@tigger back in the 90s and early 2000s my work used old controllers I don't remember which ones they was but they keep on burning out and I don't know why. It was cheaper to replace them with the same model then go to a different onces because of the cost. There was some flaw in the way they was powered up. I never know what was causing the ones that was burned out.
I left there in 2005 and they never fixed that problem. It was an add-on module that had a microprocessor on it 4 relays and other things. I was told to never touch it or figure out what was wrong with it so I never did. we replaced it at least 4 times a year. They keep a lot of spares.