Two variable frequency drives were operating blowers. One had failed. During the repair, the drive manufacturer’s service personnel applied power to the faulty drive before the problem was resolved. Powering a faulted drive in order to read the fault diagnosis read-out is often common practice by service personnel.

The resulting fault surge that was caused when the unit was turned on caused the other drive, which was operating at the time, to be damaged. Thus both drives went out of service at the same time. There was no third unit for back up.

With a suppressor on the distribution panel or one on each of the two drives, suppression would have been available for any transients that passed from one drive to the other due to component failure or motor fault.