Will a GFCI breaker nuisance-trip if a three prong outlet has a jumper between ground and neutral on an old two wire system?

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    If you have an "old two wire system" you do not connect the ground and the neutral together. As a matter of fact you do not connect them even for a more modern hookup that has safety grounds wired to the electrical boxes. This consideration may make your question moot. – Michael Karas Jun 21 '15 at 13:03

It won't nuisance trip. It will trip because you've deliberately created a ground-fault.

You should never connect the grounding conductor to the grounded (neutral), except in the service equipment. Which means you should remove the jumper between the ground and neutral.

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    FULLY agree! Jumping the ground to neutral at a receptacle is one of the dumbest things one can do electrically. It is unfortunate how often we see it though. :( – Speedy Petey Jun 21 '15 at 15:18
  • And in fact, installing a GFCI outlet is the recommended solution if you want to be able to plug grounded devices into an ungrounded circuit. The interrupter provides the protection that the missing ground wire can't. (Though at least in the US, you're also supposed to affix a label to warn folks that this is what you've done, in case they actually needed the ground to, eg, bleed away static charges.) – keshlam Jul 1 '15 at 6:38
  • @SpeedyPetey jumping the ground to neutral on a three pin outlet connected to a two-wire circuit is trading one hazard for another. Without the jumper you are vulnerable to faults where a live wire comes loose touches the metal case. With the jumper you are vulnerable to faults where the neutral becomes disconnected. – Peter Green Apr 27 '17 at 15:04
  • @keshlam a GFCI only disconnects the device after you get a shock from it. It's better than nothing but still a poor substitute for a dedicated ground wire. – Peter Green Apr 27 '17 at 15:08

Will a GFCI breaker nuisance-trip if a three prong outlet has a jumper between ground and neutral on an old two wire system?

It depends on what gets connected to the ground pin of the outlet. If it's just an isolated appliance then no, but if the appliance ground ends up connected (via water pipes, data cables, cases of two appliances touching each other or whatever) to real ground then the GFCI is very likely to trip.

Such links are a dubious practice from a safety point of view. They protect against faults where a live wire touches the case but if the neutral becomes disconnected they will cause the case of the appliance to become live. AIUI current code is that if you can't/won't rewire with a proper ground that you should fit a GFCI, leave the ground pin disconnected and label the outlet as "no equipment ground" (and "GFCI protected" if the GFCI is remote).

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