Circuit breakers include miniature circuit breakers, miniature circuit breakers, molded case circuit breakers, and vacuum circuit breakersHow to choose the model and specifications of circuit breakersIt is something that a professional electrical engineer needs to pay close attention to, and the main knowledge points are as follows:

(Low voltage)

(1) The rated current of the circuit breaker is determined by the calculated current of the circuit;

(2) Verify the breaking capacity of low-voltage circuit breakers based on the maximum short-circuit current of the circuit

(3) Verify the sensitivity of circuit breaker operation based on the minimum short-circuit current of the circuit, that is, the minimum short-circuit current of the circuit should not be less than 1.3 times the short-circuit setting current of the circuit breaker;

(4) The short-circuit setting current of the circuit breaker should avoid the normal operating starting current of the circuit;

(5) Verify the rated short-circuit making capacity (maximum expected peak current) of the circuit breaker based on the short-circuit surge current on the line (i.e. maximum instantaneous value of short-circuit full current), i.e. the latter should be greater than the former.

The selection of low-voltage circuit breakers should be based on specific usage conditions, including the type of use, rated working voltage, rated current, set current of the release, voltage and current of the shunt and undervoltage release. The protection characteristics should be selected according to the protection characteristic curve provided in the product sample, and the short-circuit characteristics and sensitivity coefficient should be verified. When there are coordination requirements with other circuit breakers or protective electrical appliances, selective circuit breakers should be selected.

1. The rated working voltage Ue of low-voltage circuit breakers with rated working voltage and rated current. And rated current Ie. It should not be lower than the normal rated working voltage and working current or calculated current of the line and equipment respectively. The rated working voltage of a circuit breaker is related to its breaking capacity and usage category. The same circuit breaker product can have several rated working voltages and corresponding breaking capacity usage categories.

2. The setting current Ir1 of the selected circuit breaker for the long delay release should be greater than or equal to the calculated load current of the line, which can be determined as 1-1.1 times the calculated load current; At the same time, it should not exceed 0.8-1 times the long-term allowable current of the line conductor.

3. The setting current Ir2 of the instantaneous or short delay release of the selected circuit breaker should be greater than the peak current of the line. The distribution circuit breaker can be determined according to the principle of not less than 1.35 times the peak current. When the action time of the motor protection circuit is greater than 0.02s, it can be determined according to the principle of not less than 1.35 times the starting current. If the action time is less than 0.02s, it should be increased to not less than 1.7-2 times the starting current. These coefficients are added taking into account factors such as setting errors and possible changes in motor starting current.

4. The rated short-circuit breaking capacity and rated short-circuit making capacity of the low-voltage circuit breaker should not be lower than the expected short-circuit current at its installation position. When the action time is greater than 0.02s, the non periodic component of the short-circuit current can be ignored, that is, the effective value of the periodic component of the short-circuit current is taken as the maximum short-circuit current; When the action time is less than 0.02s, non periodic components should be considered, that is, the full current in the first cycle of the short-circuit current should be taken as the maximum short-circuit current.

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