AFCI Vs. GFCI

Discussion in 'Electrical Forum discussion & Blog' started by Verdeboy, Dec 7, 2007.

  1. Verdeboy

    Verdeboy In the Trades

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    What exactly are the differences between the two, and is the AFCI going to replace the GFCI, or will you need both on certain circuits?
  2. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    I'll start with a cynical layman's view and let the pros fine-tune it:

    1) As you undoubtedly know, the GFCI detects any difference between the current going out the hot wire and that returning via the neutral. If that difference is greater than .006A or so, for .025 seconds, the breaker trips.

    2) The AFCI somehow (via FM?), most of the time, maybe, detects an arcing situation in the circuit downstream, whether or not (sometimes) an arc actually exists. Since arcing leads to fire, the NFPA has decided that detecting an arc, and opening the circuit is a Good Thing, and have mandated (2005NEC 210.12) that they be used to protect outlets in dwelling unit bedrooms, and in some plug-in air conditioners (440.65). Others have commented that the 2008 Code expands the mandates. AFCIs are outrageously expensive, and (as implied above) kind of flaky. As time goes on, they will undoubtedly become better and cheaper.

    While it's theoretically possible to combine both GFCI and AFCI protection on one circuit, I don't know of any devices out there (yet) that do so.

    Mike Holt's forum would be a good place to get the up-to-date and high-quality info on these things.
  3. jwelectric

    jwelectric Electrical Contractor/Instructor

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    That is in tune enough to play at the Grand Old Opery
  4. Verdeboy

    Verdeboy In the Trades

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    Last edited: Dec 7, 2007
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Where required, you'd use an AFCI in place of a normal CB in the panel. Now, you could also install a GFCI outlet on that CB. Spikes in the power consumption would seem to be what the thing is looking for. My guess is things like maybe an electric oven's thermostat kicking on and off, or other things with heating elements and stats could give them grief.
  6. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    Most of the literature I've read talks about "sophisticated" electronic devices designed to detect the "signature" of an arcing situation. As most of us know, you can hear an arc on an AM portable radio (and maybe FM), so there's RF of some sort on the line which such devices might recognize. The sophisticated circuitry probably analyzes frequency spectrum, amplitude, duration, etc. to decide whether an arc has occured. I haven't seen the UL spec, but I would expect the specification to be defined by performance, rather than by specific arc characteristics, so that suppliers would be free to develop the best way to respond.

    If you cast your mind back to the early days of radio, the transmitters used spark gaps to generate the waves they communicated with. It was very broad-band stuff, so it's not hard to detect.
  7. jimbo

    jimbo Plumber

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    A GFCI protects a person from electric shock. If any kind of malfunction, either in some equipment, in the system, or just plain doing something dumb.......causes a person to come in contact with live juice, the GFCI senses that shock and turns off the juice so quickly that a person cannot be harmed.


    The AFCI senses the general term arcing, with the idea that it detects a situation which may result in overheating of some device or some wiring, and hence a fire. This is why they were originally mandated for bedrooms. Apparently enough people were injured or killed by fires in the bedroom, from air conditioner, electric blanket, who knows......
  8. Nate R

    Nate R New Member

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    From the bit of reading I've done, AFCI looks for a specific waveform that usually is created only by arcing.
  9. BrianJohn

    BrianJohn DIY Senior Member

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    For a LAYMAN I thought Mikey gave an excellent explanation.

    And I just assumed his explanation utilizing the term FM meant Freaking Magic (or some other word that could serve in place of freaking)
  10. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    You got it. It's an old engineering term.
  11. BrianJohn

    BrianJohn DIY Senior Member

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    Dang MIkey I thought I was old...1952, Just took a look at your profile. Where in Florida are you?
  12. Alectrician

    Alectrician DIY Senior Member

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    Ground fault protects from a fault to ground. It reads the current on hot and neutral. If there is a difference, it's going somewhere it aint sposed to so it trips

    Arc fault prevents a fault via an arc. I have no idea how it works. Give me a few years and I'll figure it out.


    1952 was a banner year :D
  13. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    Well, you're only as old as you feel, they say, so I'm in big trouble, I guess. I'm in Polk City -- about midway between Orlando and Tampa. We actually have a Wikipedia entry: http://en.wikipedia.org/wiki/Polk_City%2C_Florida, and an official website: http://mypolkcity.org/. So far, no traffic lights, other than the school crossing, but there's serious talk of getting one. Time to move on.
    Last edited: Dec 8, 2007
  14. BrianJohn

    BrianJohn DIY Senior Member

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    Just curious my father is in Palm Bay and last year I was working at a Federal Institute in Leesburg FLA.
  15. Verdeboy

    Verdeboy In the Trades

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    This came from an internet source:

    "An AFCI protection device will detect any 'arcing' (i.e. sparks, where electricity is being conducted through the air and converted to heat) and shut down the outlet before any damage can be done."

    Shouldn't the AFCI device shut down the entire circuit?

    What if the arcing is occurring in a wall somewhere and the AFCI cannot sense the added heat?
  16. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    It's not a heat dectector, but it senses the arcing electronically. Since it is in the main panel, just like a CB, it shuts the whole circuit down.
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