Sorry, I'm speaking "casually". Read "terminal" as "connections" (that the installer has to make). The fourth "terminal" is typically a contact on the back/bottom side of the breaker that engages a portion of the bus bar behind it.
A "normal" breaker would be a "one terminal" device in this sense. One load wire to connect to the breaker (the "other" being this contact on the back side).
For a GFCI breaker, you have that "back side" contact plus the normal "hot to load" screw terminal. Additionally, a "neutral to load" screw terminal (not required on a regular breaker) and a *pigtail* that runs off to the "grounding" block (where *it* sits under a screw)
Receptacle hot -------------------+---| |----+ | o | | | +---| |----+ o | | neutral ----------------------------+ | [R] | Gnd
R represents a partial short to ground via spider web/crud/dirt/whatever.
With no load connected, there is no current in either hot or neutral. Therefore, there is no imbalance between them, and the GFI doesn't trip. But, when the receptacle is in use, then there is current in both wires and current in the partial short to ground. If the current through the partial short to ground is ~5mA or more, the GFI trips.
If there was a partial short to ground on the hot wire, then there would be current there without a load, and the GFI would trip, provided the current was ~5mA or more.
That answers the "ONLY WHEN IN USE" portion of your question. The failure is an environmental issue - crud getting in to the junction box - not a receptacle mechanical structure issue. The crud could just as easily have created a partial short to ground on the hot side as the neutral side.
Note that this would require a high resistance neutral path OR a significantly *low* resistance path to earth. E.g., if you assume 100 ft of #12AWG as the neutral lead (to the panel), that's ~0.2 ohms for the total run. If the circuit was not tripping until seeing a full *20A* load (note mine was tripping much before that), then you would expect 4V across the neutral. For 5mA to flow through this, you'd need to be looking into ~1K ohms. That should stand out like a sore thumb -- with even a crappy VOM!
If the circuit was tripping at 10A (or 5A) then you'd need an even lower resistance short to shunt that much current away from the neutral.
[OTOH, if there is a high-ohmic connection between the outlet and the panel (e.g., a bad splice), then you would encounter a higher "neutral potential" and a proportionately higher resistance could cause the problem]
If the crud sat on the line side (to earth, not neutral), then you're in the ~100K ballpark. But, the GFCI would trip *any* time line voltage was present!
As I don't see anything (electrically) when probing the suspect extension cord (easily isolated from the circuit in its entirety), I suspect there is something more to the problem than just some "static" partial short. E.g., something changing with the mechanical introduction of the blades of the mating plug. Some "gap" shrinking or the blades of the plug actually penetrating further into the outlet than the blades of the receptacle, etc.
I know I've got a HiPot here, someplace. That's the ideal way to check as I can then explore the "short" at various line potentials (I think up to a few KV). Trick is figuring out which box it's hiding in!! :-/ (Grrr... I *really* need to downsize!)
Right, as already stated in the post: "If there was a partial short to ground on the hot wire, then there would be current there without a load, and the GFI would trip, provided the current was ~5mA or more. "
Really? Then that kills that (trip-but-only-when-loaded) possible expnation...
What are they trying to protect against -- a high resistance neutral? Or, just being pedantic? This seems like a significant additional requirement so one would assume there is a significant practical risk...
Ah, OK.
Makes sense -- a 2 wire tester would just look like a "load".
I've still not found my HiPot (though DVM doesn't turn up anything obvious with NO load). Ah, well... I've still got a few months before I'll *need* that circuit! :-/
GFCIs will trip on a N-G short with no load current. There is a second current transformer that tries to put a small common mode current in both the H and N wire. If there is a N-G short the resulting current will trip the GFCI. The CT current is obviously not needed for a H-G trip, but if the GFCI is reverse H-N wired the common mode current will trip it if there is a actual N-to-ground short, which now goes through the GFCI on the H terminals.
Other trivia - for a GFCI receptacle the load terminals used to be connected to the receptacle. If the GFCI was reverse line-load wired the test button would appear to work but the GFCI receptacle was not protected. Wiring downstream was protected. The recommended test is to plug in a lamp and make sure it goes out when the test button is pushed. Recent GFCIs disconnect the internal receptacle from both line and load on a trip. If reverse line-load wired the receptacle is still not protected, but if tripped the GFCI can not be reset. For that reason they are shipped tripped.
As someone wrote, the internal test for GFCIs connects around the CT, and GFCIs make no connection to the ground wire. But a plug-in GFCI tester will not work if there is no ground.
I'd always used 30mA and 30-48V as the red flags. I think even
*tiny* currents can cause the heart to be stopped, though.
Being a cynic, I had assumed AFCI's included GF protection so the user wouldn't consider the AFCI vs. GFCI choice to be "one or the other" (type of protection) but, rather, see AFCI's as "GFCI *plus* arc protection"?
AFCIs include ground fault protection, typically at the 30mA level used in the UK for shock protection. In an AFCI the ground fault trip is for fire protection. The idea is that arcing may involve leakage to ground (if present), particularly in 3-wire cords. If ground fault protection was not included AFCIs probably wouldn't need the neutral wired through them (in the same way that GFCIs do).
** Nope - the differential is between active and neutral.
If there is an imbalance, current must be flowing to supply or actual ground.
Deliberate capacitance ( in an appliance) connected from active to supply ground is strictly limited in value so the current flow is only a few mA at most and will not trip a GFCI.
Yes, if the short's resistance is 1.6 ohms or less, with the neutral's resistance at .4 ohms, per UL 943. But we're talking about a partial short from neutral to ground, whose resistance can be more than that 1.6 ohms. Let's say it is 100 ohms. At 100 ohms partial short the GFI won't trip with no load.
With a 100 ohm partial short and a .4 ohm neutral, say you connect a 5 amp load. The .4 ohm neutral will carry 99.6% of the current and the 100 ohm load will carry the rest. 99.6 % of 5 amps is 4.98 amps - leaving 20 mA carried in the partial short. Bingo - the GFI trips. Obviously, YMMV - except make that your mileage Will vary. Not every circuit has a .4 ohm neutral or a 100 ohm partial short, or every load 5 amps.
So lets take another approach:
Say the neutral is 50 feet of #12 at 2 ohms per 1000 feet, yielding a .1 ohm neutral, assuming all connections are 0 ohms. 4 amps through .1 ohms is a voltage drop of .4 volts across the 50 feet of neutral wire. Lets say the partial short is 50 ohms. .4 volts across 50 ohms causes an 8 mA current. Bingo, the GFI trips, but that same 50 ohms won't cause the GFI to trip with no load connected.
How neutral to ground short circuit trips a GFI with no load:
The neutral to ground transformer forms part of an oscillator circuit that runs at 8 kHz per the RV4141A datasheet. That transformer, and the sense transformer are mutually coupled through Rg (the short) Rn (resistance of the neutral) and the neutral wire ground loop (the neutral is grounded at the panel, so with a short or partial short at the receptacle end, a ground loop is formed). No load current need be drawn for the oscillator to function. With the oscillator running, a small current is induced on both the hot and neutral wires, but the current on the neutral wire differs from the current on the hot wire, because the neutral wire has a path to ground through the short. The sense transformer "sees" a difference between hot and neutral and the sense amplifier amplifies it. If the difference is large enough, the GFI trips.
No it doesn't. See my reply to bud for the details. He's talking about a short, but what was discussed was a partial short. The short would be 1.6 ohms or less per UL 943, while the partial short would be higher, perhaps as much as 100 ohms.
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