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GFCI operation question

Discussion in 'Electrical Engineering' started by Methos, Aug 25, 2006.

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  1. Methos

    Methos Guest

    the way a GFCI functions, would it trip
    if a bad connection were made to it ?

    (if say, a terminal with the hot or
    neutral wasn't screwed down tight &
    a plugged in load drew current causing
    the terminal to warm up - would the
    GFCI trip ?)
  2. mm

    mm Guest

    I don't think so. Why would the resistance of a bad connection be any
    different from the resistance of a light bulb.

    I wouldn't assume there will be arcing just because a connection isn't
    tight enough. So I wouldn't assume that even an arc fault circuit
    breaker would be tripped by a loose connection. Am I right or wrong?
  3. LightsAREon

    LightsAREon Guest


    A GFIC is made to trip on sudden line voltage changes. A slow warm up
    would not trip the built in breaker. This very problem cause a house
    fire that burned a third of my parents house to the ground. Two wires
    in some old Romex touched and caused a slow short but it was not enough
    to trip the breakers. Therefore, it warmed up long enough to catch fire.

    GFIC's are a nice safety feature but it certainly doesn't catch all problems
  4. ehsjr

    ehsjr Guest

    There's all kinds of "bad connections".
    If the bad connection resulted in a difference
    of 5 mA or more between the current in the neutral
    versus the current in the hot wire, the GFCI
    would trip. The example you gave would not cause
    the GFCI to trip.

  5. John G

    John G Guest

    So far there has been a lot of misinformation in replies to this thread.

    Only the smoke will indicate a simple HOT connection.
    If there is any arcing a AFCI should detect the fault and disconnect.


    "The “AFCI” is an arc fault circuit interrupter. AFCIs are
    newly-developed electrical devices designed to protect against fires
    caused by arcing faults in the home electrical wiring."


    A GFCI will only protect from differences in the current flowing in the
    two conductors beyond the GFCI. It will cutoff the current when a very
    small current difference, 5 or 30 ma., is detected and should do it
    quick enough to save a person from electrocution.

    Neither device is intended to protect from voltage spikes and neither
    device can detect a simple hot connection.

    A hot connection alone will not csause a difference in current between
    the 2 legs of the circuit, only a ground fault can cause that and a GFCI
    should detect that.
  6. Methos

    Methos Guest

    thanks everyone for the responses.

    the powerpoint link was very informative.

    if i understood it correctly, any current differential
    if one terminal has contact resistance (loose),
    wouldn't the current flow on that leg be reduced
    by the amount of resistance ? and therefore trip
    the GFCI ? (since it would see less on one leg,
    vs the other - or am I misinterpreting, since a
    hot/load/neutral circuit is basically in series,
    and current is only depended on how much the
    device draws?)
  7. John G

    John G Guest

    Both wires are part of the same series circuit and the current will be
    the same whileever there is no other path (ground).

    Current in = current out.

    The resistance of the contact will be part of the impedance (along with
    the real load) that determines the magnitude of the current.
  8. mm

    mm Guest

    Let me say what I said another way. GFCI outlets and circuit breakers
    measure if there is a difference between the amount of current going
    throught the hot wire and though the neutral wire. If they are the
    same, the gfci sees no problem. Having a loose connection does not
    mean that will happen. Using an appliance that shorts to your body
    and from there to some other path than the neutral wire *would* cause
    it to trip.

    OTOH, if you took two all-metal screwdrivers, one in each hand, and
    stuck one into each of the slots in an outlett, even a GFCI outlet,
    you could burn your heart to a fine grey ash, and the breaker would
    not trip. Because the same amount of current was going through the
    two conductors.
  9. Bud--

    Bud-- Guest

    I agree with others that a GFCi wouldn't trip on a loose connection.

    A loose connection can produce enough heat to start a fire without
    arcing (a "glowing" connection). Late stages in failure are likely to arc.

    AFCIs sense "parallel" arcs - from hot-to-neutral (a "fault", the F in
    AFCI). Starting in 2008 the NEC requires them to also detect "series"
    arcs, as in a loose connection. As far as I know, no current AFCIs
    detect series arcs, so none would detect a loose connection.
    GFCIs trip on a difference in current between the hot and neutral, not
    line voltage changes. They are primarily for electrocution protection.

