Low-voltage circuit-breakers are indispensable parts for the protection of electrical circuits due to their capability of current interruption during overload and short circuit. Most important characteristic of circuit-breakers is the timely transition from the insulating state to the conductive state and vice versa. They are equipped with special switching units that are used for tripping when the current exceeds the permitted value. Final thesis will also explain the manipulation of low-voltage circuit-breakers by the cascading technique, which enables the installation of circuit-breakers with lower breaking capacities than the expected short-circuit current at the installation point and the proper coordination of low-voltage circuit-breakers by the selectivity technique which enables the operation of the protection device closest to the point of failure. The final thesis will also explain the division of lowvoltage circuit-breakers according to their mechanical design as well as the partition according to the type of switching units that are installed in the circuit-breaker itself.