Ok, here is one that stumped me for about two crucial hours today when trying stuff amongst all heck breaking loose on a before easy day broke loose within a few hour period attention span devoted period for me today. Swapped first the stage pin outlet than the GFCI, neither worked between other fans in the fire. See if you can solve the problem that did get solved but was interesting. To solve this problem, one must think in two parts and listen to specific things stated below. That's the reality of tech and how you will at times hear problems described. Tech is not always linear ... this causes this, at times one thing does not equal anything when balanced against another to confuse. Anser to this being a simple mistake, what is it? Especially what is it that proves in testing for answers a false result? Built two thought to be mirror image fixture tester power supplies over the weekend. Beyond some minor pain in the rears’s all worked and fit well enough well enough in constructing them. In reality three boxes - one also tapped off the one that worked and won’t be needed for further consideration other than in tapping off an innitial test box it also works. Overall, in the two under consideration, one worked without problems. The other and I stress this, the other would trip the GFCI when ever a fixture or Gam Check/stage pin was plugged into it’s stage pin outlet and the outlet was switched on, but specifically not when the GB Edison outlet tester was plugged into the Edison outlet or any Edison outlet than also switched on. The GB Edison tester was fine under all conditions or various outlets in not tripping the GFCI. Specific symptoms, the first tester worked fine the second in testing would work in non-switched Edison outlets and switched Edison outlets but not the switched stage pin outlet. On this, no matter what was plugged into it, it would trip the GFCI. Power to each mirror image tester comes from each own task specific 20a circuit breaker that feeds only the cable tester outlet from the sub-panel and both were the same in all ways in being a source of power that allowed the GFCI to trip. In other words, it did not matter which circuit they were plugged into, it was not the outlet, it was something about the box or tester that would work on one but not another in tripping the GFCI. Such testers are mounted to the work table by way of NEMA 1, 4"square by 12" long wireway uni-strut bolted to it that is punched for the various outlets. They are wired by a length of 10/3 SOOW cable and Edison plugged into the 20 amp Edison outlet provided by the Union electricians. It was wired as followed in tripping. For the box itself, the hot wire goes to a 20A Square D brand QO type circuit breaker which at no point tripped. It than directly feeds to a commercial grade Leviton 20A GFCI outlet. This outlet then feeds by way of Burndee splice, a second 20A duplex outlet and 20A switch. The 20A switch than feeds the test circuit by way of second (properly insulated by way of rubberized electrical tape in both instances) burndie splice from switch to single Edison outlet, single Stage pin outlet plus LED indicator light showing switch on mode tapped off the switch. Both the GFCI to Edison outlet, non-switched, and switch were by way of HT. Ideal set screw splice with ferrule or in the case of Switch to Edison, stage and indicator light to Burndie with rubber insulating tape over it fully insulated. There was nothing touching and a strain relief was provided short of each splice in banding wires into the splice together. Hot wires in other words, only had one splice per outlet than had a specific to use better than normal splice point to the conductors connection point for the outlets fed by way of the switch or GFCI. The Edisons’ were fed by the same in good condition splice as that of the stage pin that was tripping the GFCI. Ground from the line in goes to a box mounted grounding bar and is attached to the GFCI, outlet and all outlets and switches of the tester. Neutral goes to an isolated from the box ground bar in this case Marathon block and feeds the GFCI, and all outlets. Changed the stage pin plug, it did not solve the problem. Changed the GFCI and it also did not solve the problem. What’s the cause of this tripping the GFCI when ever something is plugged into the stage pin but not it’s also switched Edison outlet? Don’t expect anything such as a freyed strand of wire and remember one other tester worked perfectly well though was wired a day before hand. Hint or things thought about or perhaps an option that might or might not solve the problem: In the far past I can remember having removed the bridge link between stage pin panel mount and clamp so as to isolate the clamp from the ground terminal of the outlet. It had at the time solved some specific un-remembered problem. All flush mount stage pin outlets have a link that bridge the ground to it’s clamping mechanism. What might be the purpose or need or removing this? Two questions than, first what was the problem here in GFCI tripping, than what’s up with the stage pin panel mount bridge to the box mount clamp? Perhaps third question, why at some point might it be necessary to remove this bridge? Primary of course is the initial question of not tripping under GB Edison tester but tripping under Gam Check or fixture.