It is required for ATS22, ATS480. Not needed for ATS490 due to presence of safety function.
If the thyristors in ATS22, ATS480 are in short-circuit or bypass relay in ATS22 is stuck, Altistart will detect it but will not be able to stop the motor, despite the active stop command from whatever channel (Logic input, PLC). Not having possibility to stop the motor is not safe situation. Altistart can only provide signalization using fault relay, but there must be a device that will react on fault signal and stop the motor. Does not need to be necessarily the contactor, it can be circuit breaker with under voltage trip for example. Altistart with fault relay will interrupt the signal on under voltage trip, so breaker will cut the current to the motor.
