Low-voltage circuit breakers are composed of contacts, arc extinguishing devices, operating mechanisms and protective devices. According to the use category, there are selective type (protection device parameters are adjustable) and non-selective type (protection device parameters are not adjustable); according to the structural type, there are universal type (also known as frame type) and molded case circuit breakers; according to the arc extinguishing medium, there are air type and vacuum type (most domestic products are air type); according to the operation mode, there are manual operation, electric operation and spring energy storage mechanical operation; according to the number of poles, it can be divided into single-pole, two-pole, three-pole and four-pole types; according to the installation method, there are fixed type, plug-in type, drawer type and embedded type.
When the rated current is below 600A and the short-circuit current is not large, a molded case circuit breaker can be selected; when the rated current is large and the short-circuit current is also large, a universal circuit breaker should be selected.
The general selection principles are:
(1) The rated current of the circuit breaker ≥ the load operating current;
(2) The rated voltage of the circuit breaker ≥ the rated voltage of the power supply and the load;
(3) The rated current of the circuit breaker release ≥ the load operating current;
(4) The circuit breaker's limit switching capacity ≥ the maximum short-circuit current of the circuit;
(5) The single-phase short-circuit current at the end of the line/the instantaneous (or short-circuit) release setting current of the circuit breaker ≥ 1.25;
(6) The rated voltage of the circuit breaker undervoltage release = the rated voltage of the line.