This is a long shot but ....
Is the dealership owner or one of the techs working on a large welding project after hours? My thought is that they are using the building frame / ground as a work lead and are somehow putting high voltage spikes between the "A" phase and ground. These spikes are then taking out the contactor coils. Like I said, a long shot.
How good is the grounding system? How old are the ground rods? What phase(es) are the exterior neon lighting on? Have you tried powering the contactor coils off of another phase? Do you get a wicked voltage spike as a certain light is turned off? Have any of the lighting panel circuit breakers been replace?
What changed or was added 4 to 5 years ago? New lighting, was the building was sided or painted, did the local utility relined or replace the water or gas mains? Was the pole transformer changed? Anything that would cause or increase a High Voltage spike to the contactor coils? Did the utility upsize the feeds that the pole transformer taps off of? Are there switchable PFC caps in the building or nearby on the poles? Is the building very close to the substation? Is anything else in the building also failing frequently? Do they have phone or computer network data problems?
Larry
