The operator was standing beside the controller when it failed and heard a loud audible “pop” as the input diode bridge lost two diodes. The other pump was operating at that time and the system was not calling for a start-up of the pump controller that failed. The drive that had failed had power at the terminals and was in the standby mode but not in the run mode.
The fast-acting 90 Amp fuses had not blown, but the 100 Amp thermal magnetic circuit breaker disconnect had tripped, taking the unit off line. The design specification of that particular VFD was to co-ordinate the fuses to open before the circuit breaker on any fault condition. The purpose of the circuit breaker was to serve as a manual disconnect for the drive and, consequently, the breaker trip characteristics was such that the breaker was essentially out of the electrical circuit.
The breakdown of the diodes caused a phase-to-phase fault condition and a resulting surge current which caused the breaker to trip. A breakdown of the diode due to normal diode failure would have caused a steady-state current flow that would have tripped the fuse (nominal trip time for a fast-acting fuse is 3 milliseconds).
However, if the diodes failed due to a high incoming transient voltage, a very high amperage surge with duration of less than 3 milliseconds would have resulted. A current surge of that short a duration would not cause a heating effect on the fuses and, thus, the fuses would not “see” the current that resulted from the breakdown of the diodes. Only a high transient surge current could cause the magnetic sensor of the breaker to trip before the fuses would blow. Thus only the circuit breaker with a high current level instantaneous trip was able to see the surge current, a current that could only have been initiated by a transient voltage that both caused the diode to break down and trip the breaker while leaving the fuses intact.
A similar failure is discussed in Transients: Equipment Malfunction. The drives in both cases were identical and made by the same manufacturer. In both cases the circuit breakers tripped and the fast-acting fuses did not blow.
The characteristics of a PN junction are such that a high reverse voltage transient on a non-conducting junction will cause the junction to avalanche or cause an excessive leakage current. In the case of excessive leakage current the damage may be such that the junction blocking voltage capability is lowered to a level that is below the device’s normal operating voltage. Thus, if a normal voltage of 480 VAC is applied to the device, the junction will
conduct in the reverse direction, creating a fault condition. 
 “Transient Voltage Suppression Devices,” published by Harris Semiconductor Corporation,
Melbourne, FL, 1995, page 1-15.