The Breaking Capacity of Molded Case Circuit Breakers

The selection of the breaking capacity of the molded case circuit breaker depends on the short-circuit current value at the installation location. The breaking capacity of the circuit breaker is required to be greater than the short-circuit current value at the installation position, which is independent of the load size. Please calculate the short-circuit current level (KA) at the installation point and select the circuit breaker. Assuming no calculation is conducted, it is assumed that the circuit breaker is installed in the middle of a high-power capacity such as a power plant, and 50KA can be selected for greater reliability. Assuming it is a regular industry, 25-35KA can also be chosen. The selection of the rated current of a circuit breaker depends on the current of the circuit load.

Working principle:
The main contact of the low-voltage circuit breaker is manually or electrically closed. After the main contact is closed, the free release mechanism locks the main contact in the closed position. The coil of the overcurrent release and the thermal element of the thermal release are connected in series with the main circuit, while the coil of the undervoltage release is connected in parallel with the power supply.
When the circuit is short circuited or severely overloaded, the armature of the overcurrent release is pulled in, causing the free release mechanism to operate and the main contact to disconnect the main circuit.
When the circuit is overloaded, the heat released by the thermal element will heat and bend the bimetallic, pushing the free release mechanism to operate, and the main contact point will disconnect the main circuit.
When the circuit is under voltage, the armature of the under voltage release device is released, which also causes the free release mechanism to operate and the main contact opens the main circuit.
When the shunt release button is pressed, the armature of the shunt release is engaged, causing the free release mechanism to operate and the main contact to disconnect the main circuit.
Traditional distribution systems require circuit breakers to not only provide switching current, but also provide isolation and protection functions to complete circuit control and simple short circuit and overload protection, especially for the reliability of personnel and equipment in the distribution system. All parties have put forward new requirements. Therefore, the development, design, and procurement of products also focus on the following aspects:
(1) Maintenance of circuits and equipment
(2) Reliable and discontinuous power supply