 Even with “closed transition” there is a transient produced at contact separation of the Star point switch.The transient is of a very short duration, less than 5 microseconds and occurs when the motor current from the secondary of the autotransformer is forced to the line voltage.This is a very fast event and is the origin of the destructive voltage transients that have caused major breakdowns of large medium and high voltage starters >1000 kW.