Whether you are an electrical engineer or an ordinary electrician, the most commonly used electrical component in your work is definitely a circuit breaker. Basically, no electrical cabinet does not require a circuit breaker. Even in family life, every household has a circuit breaker. It can be said that Circuit breakers are used wherever electricity is used. How should we choose the circuit breaker (low voltage) correctly?

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

(2) Verify the breaking capacity of the low-voltage circuit breaker according to the maximum short-circuit current of the line;

(3) Verify the sensitivity of the circuit breaker action according to the minimum short-circuit current of the line, that is, the minimum short-circuit current of the line 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 line;

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

When selecting a low-voltage circuit breaker, you should select the usage category according to the specific usage conditions, select the rated operating voltage, rated current, release setting current and shunt, voltage and current of the undervoltage release and other parameters, refer to the protection characteristics provided in the product sample The curve uses protection characteristics, and the short-circuit characteristics and sensitivity coefficient need to be verified. When there are coordination requirements with other circuit breakers or other protective appliances, a selective circuit breaker should be selected.

1. Rated working voltage and rated current

Rated operating voltage Ue of the low-voltage circuit breaker. and rated current Ie. Should be no less than the normal rated operating voltage and operating current or calculated current of the line and equipment respectively. The rated working voltage of a circuit breaker is related to the breaking capacity and usage category. The same circuit breaker product can have several rated working voltages and corresponding breaking capacity usage categories.

2. Long time delay release setting current Ir1

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

3. Setting current Ir2 of instantaneous or short-delay release

The instantaneous or short-delay release setting current Ir2 of the selected circuit breaker should be greater than the line peak current. 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 It is not less than 1.7-2 times of the starting current. These coefficients are added to take into account factors such as tuning errors and possible changes in motor starting current.

4. Short-circuit making and breaking capability and short-time endurance capability verification

The rated short-circuit breaking capacity and rated short-circuit making capacity of the low-voltage circuit breaker should not be less than the expected short-circuit current at its installation location. 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 regarded as the maximum short-circuit current; when the action time is less than 0.02s, the non-periodic component should be considered, that is, the short-circuit current should be regarded as the first The full current in the cycle is taken as the maximum short-circuit current. If the verification results indicate that the circuit breaker's switching capacity is insufficient, the following measures should be taken.

a) Add other protective appliances (such as fuses) on the power side of the circuit breaker as backup protection.

b) When using a current-limiting circuit breaker, the corresponding product can be selected according to the let-through current characteristics or current-limiting coefficient (i.e., the ratio of the actual breaking current peak value to the expected short-circuit current peak value) provided by the manufacturer.

c) A circuit breaker with a larger capacity can be selected. Various short-circuit protective circuit breakers must be able to carry unrestricted short-circuit current transients in the closed position and must also be able to carry the short-circuit current within a specified delay range. The short-circuit current value carried by this short-term load should not exceed the rated short-time withstand capacity of the circuit breaker. Otherwise, measures should be taken or the specifications of the circuit breaker should be changed. Circuit breaker product catalogs generally give the product's rated peak withstand current and rated short-time withstand current (1s current). When AC current is used, the short-time withstand current shall be based on the effective value of the unrestricted periodic component of the short-circuit current.

5. Sensitivity coefficient calibration

The selected circuit breaker should also be calibrated for sensitivity coefficient based on short-circuit current. The sensitivity coefficient is the ratio of the minimum short-circuit current in the line (generally taken as the two-phase or single-phase short-circuit current at the motor terminal or the end of the distribution line) and the instantaneous or delayed release setting current of the circuit breaker. The sensitivity coefficient during two-phase short circuit should not be less than 2. The sensitivity coefficient during single-phase short circuit can be 1.5 for DZ type circuit breaker and 2 for other types of circuit breakers. If the verified sensitivity coefficient does not meet the above requirements, in addition to adjusting the setting current, a time delay release can also be used as backup protection.

6. Determination of parameters of shunt and undervoltage release

The rated voltage of the shunt and undervoltage release should be equal to the rated voltage of the line, and the power supply category (AC, DC) should be determined according to the control line conditions.

XZT 7111 High Voltage Circuit Breaker Tester