Current-limiting circuit breaker having a selective solid state trip unit

A current-limiting circuit breaker having a pair of separable stationary and movable contacts for interconnecting an electrical circuit thereto, an operating latch machanism for opening and closing said separable contacts, a selective trip unit cooperating with an actuator of said mechanism for actuating said contacts in a definitive open position upon occurrence of an overcurrent condition, and additional electrodynamic or electromagnetic means responsive to the value of the current in said circuit for independently actuating said movable contact which is automatically urged towards the open position when the fault current exceeds a first predetermined threshold, and further to the closed position when the fault current falls below said first threshold, so as to define an opening and closing cycle of said contacts, said selective trip unit comprising:

• a tripping control device including a cycle counter which counts successive said opening and closing cycle effected by said additional means;
• counter setting means cooperating with said cycle counter for incrementing the latter and for supplying a tripping order to said actuator so as to trip said mechanism such that the latter causes a positive opening of the circuit breaker after a selected number of cycles;
• zero resetting means for clearing said cycle counter when said succession of cycles is interrupted after supplying of said tripping order;
• a first, time-delay solid state tripping circuit connected to said actuator;
• a second, instantaneous solid state tripping circuit having an operating time lower than that of said first tripping circuit, and cooperating with said cycle counter so as to ensure instantaneous tripping of said mechanism as soon as the value of the current exceeds a second instantaneous tripping threshold, as well as tripping of said mechanism after counting the selected number of cycles by the counter setting device;
• a current sensor system including a plurality of current transformers;
• and
• wherein each current transformer of the current sensor system comprises an induction coil wound upon a non-magnetic toroid form surrounding a current conductor of said electrical circuit, said coil providing a di/dt measurement signal corresponding to the time derivative of current in said conductor, and wherein said cycle counter comprises a threshold detector circuit which commences fault current pulse counting when the measurement signal of said sensor exceeds the threshold of said detector circuit.