I would love to diagnose your original switch and see exactly what went wrong. The layout is similar to the transfer switches I built for the Kelley II
Their burden was 35 amperes 240 AC and the first set of relays lasted 5 years or so (heavily salt laden marine atmosphere). A really crummy circuit at the fish processing plant defeated even a reasonably spread hysteresis gate.
The transfer switches I used to build used 1st generation 5mm diagnostic LEDs - one glance and any problem evident simply screamed its confession.
