Originally Posted by P-J
Please keep in mind that the E-Stop button switch is a lock-on switch and must be reset after use. If you do not reset it the GFCI circuit protection will instantily trip again as soon as power is applied to the power line.
(He said thinking out loud)
I have a momentary, non-locking e-stop, and according to the continuity test it is working fine.
I started working backwards and have pinpointed it to the contactor / SSR.
Here is a quick sketch on how it is wired.