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Is the ground socket of a GFCI receptacle that isn’t connected to ground useful? (I'm in the United States, if it matters.) If the connected appliance had current on its ground pin, where would this current go in the ungrounded GFCI? If the current on the GFCI's hot and neutral are equal, including the case in which both are zero, will this ground-pin current cause the GFCI to trip and disconnect its internal hot, neutral, and/or ground? Which one(s) will it disconnect? Can current on the ground pin keep draining somewhere even after this tripping? If so, does that mean GFCI automatically internally bootlegs/bridges its open ground to neutral under these tripping conditions?

Can the ground socket be used for grounding the following ground-pin-only items?

  1. a shielded ethernet cable's drain wire that's inserted into the socket via a ground-only plug,

  2. an ethernet surge protector that uses only the ground pin,

  3. surge protector power strip,

  4. anti-static wrist strap

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Despite its name, a GFCI does not look at ground current - it only looks for a difference between the line and neutral currents passing through it. If it finds a difference of 5 mA or so (maybe 15 mA??) it assumes some current is going where it shouldn't, and trips, disconnecting the Line and Neutral.

The Ground pin on an outlet is intended to connect non-current-carrying parts of an appliance to Ground, to ensure the safety of users. If the Ground pin on an outlet is not connected to anything it cannot connect anything to Ground, and is useless.

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  • \$\begingroup\$ Does ungrounded GFCI look at ground faults, like if a toaster’s hot line shorts to its chassis? Or does it only notice this if there’s current flowing out of this chassis and into the environment, like through a person who’s standing on the ground? \$\endgroup\$
    – CodeBricks
    Jun 8, 2017 at 23:33
  • \$\begingroup\$ As I said, a GFCI only looks for an imbalance between the Line and Neutral currents. If there is an imbalance, the GFCI doesn't care what caused the imbalance - it will trip regardless of where the "misisng" current went. \$\endgroup\$
    – Peter Bennett
    Jun 8, 2017 at 23:37
  • \$\begingroup\$ By your answer’s last sentence: “If the Ground pin on an outlet is not connected to anything…”, do you mean (a) if the outlet’s ground socket isn’t connected to the breaker panel’s ground; or (b) if the plugged-in appliance’s plug is missing a ground prong/using a cheater plug? \$\endgroup\$
    – CodeBricks
    Jun 8, 2017 at 23:47
  • \$\begingroup\$ I'd connect the ground pin to neutral inside the wall. That is certainly suboptimal (if the neutral cable breaks, the "ground" will be connected to hot through the other appliances on the same circuit), but the cable breaking inside the wall is less likely than a faulty appliance, and the return path through the "ground" that bypasses the GFCI will trigger it. \$\endgroup\$
    – Simon Richter
    Jun 8, 2017 at 23:55
  • \$\begingroup\$ You asked "Is the ground socket of a GFCI receptacle that isn’t connected to ground useful?" I am answering (or trying to) that question. Perhaps I should have said "Ground socket" rather than "Ground pin"... \$\endgroup\$
    – Peter Bennett
    Jun 8, 2017 at 23:56

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