Main Contactors: when and how should they be used? 

Soft starters can be installed with or without a main contactor.

A main contactor:
- may be required to meet local electrical regulations.
- provides physical isolation when the starter is not in use and in the event of a soft starter trip.

Even in the off state SCRs do not offer a high degree of isolation due to leakage through the SCR and protection networks.

- protects the soft starter SCRs from severe overvoltage situations (eg lightning strikes).

SCRs are most susceptible to overvoltage damage when in the off state. A main contactor disconnects the SCRs from the supply when the motor is not running, preventing possible damage.

Main contactors should be AC3 rated for the motor FLC.