Ok, I've got two ERVs, OEMed Reznors, that keep blowing fuses. Always A & B fuse, never C. 12A time delay fuses, 6 and change running amps, 480 volts. Two 3 HP blower motors and one wheel motor fed via an xfrmr on A & C. It happens more often than once a week in warm weather, getting less frequent as the temperature drops. One does it twice as often as the other. Fed from different panels, from different switchgear, from possibly different services, from (I imagine) the same primary. Industrial, three or maybe 4 2000A services. We dropped coin on a DM-5 & it said that the voltage holds steady & yup we hit 30-40A when the fuses let go. The motors meg out better than my Fluke 287 can read, winding resistance is ±0.1 Ω of each other. There's no burns that indicate a reoccuring fault, it happens with the wheel motor completely out of the circuit (Stakons unplugged).
Where could I be getting 40+ A of fault current? The motors start fine, a damper interlock staggers them. The place runs many million dollars worth of equipment, I can't believe that there are significant power quality issues. It's two separate units on different ends of the building, although they are sisters off the same line, one digit apart on the serial number.
Like I said, I'm stumped.