All the PM42 zones were programmed to trip at 3A current load. So why were some zones tripping when any train ran, while others did not?
That kind of problem demanded a scientific investigation. What I did was to purchase a set of 47Ω power resistors (rated at I think 3W each). Each of these added across the rails would add a 0.25A load, approximately; with some accurate voltage measurements and a spreadsheet I would know the truth. As it turns out, my multimeter when set to "AC Volts" gave a near exact voltage reading: I calibrated it against true peak-peak rail voltage using an oscilloscope.
The results, when I got them, were surprising: two of my PM42 modules had current trips close to the "brochure" figures; the other two - at the fiddle yard end- had trip points significantly lower than the others. I'd bought them at different times; I don't know if there has been a design change. However armed with these measurements I was able to balance the current limits by reprogramming the over-sensitive PM42 modules.