Line Contactor - Danfoss VLT Pocket Manual

Line Contactors: When should a line contactor be used?
Soft starters can be installed with or without a line contactor.
A line contactor disconnects the SCRs from the supply when the motor
is not in use. This isolates the soft starter, and protects the SCRs from
damage due to severe overvoltage (e.g. lightning strikes) – SCRs are
most susceptible to overvoltage damage when in the off state. The soft
starter is also isolated from the supply in the event of a trip.
A line contactor may be required by local electrical regulations and
should be AC3 rated for the motor FLC.
The line contactor can be controlled via the soft starter's
relay output. The inrush VA rating of the contactor coil must
not exceed the rating of the soft starter's relay output.
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VLT Soft Starter Pocket Guide