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# How to choose the rated current of the circuit breaker

How to choose the size of the switch (breaker, etc.)? Generally, how much is more appropriate than the rated current?

Generally choose 1.5 times to 2 times the load, the load capacity is large, the starting current is large and the starting current is large, and the starting current is selected 1.5 times.

(1) The setting value of the circuit breaker's extended operating current ≤ the allowable current carrying capacity of the wire. For the use of wires and cables, 80% of the allowable current carrying capacity of the wires and cables can be taken.

(2) The returnable time of the 3 times long delay action current setting value ≥ the starting time of the motor with the maximum starting current in the line.

(3) Instantaneous current setting value ≥ 1.1X(Ijx+k1kIedm)

• Ijx line calculates the load current;
• k1 is the impulse coefficient of the motor starting current, generally k1=1.7-2;
• k motor starting current multiple;
• Icdm the rated motor Current

The rated current selection for 220kV circuit breaker breaking is the short-circuit current calculation in the textbook.The rated open-end current is the effective value, and the rated peak withstand current is the impulse current.

The breaking time of the circuit breaker should be less than 50ms (national regulations).

220 line, load current and short-circuit current are not necessarily, depending on whether you are a tie line or a load line, the current of the tie line is determined by the system parameters, and the current of the load line is determined by the size of the load supplied.

The selection of the rated current of the circuit breaker is based on the different grounding methods of the neutral point of the system. Generally, the prime start coefficient of 1.3-1.5 is multiplied on the basis of the calculated current of the line.

In short, the rated current of 220kv circuit breakers is generally 1250A-4000A. (State Grid equipment bidding parameters)

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