First calculate the total of all electrical power P, the calculated total current number of A I = P / U, when the switch is selected to be greater than the total amount of current leakage, cut to have a certain margin, or installed in other words on the electrical will bear the remaining leakage current can be a home 30MA., GFCI is generally no overload protection, if they wish to protect the circuit if small circuit breakers, miniature circuit breaker can not be selected too large. in general the choice of type C small circuit breakers used in some error is not very large and can project the simple calculation method is acceptable:

1. the miniature circuit breaker 10 / 0.4KV voltage level of the high-pressure side of the short-circuit capacity can be considered as infinite (10KV side short circuit capacity is generally 200 ~ 400MVA even larger, so considered by infinity, the error is less than 10% 25).

2. GB50054-95 "low voltage power distribution design specifications" 2.1.2 article states: "When the short circuit point 25 near the connected motor rated current and the short-circuit current exceeding 1%, should be included in the motor feedback current Effect "when the short-circuit current 30KA, whichever is 1% 25, ​​should be 300A, the total power of the electric motor about 150KW, and simultaneously start the feedback current used at this time should be accounted 6.5ΣIn.

3. impedance voltage miniature circuit breakers represent the UK shorted secondary (road), when the secondary has reached its rated current, primary voltage percentile for their rated voltage miniature circuit breakers. Therefore, when the primary voltage is the rated voltage, the secondary current is its prospective short circuit current.

4. MCB secondary rated current Ite = Ste / 1.732U wherein the capacity Ste MCB (KVA), Ue is a secondary nominal voltage (load voltage), Ue at 10 / 0.4KV = Thus a simple calculation 0.4KV MCB secondary rated current of the circuit breaker should be small capacity x1.44 ~ 1.50.

5. According to (3) the definition of Uk, secondary short-circuit current (three-phase short circuit) to I (3) of the definition of Uk, secondary short-circuit current (three-phase short circuit) to I (3) = Ite / uk, this is AC rMS.

6. in the same small capacity circuit breaker, if a short circuit between the phases, then I (2) = 1.732I (3) /2=0.866I (3)

The above calculations are small circuit breaker outlet end when the short-circuit current value, which is the most serious short-circuit accident. If there is a short circuit point from the miniature circuit breakerA certain distance, we need to consider the line impedance.