Although it is common practice to use solid state starters without an isolation contactor, it is better practice to use such a contactor. A solid state starter employs SCRs to control the current to the motor. When these SCRs are subjected to a sever fault situation, they can fail and if they do, they will fail closed which could cause major damage to the motor, especially if the motor is effectively left connected to two phases only.
The contactor used must be AC3 rated because it carries the starting current to the motor. In some situations where the start duty is very light and the start torque is particularly low, the start current may be less than 300% current in which cases it may be acceptable to use an AC2 rated contactor. (The ACx rating is determined byt the start current required.)
It is preferable to open the contactor when the motor is not running to reduce the voltage transients blocked by the SCRs. This will result in a much reduced potential of SCR failure due to the supply born transients causing an excess voltage across the SCRs.
The contactor is not neede to make the motor and/or starter run, it is an additional safety device givong better isolation in the off state, reducing the potential of the starter and motor being damaged by supply transients and surges.
Note: the leakage current through the SCRs and snubbers in the OFF state can be considerable, many tens of milliamps and in some cases could exceed 100 mA without exceeding any component specifactions. This could be very dangerous for service personell!!
Best regards, Mark Empson