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What is the key difference between a Ground Fault Circuit Interrupter and an Arc Fault Circuit Interrupter?

The Acronyms AFCI/GFCI kind of tell us that they do, detect ground faults and arc faults, But I only have GFCI's on 5 different outlets throughout my house and in the past couple of weeks I've had a few problems that made me think about this question, A projector had an arc that blew the GFCI, an old outdoor receptacle so old that it was falling apart ( not rated for outdoor use ) and the wires were also loose so when my wife went to use the hairdryer it blew the GFCI that was in series with it, then there was an outlet where the hot wire broke off at the receptacle in one of the bedrooms and It blew the GFCI

wouldn't that mean that a GFCI is basically a AFCI ?

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What can happen...

If you look closely at the way many AFCIs behave, they will trip on a gross ground fault. This is no accident; instead, they rely on this behavior to catch arcs-to-ground specifically (it's the same reason IEC systems use RCD protection panelwide). The converse of that is that an arc-to-ground will trip a GFCI as well. This is one reason why Ground Fault Protection of Equipment (GFPE) is called for by certain parts of the NEC -- GFPE devices are like GFCIs, only with a higher trip threshold (30mA or 100mA vs. the 6-10mA of a GFCI), and are only seen in breaker form due to their limited scope of usage.

But, you still need AFCI

However, the discussion above does not cover arcs between hot and neutral. These arcs will not trip a GFCI or a regular breaker, since they simply bypass the normal load path at a level too low for a normal breaker to trip on, and are why AFCIs were originally developed. These use electronics to "listen" for the broadband radio-frequency signature (snap crackle pop) created by arcs, and distinguish harmless arcs (from switching action, hot-plugging of receptacles, light-bulb burnouts, or the commutating of motor brushes) from hazardous ones.

  • 3
    It might also be worth mentioning there are whole other classes of arcs, arcs between hot and neutral, and in line arcs in either hot or neutral, that GFCIs will not catch, but AFCIs will. So a GFCI alone covers some, but not all, of what an AFCI does. – Nate S. Jan 8 at 0:59
  • Thank you, this is actually pretty frightening considering an AFCI youtube.com/watch?v=-rfqqNg-uVE – hello moto Jan 8 at 5:22
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    @hellomoto -- the "series arc" requirements in the UL standard for AFCIs are a bit...half-hearted from a technical standpoint; there were some weird technical-political arguments around the time the AFCI req was adopted into the NEC that led to the "branch/feeder" vs "combination" AFCI thing, and I suspect did the adoption of AFCIs very few favors... – ThreePhaseEel Jan 8 at 5:28
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GFCI and AFCI are as different as a seat belt, vs. the barrels on the highway that absorb impact energy if you would otherwise smash into a concrete bridge abutment. They are both vaguely about safety, but do totally different things.

ThreePhaseEel discusses how some AFCIs include a limited GFPE capability, since it's a cheap way to detect some arc faults.

However, your fault is completely unrelated to AFCI

and may not be an arc fault at all. It is quite possible that your device's hot or neutral wire shorted to ground directly and definitively, with no partial contact, hesitation or sputtering. Then there wouldn't be any arcing to speak of, so not much for an AFCI to detect, if you had one, which you don't.

But it's definitely a diversion of current. Now, current is flowing from hot to ground, or from neutral to ground, causing currents to take three paths: out and back the normal hot/neutral, and also back the safety ground. That means the hot/neutral currents are unequal, which is what the GFCI is looking for.

This misdirection of current is what tripped your GFCI.

You could build a device whose entire purpose was to create snap-crackle-pop harmonics (the "sound" of arcing, you've heard it, which is what AFCIs listen for). let's call this machine Alice. The GFCI would not be bothered by Alice. The GFCI would cheerfully let Alice operate indefinitely, as long as hot current equaled neutral current.

It's also possible - really quite likely - that your device did produce some arc-fault "noises" of sputtering as it failed. Nobody knows, because nobody was listening.

An AFCI doesn't care about ground faults (unless it does)

Conversely, you could build a machine called George, whose entire thing is to induce ground faults: returning neutral current on the ground wire, or having leakage from hot to ground (current-limited so it doesn't trip the overcurrent protection).

As ThreePhaseEel discusses, some AFCIs will notice and trip, because they include a weaker, equipment-protection-only GFCI (GFPE) as a cheap way to detect arc faults to ground. This machine would detect George by virtue of its GFPE feature.

However, there are also AFCIs on the market which operate solely by listening: they do not provide GFPE; they make up for it by being really good listeners. These units don't even need a neutral wire! They wire like a normal breaker, and the manufacturer doesn't even make a 2-pole AFCI; they tell you to use two 1-poles and handle-tie them. Without a neutral wire, these AFCIs are oblivious to any ground fault. These AFCIs would never even hear George, because George doesn't make snap-crackle-pop sounds.

  • I built George and was prepared to return any AFCI it tripped as defective. The ones in my house have neutral wires. The manufacture wasn't willing to tell me how they worked, so I had to test. – Joshua Jan 8 at 16:39
  • @Joshua Well, like ThreePhaseEel and I say, I would expect George to trip many AFCIs, owing to their inclusion of a "lite" GFCI function for the purpose of detecting arc-faults to ground. That does not make them a bad AFCI. – Harper - Reinstate Monica Jan 8 at 16:49
  • I've worked in labs with 'Alice' machines for testing fault monitoring equipment in electricity substations, didn't know there were related devices for residential use, they don't seem to be common in the UK. – Pete Kirkham Jan 9 at 10:26
  • @PeteKirkham Alice and George machines are a fabrication of my mind :). – Harper - Reinstate Monica Jan 9 at 20:00
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What's probably confusing you is where they are used

GFCIs are most commonly found in receptacles. This is because you generally want to be able to reset a ground trip easily, and the GFCI is typically required where you are most likely to have them (bathrooms, kitches, outside, etc.) Ground faults typically don't happen because of wiring problems.

AFCIs are newer and more commonly found in circuit breakers (and NEC 2014 406.4(D)(4) requires them). AFCIs are used to protect you from wiring faults, which you might not otherwise see. While you can buy AFCI receptacles, you generally want the whole circuit covered. The receptacles are mainly for retrofit, where you can't add a AFCI breaker.

What will get you is that AFCI breakers will sometimes do GFCI as well, and they don't always make it evident when they do. As such, it's possible you have an AFCI labeled breaker that trips on ground faults. Many do tell you they are combo, however, but the labeling there gets funny (CAFCI, Combo AFCI, CAFCI + GFCI, etc). You'll need to find your breaker model and look it up to be sure.

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An AFCI detects arcs, even when they are not to ground. An arc has a certain "signature" of rapidly changing current, and the breaker/outlet (somehow) detects that. Generally it also incorporates a GFCI, and the arc fault detection logic shorts hot to ground, triggering the GFCI (and resulting "disconnect"), when an arc fault is detected.

Arc faults can develop, eg, when a wire comes loose inside a lamp. These are the types of faults that would cause flickering in some cases, though in other cases the first sign of failure (absent the AFCI) is a fire.

  • Generally it also incorporates a GFCI, and the arc fault detection logic shorts hot to ground, triggering the GFCI (and resulting "disconnect"), when an arc fault is detected. While that might be true of some devices (like circuit breakers), It's not generally true of all AFCI devices. This Leviton AFCI outlet mentions nothing about GFCI – Machavity Jan 8 at 23:20

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