit Interrupter (AFCI). These devices were based on a mixture of analog and digital technologies, and proved highly effective at detecting house-burning parallel arcs, but in order to detect other fire-starting conditions, such as glowing connections, they also had to add the equivalent of GFPE protection into the device. While not enshrined in the UL standards for AFCIs (a rather...debatable requirement for "series arc detection" was substituted for it, at least for the combination type AFCIs commonly deployed), most AFCI makers incorporated this functionality into their devices. (Branch/feeder AFCIs lack series arc functions, relying solely on the GFPE trip to provide protection against glowing connections and other leakage inducing faults.)

However, the AFCI/GFPE combination had a drawback -- people were expecting it to behave like a regular single pole breaker, not a GFPE, and were perplexed by mystery trips when they started installing it. While some of these mystery trips were due to arc-generating devices not being distinguished from real arcs, or EMI, some of them were a function of shared and looped neutrals in house wiring as a result of wiring errors. This, of course, was all blamed on the AFCIs, rightly or wrongly, and also led to GE eventually redesigning their AFCI products to remove the GFPE functionality, replacing it entirely with microcomputer-based series arc detection.

Fast forward another 10 years. The AFCI requirement, while still debated at the local level, has become permanently enshrined in the NEC, and is being expanded much like the GFCI requirement was back in the 80s. One spot AFCI protection was expanded to during this was the kitchen -- with high power appliances, extensive usage, and all sorts of cord damage possibilities, the possibilities for electrical fires abounded here. This, however, collided with the existing requirement for GFCI protection -- you either had to put one of the protection devices in a receptacle form factor (usually the GFCI), or use a subpanel to house a second, series-connected breaker with its attendant hassles. Hence, in the early 2010's, manufacturers started to introduce Dual Function Circuit Interrupter (DFCI) devices that combined combination-type AFCI protection with GFCI protection for personnel. While only available for single phase, 15 and 20A branch circuits at this time, they represent the ultimate in protection.

Furthermore, receptacle (called "outlet branch circuit") AFCIs were developed for retrofit and other limited (such as part of what is called a "system combination AFCI" using a specially listed circuit breaker, supplemental arc fault circuit breaker, or branch/feeder AFCI) applications. There is even a receptacle DFCI on the market, but its application scope is unclear.

Can you have more protection than the bare minimum?

The answer to this question is almost a resounding yes. The main caveats with installing AF/personnel GF protection throughout a building are leakage currents, EMI, and shared neutrals/MWBCs. Once these are licked, then full ground and arc fault + overcurrent and short circuit protection throughout a building can be a reality.

First, some appliances have poor AF/GF compatibility. They generate arcs internally that are mistaken for arc faults, spit EMI onto the power line that confuses sophisticated trip sensors, or simply leak too much current to ground. Once again, though, instead of taking, say, the vacuum back because it's tripping the arc fault breaker for their bedroom, people blame the breaker for the problem -- this has been a cause of serious pushback against AFCI mandates, and is even a problem with plain GFCIs in corner cases.

The other problem is shared neutrals and MWBCs. Shared or looped neutrals are the result of common wiring errors, usually "nut all the whites together" in boxes which are fed by more than one circuit. A simple solution is to keep separate circuits completely separate, but this isn't always practical (say for kitchen small appliance branch circuits). Fortunately, another option is available -- nowadays, you can buy two circuit (/2/2) NM cable that has two distinctly identified neutral wires in it, which can help with this, and also provides a suitable substitute for the use of MWBCs, as two-pole DFCIs are not available despite two-pole AFCIs and GFCIs being stock items.

