Breaker selection should comply with the basic principles of

A circuit breaker must be greater than or equal to the rated voltage of the line voltage.
Load or rated supply voltage is greater than or equal to the rated voltage of the switch, because this thing off product safety performance. Higher than the rated voltage switch may cause the product insulation performance, there is an accident hidden.
Two circuit breakers rated short-circuit breaking capacity ≥ lines maximum possible short circuit current.
A phase line and the line-line or between line and neutral circuit current is very large, the closer side of the current power distribution greater, because the short circuit impedance smaller. Thus requiring the circuit breaker must have a breaking capacity, when the short-circuit breaking capacity greater than or equal to the maximum possible line short-circuit current, the role of the instantaneous release, the switch can be  Air circuit breaker  extinction disconnected. If the switch is rated short-circuit breaking capacity ≤ lines maximum possible short-circuit current extinction because the switch does not, caused by the high temperature of the arc to make contact adhesive (short circuit) and thus destroy the distribution lines that device.
3 circuit breakers rated current ≥ line load current.
The rated load current must be equal to or less than the rated current of the switch, the switch under normal circumstances is less than the rated current, taking into account leave some margin, the general election switch rated current than the actual load current 20 ℅ so, do not choose too Large, overload and short circuit protection must be able to consider action, select too large rated current, overload protection useless, because the line thickness and length of the relationship, the load side of the short-circuit current of less than instantaneous release tuning action value, thus failure of the short-circuit protection.
4 leakage circuit breakers rated residual operating current must be ≥ 2 times of the existing line leakage current.
In the distribution line insulation resistance due to line growth will decline over time and ground wiring distributed capacitance, the line more or less on the ground there is a certain leakage current, and some relatively large, so the selection circuit leakage the rated residual operating current must be greater than twice the actual leakage current in order to ensure the switch will not malfunction, which is the national standards with a rated residual non-operating current to half the rated operating current is consistent.
5 breaker relatively short end to make a single choice B, C, D type instantaneous tripping switching action, for different types of load (electrical equipment) use different instantaneous release and the corresponding current level of product . Depending on the load devices use different types of instantaneous release and the rated current, B, C, D type instantaneous release described the use of an object in front. Select the rated current and the corresponding instantaneous release must be considered when the rated load current and the maximum possible output short-circuit current. When the maximum short-circuit current is greater than or equal to B, C, D type tuning instantaneous tripping action value, short circuit protection to work.
6 mounted leakage protection must find out before the original supply protection type, in order to determine whether you can directly install or need to change.
Supply protection has been described in detail in the previous type. Before leakage breaker is not installed, some of the equipment has taken some type of power protection, but there are some type of protection is inappropriate, if not directly change leakage circuit breakers installed, otherwise it will cause the switch malfunction or refuse to move. Specific use case will be analyzed in the following.
7 has inlet and outlet must be strictly in accordance with the specified product requirements wiring, inlet and outlet can not be reversed.
Leakage circuit breaker must be required wiring, otherwise it will cause damage to the switch leakage protection, leakage  mcb circuit  circuit board due to the work of the power drawn from the outlet end of the switch, such as taking anti-wiring, the power supply circuit board work long, once leakage protection action, internal electromagnetic trip coil damage due to long-term power (electromagnetic instantaneous tripping coil is designed to work), leakage function damage.