Installation, Operation, and Maintenance of Safety Interlocks in Medium-Voltage Motor Starters

This paper describes the benefits and techniques for performing maintenance on the electrical and mechanical interlocks in medium-voltage (MV) motor starters. These interlocks are safety features built into MV motor starters to prevent the isolation switch from being operated while the motor is running, which could result in a serious injury because the isolation switch is not rated to operate under a load. The electrical interlock is designed to open the contactor if an attempt is made to rack the isolation switch on or off the bus while the contactor is closed. The mechanical interlock is designed to prevent the isolation switch from physically being opened or closed if the motor is running or the contactor is closed. These interlocks are part of the equipment design and are defense mechanisms to prevent a potentially serious injury. During a maintenance turnaround, the operation of the mechanical and electrical interlocks was function tested. The interesting findings from these tests are outlined hereinafter.