Miniature circuit breaker, referred to as MCB, is the most widely used terminal protection electrical appliance in building electrical terminal power distribution devices. Used for single-phase and three-phase short-circuit, overload, overvoltage protection under 125A, including single-pole 1P, two-pole 2P, three-pole 3P, four-pole 4P, etc.
The miniature circuit breaker is composed of operating mechanism, contacts, protection devices (various trip units), arc extinguishing system, etc. The main contact is manually operated or electrically closed. After the main contact is closed, the free trip mechanism locks the main contact in the closing position. The coil of the overcurrent release and the thermal element of the thermal release are connected in series with the main circuit, and 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 pulls in, causing the free tripping mechanism to operate, and the main contact disconnects the main circuit. When the circuit is overloaded, the heating element of the thermal trip unit will bend the bimetal and push the free trip mechanism to move. When the circuit is under-voltage, the armature of the under-voltage release is released. Also make the free trip mechanism act
In the design of civil buildings, low-voltage circuit breakers are mainly used for line overload, short circuit, over current, loss of voltage, under voltage, grounding, leakage, automatic switching of dual power supplies, and protection and operation of the motor during infrequent starting. Its selection Principles In addition to complying with the basic principles such as the use environment characteristics of low-voltage electrical equipment (see Industrial and Civil Power Distribution Design Manual), the following conditions should be considered:
1) The rated voltage of the circuit breaker should not be less than the rated voltage of the line;
2) The rated current of the circuit breaker and the rated current of the overcurrent release are not less than the calculated current of the circuit;
3) The rated short-circuit breaking capacity of the circuit breaker is not less than the maximum short-circuit current in the line;
4) The selection of type distribution circuit breakers needs to consider the short-time short-circuit making and breaking capacity and the coordination between the time-delay protection levels;
5) The rated voltage of the undervoltage release of the circuit breaker is equal to the rated voltage of the line;
6) When it is used for motor protection, the starting current of the motor should be considered when selecting the circuit breaker and it should not operate during the starting time; for the design calculation, please refer to the "Industrial and Civil Power Distribution Design Manual";
7) The selection of the circuit breaker should also consider the selective coordination of the circuit breaker and the circuit breaker, and the circuit breaker and the fuse.