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GFCI operation question

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

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  1. John Gilmer

    John Gilmer Guest

    And it saves a little money. It's looks like the GFCI/CB doesn't have the
    extra coil that injects the signal to detect a Neutral/Ground cross. Is
    that true?

    EMWTK
     
  2. Guest

    18:38 GMT, wrote:
    |
    |>So why is the neutral opened?
    |
    | In receptacle devices they open both sides because this may be used to
    | extend older wiring where there may be a polarity reversal. In K&T
    | polarity was far from a priority and you pretty much have a 50:50
    | chance on which wire is hot. With no other ground reference handy it
    | is hard for a contemporary installer to figure it out

    So basically they will work miswired (at least without a ground attached,
    if there is a way for them to detect the miswire with a ground reference).
     
  3. Guest

    | What makes you so certain that a GFCI circuit breaker does not open the
    | neutral? Have you checked with several manufacturers.

    I've looked at the engineering diagrams, cut-aways, and schematics.
    There are no contacts for interrupting the neutral wire.


    | One reason why it might be OK for a breaker to leave the neutral alone
    | is that it is far less likely and in fact rather difficult for a breaker
    | to be revere wired. When a breaker type GFCI operates it will nearly
    | always open the ungrounded conductor. There are a lot more ways a
    | receptacle type of GFCI can be supplied with the ungrounded conductor
    | controlled by the grounded conductor leg of the GFCI mechanism.

    So basically, there is no goal or interest in specifically opening the
    neutral. It's just a case of opening both in situations where either
    might be the neutral.
     
  4. Guest

    On Sun, 01 Oct 2006 21:30:31 -0400 wrote:

    | On Sun, 01 Oct 2006 17:14:18 GMT, "Member, Takoma Park Volunteer Fire
    |
    |>What makes you so certain that a GFCI circuit breaker does not open the
    |>neutral? Have you checked with several manufacturers.
    |
    | Square D doesn't
    | http://members.aol.com/gfretwell/gfci.jpg
    |
    | There is no real need for it in a breaker since you know which leg is
    | hot.

    MAYBE there is no real need for it. The fact that GFCI receptacles ARE
    opening the neutral because of the chance of miswire or usability on old
    wiring like K&T, we might not know if there are hazards to leaving the
    neutral connected. Statistics for this would also be harder to gather
    since if there are such hazards, they would be less often and/or less
    severe than with the hot left connected.

    Still, I personally believe there is a small NON-ZERO hazard, generally
    associated with other faults or conditions. One is the possibility of
    higher than expected voltage on the neutral due to a loose/open neutral
    condition upstream. The other is that even with a normally very low
    neutral voltage, a solid fault from neutral to ground could still pull
    the 6ma that would trip the breaker. The hazard exists *IF* the breaker
    would continue to be activiting its mechanism to open the circuit even
    after it is already open, if the mechanism is not rated for continuous
    duty heat dissipation.

    And speaking of heat, look forward, post 2008, for a lot of new home panels,
    the ones now FULL of AFCI breakers, to be more crowded with pig-tail wires,
    AND getting extra hot due to all those AFCI breakers continuously (24x356xN)
    using a small bit of power.
     
  5. PPS

    PPS Guest

    A "neutral" is not defined in the NEC, but is described in Article
    310.15(B)(4) as carrying "only the unbalanced current from other
    conductors...".

    In a 120 volt (lighting) circuit, the current is carried on both the white
    (or "grounded") conductor and an "ungrounded (usually black, but not
    necessarily) conductor.
    The term "neutral" refers to the neutral connection at the transformer; the
    center-tap. In a pure 240 volt circuit, current flows on the two phase
    conductors and a white (or grounded) conductor in not even needed. By
    introducing 120 volt circuits, the white forms one of the return legs, and
    carries current. (240 v between the ungrounded legs, 120 v from either leg
    to the neutral.)

    In Europe, the term "neutral" does include a grounded conductor in a 120 v
    circuit. In the states the term is used interchangeably but in error.
     
  6. Bud--

    Bud-- Guest

    Easy to find the hot with a neon test light by touching one lead and
    probing with the other.

    Opening both wires also protects from miswiring by people who don't know
    what they are doing. GFCIs are now fairly idiot proof.

    They don't need and don't use the ground wire, except to pass it through
    to the receptacle, so they work with line H-N reversed. One would
    presume the detection of a downsteam N-G connection at no load would not
    work as the current injection is in the hot wire, but downstream N-G
    connection would cause a trip when there is a load.

    bud--
     
  7. Guest

    | A "neutral" is not defined in the NEC, but is described in Article
    | 310.15(B)(4) as carrying "only the unbalanced current from other
    | conductors...".

    "Neutral conductor" is defined in the NEC. The term is used in many places.
    I fully understand what a neutral is. Are you raising an issue about its
    common or formal usage?


    | In a 120 volt (lighting) circuit, the current is carried on both the white
    | (or "grounded") conductor and an "ungrounded (usually black, but not
    | necessarily) conductor.

