Glen, I solved the mystery. (about the firing system tripping on overcurrent with small currents, on a 200A welder)
Here's what it was all about:
The current sensor (connected to DC shunt) was NOT, contrary to a statement by their engineer, a differential sensor.
What it was actually measuring was difference between the negative bus and the plus side of the shunt. Even though I properly wired two ends of the shunt where they belonged on the firing system, the sensor was not taking the difference between them, it was taking the difference between the + side of the shunt and the negative bus, ignoring the minus side of the shunt.
It is completely contrary to the impression that they were trying to make.
The problem was that there was too much wire between the minus bus and the old shunt in the welder, which was connected to the current meter and was located near the commutator. Between the negative bus and the shunt, there was an interphase transformer and also the "reactor". Too much resistance was all a part of the "virtual shunt" that was in reality being sensed.
My solution: put another shunt right on the negative bus. It is a300A, 50mV shunt, so the system can produce more current than allowed, I will have to adjust for it, but it happily puts out A LOT of amps.
I pronounce this firing system to be perfectly functional.
I am very happy right now.
I think that I can adjust the firing board (there is a trimpot for current sensing), so that it would produce exactly 200A when the current control potentiometer is at the max. I will do that before going to be tonight.
When everything works, I will buy a huge box of donuts for PCTI.i