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  • NEC requires AFCI protection for refrigerators. I have read much about not putting refrigerators on a GFCI breaker, due to the risk of food spoilage with nuisance tripping. Are AFCI's less likely to nuisance-trip a refrigerator circuit? And if so, is it because the ground current threshold for tripping a GFCI breaker is 4-6mA, as opposed to 30mA for an AFCI breaker? – PhilPDX Dec 31 '16 at 22:47
  • @PhilPDX -- a properly functioning fridge shouldn't leak current to ground outside the UL limits (which are at the sub-milliamp level). That gives me an idea though...also, your question re: AFCIs and fridges is a good one that merits a question of its own (post it as a full question and I'll toss you an upvote bone) – ThreePhaseEel Dec 31 '16 at 23:57
  • @ThreePhaseEel: I have discovered that AFCI is incompatible with Ethernet due to the AFCI trying to provide GFCE which Ethernet does not tolerate. Check the schematics of cable modems. They dump DC power onto the neutral line that originated from another circuit altogether. – Joshua Oct 18 '18 at 0:49
  • @Joshua -- you have a really weird cable modem if it's injecting DC onto the neutral, dude. what make and model do you have? – ThreePhaseEel Oct 18 '18 at 1:12
  • @ThreePhaseEel: It's the connection sense wire in the Ethernet port. It's wired straight to ground, only the device only has two power wires so the ground is really the neutral. Every single Ethernet device I've looked at has this property of wiring the connection sense to ground; ergo every two power wire device that uses Ethernet dumps DC to neutral. – Joshua Oct 18 '18 at 1:18

Half of all electrical fires in the US are caused by shorts or intermittent "flexing and weak" breaks in wiring where the impedance is increased at that point, causing heating and eventual fire. In 1999, the NEC formally announced the first new standard to begin a gradual introduction of a new class of circuit protection devices: AFCI.

Actually, the broad term AFCI covers several kinds of protections. A combination AFCI breaker protects against parallel and ground arcing (such as from line to neutral, line to ground, or neutral to ground), series arcing (caused by loose or broken or otherwise high impedance points in a single line), and well as the main function of overload protection.

But the NEC didn't feel they could do everything all at once, so they set out a planned schedule of changes to take place over time. The first was to only require AFCI to cover parallel arcing and NOT to cover series arcing, as parallel arcing protection was more urgent and the series arcing protection was "more expensive" to do and would take time to develop both good technology that was tested to work well as well as to develop consumer acceptance. They also choose to first begin requiring such protection in bedrooms (nope, not kitchens, not bathrooms, but bedrooms.) This is partly because they'd already required GFCI in wet rooms many years earlier. But also partly because bedrooms were the first priority based on actual home fire experiences.

Over time, these protections have rolled out to cover other rooms. They are very close, if they haven't already crossed the date by now, to requiring new construction to cover the entire home with combination-AFCI support and, where appropriate, GFCI.

So to answer your last edited question, I think that today (or very soon) the NEC requires AFCI and, as appropriate, GFCI protection for EVERY single room in the home.

There's more, having to do with MC wiring. For example, when installing new wiring and if the breaker box isn't provided with AFCI/GFCI breakers for some circuit (and for some [probably dumb] reason), then the NEC code requires the circuit wiring leaving the panel box to be metal clad until such protection occurs in the circuit. (To protect it from someone hammering a nail into the wall to hang a picture and accidentally causing a short.) Or, alternatively, it must be covered by at least two inches of concrete.

And more still, such as the fact that bathrooms are to be on their own circuit branch and NOT to be combined with any other branches, such as kitchens or utility rooms or any other kind of room.

This is all for the US. I don't know if it covers Canada or Mexico or anywhere else.

And finally, this just comes from the fact that I sat down and actually read the new NEC code book, about a year ago, since I was getting ready to tear down and completely replace some of the electrical wiring in my home.

I am NOT a trained electrician and what I wrote above is my own interpretation of what I read and listened to. It certainly may differ in some ways from how an electrician is trained. But that's how it read to me. I also watched electrician training videos -- videos capturing meetings with NEC personnel visiting local electrician chapters to explain the new rules. I also went back and read some of the NEC meeting minutes to see why they chose to do things as they did. That's my take on it, anyway.

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