Selecting low voltage circuit breakers is a critical task that requires careful consideration of multiple factors to ensure the chosen breaker is suitable. Below are the key aspects to consider when selecting a low voltage circuit breaker.

Key Parameters for Selecting Low Voltage Circuit Breakers

1. Determine the Rated Current

The rated current of the circuit breaker should be equal to or greater than the maximum load current in the circuit. This ensures that the circuit breaker can effectively protect the circuit. The rated current should be determined by the calculated current of the line.

2. Verify the Breaking Capacity

The breaking capacity of the circuit breaker is crucial as it determines the breaker’s ability to respond to short circuits in the circuit. To ensure safety, the breaking capacity should be sufficient to withstand the maximum short-circuit current that may occur in the circuit.

3. Sensitivity to Minimum Short-Circuit Current

The circuit breaker’s sensitivity should be verified by ensuring that the minimum short-circuit current in the circuit is at least 1.3 times the short-circuit trip current setting of the breaker. This ensures that the breaker can respond effectively to short circuits.

4. Short-Circuit Trip Current

The short-circuit trip current of the breaker should be set to avoid normal operational starting currents of the circuit. This means the breaker should not trip during normal start-up conditions, but should trip during actual short-circuit conditions.

5. Verify Short-Circuit Making Capacity

The short-circuit making capacity, which is the peak value of the short-circuit current the breaker can withstand, should be greater than the maximum prospective short-circuit current in the circuit. This ensures that the breaker can handle the highest instantaneous current during a short circuit.

Key Parameters for Selecting Low Voltage Circuit Breakers

Key Parameters for Selecting Low Voltage Circuit Breakers

Rated Operational Voltage and Current

The rated operational voltage (Ue) and rated current (Ie) of the low voltage circuit breaker should not be lower than the normal operational voltage and current or calculated current of the line and equipment. Different rated operational voltages and corresponding breaking capacities may be available for the same breaker product.

Long-Time Delay Trip Unit Setting Current (Ir1)

The setting current of the long-time delay trip unit (Ir1) should be greater than or equal to the calculated load current of the line, typically set at 1 to 1.1 times the calculated load current. It should not exceed 0.8 to 1 times the long-term allowable current of the line conductors.

Instantaneous or Short-Time Delay Trip Unit Setting Current (Ir2)

The setting current of the instantaneous or short-time delay trip unit (Ir2) should be greater than the peak current of the line. For distribution breakers, it can be set at no less than 1.35 times the peak current. For motor protection circuits, if the action time is greater than 0.02s, it can be set at no less than 1.35 times the starting current. If the action time is less than 0.02s, it should be increased to no less than 1.7 to 2 times the starting current to account for setting errors and possible variations in motor starting currents.

Short-Circuit Breaking and Withstanding Capacity Verification

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 the installation location. For action times greater than 0.02s, the effective value of the periodic component of the short-circuit current can be considered the maximum short-circuit current. For action times less than 0.02s, the total current in the first cycle of the short-circuit current should be considered. If the verification shows insufficient breaking capacity, measures such as adding backup protection devices (e.g., fuses) on the power side of the breaker or selecting a current-limiting breaker should be taken.

Sensitivity Verification

The selected circuit breaker should undergo sensitivity verification based on the short-circuit current. Sensitivity is the ratio of the minimum short-circuit current (typically two-phase or single-phase short-circuit current at the motor terminals or end of the distribution line) to the instantaneous or delayed trip current setting of the breaker. For two-phase short circuits, the sensitivity should not be less than 2, and for single-phase short circuits, it should be 1.5 for DZ type breakers and 2 for other types. If the sensitivity requirements are not met, adjust the setting current or use the delayed trip unit as backup protection.

Parameters for Shunt and Under-Voltage Trip Units

The rated voltage of the shunt and under-voltage trip units should equal the rated voltage of the line. The power category (AC or DC) should be determined based on the control circuit. National standards specify rated control power voltage series as follows: DC (24), (48), 110, 125, 220, 250V; AC (24), (36), (48), 110, 127, 220V. Values in parentheses are not recommended.

By considering these factors and parameters, you can select the appropriate low voltage circuit breaker to ensure the safety and reliability of your electrical system.