    If you are wanting to get very specific, it's the insulators that have the
    color. The current is carried on the (usually) copper metal (with a magnetic
    field, of course).

    The code requires the grounded conductor be identified well (e.g. continuous
    color, not just marked at each end). Others have more leeway.


    | The term "neutral" refers to the neutral connection at the transformer; the
    | center-tap. In a pure 240 volt circuit, current flows on the two phase
    | conductors and a white (or grounded) conductor in not even needed. By
    | introducing 120 volt circuits, the white forms one of the return legs, and
    | carries current. (240 v between the ungrounded legs, 120 v from either leg
    | to the neutral.)

    You could tap the transformer off-center a bit if you wanted to and have
    115 volts on one side and 125 volts on the other side. Would you call that
    a "neutral"?

    So tell me ... what happens if you have a couple of very low power factor
    loads, one on each 120 volt side, where one is very inductive and the other
    is very capacitive? Now how much current flows on the "neutral"?


    | In Europe, the term "neutral" does include a grounded conductor in a 120 v
    | circuit. In the states the term is used interchangeably but in error.

    I see very little use in error in the US. Neutral does not mean grounded,
    but it generally implies that because that is the required way to wire it
    up. See NEC 250.26(2). The two terms "neutral conductor" and "grounded
    conductor" do have different meanings, but are associated with the same
    wire because that is the required way.

    Single phase in Europe is generally 2-wire service. You can still call one
    wire neutral because it may well be the wire connected to the real neutral
    point in either a single phase transformer (center tapped 230/460) or a
    three phase transformer (connected to the star common). But it is grounded
    and thus (also) correct to call it a grounded conductor. If the service is
    coming from a 2-wire transformer all by itself, then it's not really neutral;
    it's just grounded.
     
  8. Guest

    | wrote:
    |> On Sun, 01 Oct 2006 13:07:48 -0400 wrote:
    |> | On 1 Oct 2006 10:18:38 GMT, wrote:
    |> |
    |> |>So why is the neutral opened?
    |> |
    |> | In receptacle devices they open both sides because this may be used to
    |> | extend older wiring where there may be a polarity reversal. In K&T
    |> | polarity was far from a priority and you pretty much have a 50:50
    |> | chance on which wire is hot. With no other ground reference handy it
    |> | is hard for a contemporary installer to figure it out
    |>
    |
    | Easy to find the hot with a neon test light by touching one lead and
    | probing with the other.
    |
    | Opening both wires also protects from miswiring by people who don't know
    | what they are doing. GFCIs are now fairly idiot proof.
    |
    |
    |> So basically they will work miswired (at least without a ground attached,
    |> if there is a way for them to detect the miswire with a ground reference).
    |>
    |
    | They don't need and don't use the ground wire, except to pass it through
    | to the receptacle, so they work with line H-N reversed. One would
    | presume the detection of a downsteam N-G connection at no load would not
    | work as the current injection is in the hot wire, but downstream N-G
    | connection would cause a trip when there is a load.

    If there is a voltage imbalance in the system, there could be enough to
    have at least 6 milliamps flowing down the N of a circuit and back up
    the G of that circuit. Remember, electricity does NOT take the best
    path to return ... it takes ALL the paths. So you really could see a
    wiswired GFCI just up and trip with no load downstream of it.
     
  9. Guest


    Phil, give me an example of a neutral that is not grounded in an NEC
    compliant circuit.
    I do understand in a corner grounded delta the grounded conductor does
    not carry the imbalanced current but that is the only example I can
    think of when a grounded circuit conductor is not a neutral. It is
    still identified white or grey.
     
  10. daestrom

    daestrom Guest

    Uh, I thought the 'current injection' was on the groundED conductor. No
    need to inject a signal in the ungrounded conductor ('hot') as a fault there
    will be sensed immediately.

    A 'current' signal is injected on the groundED conductor (neutral) so if it
    is connected to the groundING conductor downstream, the GFCI will trip
    *immediately*, not just when a load is connected (been there, done that :-(

    daestrom
     
  11. daestrom

    daestrom Guest

    Don't see what your thinking there. If the groundED conductor has no load
    on it, then it's at the same potential at the GFCI as it is in the service
    panel. Ditto for the groundING conductor, so there is no current flow
    through a groundED - groundING conductor fault downstream of the GFCI.

    Except... GFCI's deliberately inject a small signal into the 'hot' and
    'neutral' that is the same in both. Doesn't affect the load normally
    (common mode signal), but it *will* generate enough current through a
    'neutral-ground' fault downstream to trip the GFCI with no load attached.
    So a groundED - groundING fault downstream is detected and trips the unit,
    regardless of load.
     
  12. Guest

    09:46 GMT, wrote:
    |
    |>I see very little use in error in the US. Neutral does not mean grounded,
    |>but it generally implies that because that is the required way to wire it
    |>up. See NEC 250.26(2).
    |
    |
    | Phil, give me an example of a neutral that is not grounded in an NEC
    | compliant circuit.

