Circuit breaker current calculation method

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Circuit breaker current calculation method
02 19 , 2022

First calculate the total power P of all electrical appliances, in the calculation of the total current A number I=P/U, the leakage switch should be greater than the total current, there must be A certain margin, or in the installation of other electrical appliances will not be able to withstand. Residual leakage current household 30MA can. Leakage switch is generally not overload protection function, if you want to protect the line to small circuit breaker, small circuit breaker can not choose too big. In general, type C is used.

Miniature circuit breakers use a simple calculation method that has some errors that are not very large and can be accepted in engineering:

(1) for 10/0.4kV voltage grade of small circuit breaker, can consider the high voltage side of the short circuit capacity is infinite (10KV side of the short circuit capacity is generally 200~400MVA or even greater, so according to infinite to consider, the error is less than 10%).

(2) Gb50054-95 “Low-voltage power Distribution Design Code” 2.1.2 provisions: “When the sum of the rated current of the motor connected near the short-circuit point exceeds 1% of the short-circuit current, the influence of the motor feedback current should be included. If the short-circuit current is 30KA, take 1% of it, which should be 300A. The total power of the motor is about 150KW, and when it is started at the same time, the feedback current included should be 6.5∑In.

(3) the impedance voltage UK of the mini-circuit breaker represents the side short circuit (circuit), when the side reaches its rated current, the original side voltage is the hundred point value of its rated voltage. Therefore, when the primary voltage is the rated voltage, the secondary current is its expected short-circuit current.

(4) Minor side of the circuit breaker rated current Ite=Ste/1.732U Ste is the capacity of the minor circuit breaker (KVA), Ue is the minor side rated voltage (no-load voltage), Ue= 0.4kV when 10/0.4kV, therefore, the secondary side rated current of the mini-circuit breaker should be calculated simply as the capacity of the mini-circuit breaker X1.44 ~ 1.50.

(5) According to the definition of Uk in (3), the short-circuit current of the auxiliary side (three-phase short-circuit) is the definition of I(3) for Uk, and the short-circuit current of the auxiliary side (three-phase short-circuit) is I(3)=Ite/Uk, which is the effective value of AC.

(6) Under the same mini-circuit breaker capacity, if there is a short circuit between the two phases, then I(2)=1.732I(3)/2=0.866I(3)

The above calculation is the current value of the short circuit at the outlet of the miniature circuit breaker, which is the most serious short circuit accident. If there is a certain distance between the short circuit point and the mini-circuit breaker, consider the line impedance.

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