Lets assume contactor is OK. What changes from day (when you tested it) and night (when it lets out the magic smoke).
Is it possible that the neutral for the lighting circuits shifts under load? This might overvoltage the coil. Perhaps the neutral ground bond is missing? Are the lighting circuits fed from a dedicated transformer? Is there the possibility that one of the lighting circuits is accidentally tied into another power source?
I would suspect that if the factory has been involved and they are coming up blank, then perhaps it is a facility issue.
What is downstrean of this contactor?
