A circuit breaker with electrical contacts adapted to open or close thereby preventing or allowing respectively current to be supplied from one part of an electrical system to a further part of the electrical system. The circuit breaker is provided with a storage device for storing data indicative of the zero crossing points of the current signal and an actuator arrangement for causing the electrical contacts of the circuit breaker to be opened or closed in accordance with the data indicative of the zero points of the current signal. The system synchronises the zero energy state of the a.c. system with the rapid opening of the contacts thereby circumventing an arc in the circuit breaker. preferably, the actuator is an electrical linear motor whose armature is provided with a position detector to provide feedback control of the motor.