The working principle of miniature circuit breaker is arc extinguishing principle. Circuit breaker is generally composed of contact system, arc extinguishing system, operating mechanism, tripper, shell, etc. The function of miniature circuit breaker is to cut off and connect the load circuit, and cut off the fault circuit, prevent the accident from expanding, and ensure the safe operation. When a short circuit occurs, the magnetic field generated by a large current (usually 10 to 12 times) overcomes the reaction spring, the release pulls the operating mechanism, and the switch instantaneously trips. When overloaded, the heat is increased, the electrorheological (ER) is increased, and the deformation of the bimetal sheet to a certain extent promotes the action of the mechanism (the greater the current, the shorter the action time).

Miniature circuit breakers are used to switch on and off the circuit, it can not only switch on and off the normal working state of the circuit, but also can break the fault circuit, the most serious of which is short circuit. The 63 amp mcb consists of the following main parts:
1. Breakable contact connection. It consists of fixed contacts and movable contacts.
2. A device consisting of a metal cylinder or an insulating cylinder is equipped with a breakable contact connection and arc extinguishing device and a transmission device.
3. The drive device to drive contacts.
4. Arc extinguishing equipment. When the 10ka mcb is used to break the circuit, the dynamic contact between the breaker and the fixed contact is separated by mechanical means. When the brake is closed, the opposite contact is used to make the moving contact closed with the fixed contact. During the operation of switching on and off the load circuit, an arc will occur between the fixed contact and the moving contact. The arc generated during the breaking process is much more serious than the closing process.

When the breaking current is very large, especially when the short circuit is broken, the arc is very large, and it is often difficult to open the circuit. In the 63 amp mcb of the modern structure, the arc extinguishing principle is common, that is, the current flows through the electric appliance. At zero (that is, natural zero crossing), a strong de-freeing is performed. This strong de-freeing action is accomplished by an arc extinguishing device in a 10ka mcb.

The rated current of the load of a 63 amp mcb must be equal to or less than the rated current of the switch, generally less than the rated current of the switch. Considering that there is a certain margin left, the rated current of the selected switch is about 20% larger than the actual load current, so the overload protection and short circuit protection must be considered to operate. If the rated current is too large, the overload protection will lose its function. Because of the relationship between the size and length of the line, the short-circuit current at the load side can not reach the setting action value of the instantaneous tripper, thus making the short-circuit protection invalid.

Because the insulation resistance of the line will decrease with time and the distributed capacitance of the ground wiring exists in the distribution line of the 10ka mcb, the leakage current of the line more or less exists to the ground, and some are relatively large, so the rated leakage current of the leakage circuit breaker must be greater than the actual leakage current. Only by this way can the switch be guaranteed not to malfunction, which is also in accordance with the national standard stipulated that the rated leakage non-operating current is half of the rated operating current.