    The "slow short" you describe, an arc that trips a breaker slowly, if at
    all, is exacty what AFCIs are designed to protect against. A more likely
    cause is probably an abused extension cord.

    AFCIs also include 30mA ground fault protection (GFCIs have 5mA
    protection). The idea is, I think, that if a ground wire is adjacent, a
    hot-to-neutral arc is likely to also become hot-to-ground.

  10. LightsAREon

    LightsAREon Guest

    It sounds like bud has the right answer. I hadn't heard about the
    upcoming AFIC's but will certainly pass the info on to everyone at my
    office (I'm a construction administrator for a large architectural
    firm). Thanks bud for the education.
  11. PPS

    PPS Guest

    I don't believe it's "upcoming". I think that all AFCI's currently have 30ma
    ground fault protection for equipment.
  12. Guest

    They call it Ground-Fault Protection of Equipment when it is at the
    30ma level.
    BTW get used to AFCIs. They will be on all 120v 15 and 20a circuits
    in a dwelling in 2008 if the code goes as drafted. Comments are still
    open until October.
  13. Bud--

    Bud-- Guest

    Yea - it is to detect the arc, not protect people. AFCIs can have a 5 mA
    ground fault trip and be used as both AFCI and GFCI. It would require 2
    test buttons. Probably would see them if the code change below goes into

    To PPS - I presume "upcoming" is the requirement to detect "series" arcs
    starting 2008, not 30mA ground fault trip.
    Considering the new AFCIs aren't on the market (or are they recently
    out?), and it is only about 1.5 years for field experience until they
    are required in 2008, it seems like requiring the new AFCIs in all
    15/20A dwelling circuits is not a great idea (whatever the wisdom of
    requiring the current AFCIs is). Bet there are plenty of comments on
    that code change.

    An interesting piece on AFCIs, including why normal breakers are
    inadequate for arcs and arcs that shouldn't trip them is at:

  14. Guest

    Usually it would be all the 120v ones.
  15. Guest

    I believe there may be 240v AFCIs but they are not mandated in the
    code yet.
  16. PPS

    PPS Guest

    Only requirement I'm aware of where GFPE's (30 ma GFCI) are required in the
    2005 NEC is Section 426.28.

    "Ook" <Ook Don't send me any freakin' spam at zootal dot com delete the
    Don't send me any freakin' spam> wrote in message
  17. David Combs

    David Combs Guest

    What is an AFIC?


  18. Sam E

    Sam E Guest

    An Arc Fault Interrupted Circuit. A circuit that has no current in it
    because you didn't use an AFCI, your house was on fire, and the fire
    department cut off the power.
  19. Bud--

    Bud-- Guest

    Arc Fault Circuit Interrupter. The AFCI breakers look like, and wire
    like GFCIs. AFCIs trip on arcs. The NEC requires them on new circuits to
    bedrooms. The proposed 2008 NEC requires them for all residential 15 and
    20A circuits IIRC (could still be changed though).

    A good paper from the Consumer Product Safety Commission on AFCIs is at
    It explains the rationalle for using AFCIs describes how they work.

  20. Guest

    | Arc Fault Circuit Interrupter. The AFCI breakers look like, and wire
    | like GFCIs. AFCIs trip on arcs. The NEC requires them on new circuits to
    | bedrooms. The proposed 2008 NEC requires them for all residential 15 and
    | 20A circuits IIRC (could still be changed though).

    And AFCI-only device could be made to work without accessing the neutral
    of the circuit involved. The issue is the AFCI device needs to use power
    to function. Possibly that is the only purpose of the neutral pigtail if
    the device does not include any GFCI function.

    I hope the change goes through. But I would like to see local AHJ rules
    that permit case-by-case exceptions to be made where AFCI devices are
    found to be incompatible with certain appliances.

    And regarding the issue of putting smoke detectors on AFCI protected
    circuits. The simple solution is keep receptacle circuits and lighting
    circuits separate, and put the smoke detectors on the lighting circuits.
    Those circuits should have much less instance of nuisance trips, and
    would more readily be noticed if they are opened, in case the smoke
    detector false to alarm.
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