    I know this isn't exactly what you might be thinking of, but since this
    is more about term definitions than about practical wiring, let me point
    to 411.5(A):

    Grounding. Secondary circuits shall not be grounded.

    Of course that's part of article 411 covering lighting systems operating
    at 30 volts or less. Typical systems operate at 12 volts. It could be
    quite practical to construct such a system using a transformer with a
    secondary supplying 24 volts which is center tapped, giving 12 volts in
    two separate poles, e.g. a 12/24 volt system, very much like the common
    North American single phase 120/240 volt system, except that the neutral
    is not grounded per 411.5(A).

    OK, so there, I met the letter of your request.

    And we might also consider 250.21 and 250.22, although I don't know that
    I would want anything more than a 2-wire system for any of those.

    What is not clear to me, yet, is whether white or gray is a valid color
    for the insulation of that UNgrounded neutral wire on the 12/24 volt
    ungrounded lightning system described above. 200.7(B) and 250.20(A)
    don't make this obvious for an article 411 system.


    | I do understand in a corner grounded delta the grounded conductor does
    | not carry the imbalanced current but that is the only example I can
    | think of when a grounded circuit conductor is not a neutral. It is
    | still identified white or grey.

    Just because the voltage and current phases are systematically not at a
    nice 180 degrees apart, is not a cause for me to not refer to it as a
    neutral. Consider the rather common case in many locations where single
    phase service is provided by supplying 2 phases of a 3 phase system at
    120/208 volts. NEC 250.26(5) refers to this as a neutral conductor.
    Why would two phases at 120 degrees consider it as a neutral and two
    phases at 60 degrees not? Of course there is an imbalance present as
    310.15(B)(4)(b) addresses by requiring it to be counted as a current
    carrying conductor for derating purposes, where at 180 degrees is would
    not need to be counted.
     
  13. Guest


    Careful now, all that twisting around and you might hurt yourself.

    ;-)
     
  14. Guest

    In a corner ground there is 240v between all current carrying
    conductors and there is no unbalanced current in the center tapped
    sense of the word. This will actually look a lot like single phase
    until you check the voltages . (2 blacks and a white with a 2 pole
    breaker.)
    It is used when it is all balanced 3 phase loads, typically a motor.
     
  15. Guest

    | Don't see what your thinking there. If the groundED conductor has no load
    | on it, then it's at the same potential at the GFCI as it is in the service
    | panel. Ditto for the groundING conductor, so there is no current flow
    | through a groundED - groundING conductor fault downstream of the GFCI.

    If the groundED conductor splits off somewhere between the point it is
    bonded to ground, and where the GFCI is, then a load on another circuit
    at that point can create the voltage drop on the feeder to that point,
    and give you the voltage. Think of a subpanel.

    My home design plan currently involves keeping subpanels as close to the
    main panel as possible, and oversizing the feeder to the subs. A more
    extreme solution would be to have separately derived systems for subs.


    | Except... GFCI's deliberately inject a small signal into the 'hot' and
    | 'neutral' that is the same in both. Doesn't affect the load normally
    | (common mode signal), but it *will* generate enough current through a
    | 'neutral-ground' fault downstream to trip the GFCI with no load attached.
    | So a groundED - groundING fault downstream is detected and trips the unit,
    | regardless of load.

    I wonder if this is the cause of being able to dangerously trip GFCI
    receptacles with a radio transmitter.
     
  16. In Europe, there are no 120V circuits, and "neutral" is a supply
    current carrying conductor which is at or near ground potential.
     
  17. It is on both hot and neutral, common mode. For N-G detection it is
    only required on the N as you say, but unless it is also applied to
    the hot in common mode form, it would also appear across all loads
    downstream, which would require a much more powerful current source
    in the first place.
     
  18. And just to correct myself, it's a voltage source, not a current source.
    Current only flows when there's a N-G fault.
     
  19. Guest

    | In article <[email protected]>,
    |> In Europe, the term "neutral" does include a grounded conductor in a 120 v
    |> circuit. In the states the term is used interchangeably but in error.
    |
    | In Europe, there are no 120V circuits, and "neutral" is a supply
    | current carrying conductor which is at or near ground potential.

    But that doesn't really change the meaning's origin. The first power
    systems were three phase to drive motors. I don't know if delta was
    used much way back when, but with star/wye configurations, you do have
    a genuine neutral. When single phase at 240v is taken from that, the
    neutral is still there. It just doesn't have enough phases brought
    in to take the neutralizing role there.
     
  20. The neutral role is still there, i.e. it's still at or near ground
    potential.

    Now there are some single phase supplies in europe which don't have
    a neutral, but they are much less common and only in a few countries
    (not UK). An example is a single phase supply from a corner grounded
    delta, where both of the lines are taken from a non-grounded corner.

    There are also IT supplies which are isolated with just a resistance
    to ground to prevent the secondary capacitively floating up to the
    much higer primary voltage. Strictly the side with the resistor to
    ground is still called a neutral, although it might be some way from
    ground potential. Again, I believe some parts of Europe use this,
    but it only occurs in the UK on standalone generators, not from the
    public supply.
     
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