A three contactor bypass system offers the ability to isolate the inverter in order to service the unit while in the bypass mode. The service contactor can be closed to allow repairs and testing prior to switching out of the bypass mode. The bypass contactor and isolation contactor are mechanically interlocked so the inverter will not be damaged by back feed from the AC line voltage.

The two contactor system allows the motor to be by-passes for emergency use. However, the drive cannot be isolated for service purposes. This configuration is used where system cost is a consideration.