The mechanical lock between the two contactors prevents one of the contactors from switching on, as long as the other is still switched on and vice versa. Switching over reversing contactors too quickly poses the risk of a phase short circuit. This is the case if the arc of the contactor switching off has not yet extinguished and the contacts of the contactor switching on are already closed. To avoid this risk, the two contactor coils must be switched to idle for a minimum of 50 ms during the switchover process and electrically locked against each other.
