What are main contactors?

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.