01. Selection of the Circuit Breaker Breaking Capacity

The breaking capacity of the circuit breaker should be chosen based on the calculated expected short-circuit current of the line. Calculating the expected short-circuit current accurately can be quite complex. Therefore, simplified calculation methods are often used in engineering practice, which have minimal errors:

1.Transformers with a voltage rating of 10/0.4 kV: The short-circuit capacity on the high-voltage side can be considered infinite. The short-circuit capacity on the 10 kV side is typically between 200-400 MVA, or even larger. Assuming infinite short-circuit capacity results in less than a 10% error.

2.Motor Feedback Current: According to GB50054-95, “Low Voltage Distribution Design Code,” if the sum of the rated currents of motors connected near the short-circuit point exceeds 1% of the short-circuit current, the influence of motor feedback current should be considered. For a short-circuit current of 30 kA, 1% is 300 A. The total power of the motor is around 160 kW. When the motor starts simultaneously, the feedback current should be 6.5 times the rated current.

3.Impedance Voltage (UK%): Impedance voltage refers to the percentage of primary side voltage needed when the transformer’s secondary side is short-circuited. The secondary current reaches its rated current when the primary voltage reaches the rated voltage. This provides a method for estimating the expected short-circuit current.

4.Rated Current of the Transformer: The rated current on the secondary side of the transformer is calculated as:

Where:

Ste= Transformer capacity (in KVA)

Ue = Rated secondary voltage (0.4 kV for a 10/0.4 kV transformer)

Thus, the rated current can be approximated as the transformer capacity multiplied by a factor between 1.44 and 1.5.

5.Short-Circuit Current: The short-circuit current on the secondary side of the transformer can be calculated as:

For a two-phase short circuit, the current would be:

6.Line Impedance and Distance Impact: When the short-circuit point is at a distance from the transformer, the line impedance reduces the short-circuit current. For example, if the transformer capacity is 200 KVA and the short-circuit point is 100 meters away, the three-phase short-circuit current drops by 35% compared to a short-circuit at the transformer output terminal.

In practice, when selecting a circuit breaker, ensure:

The rated current In of the circuit breaker is at least equal to the rated current IL of the line.

The rated ultimate short-circuit breaking capacity Icu is greater than or equal to the expected short-circuit current.

02. Rated Short-Circuit Breaking Capacities

According to IEC 947-2 and GB 4048.2, the rated short-circuit breaking capacities of a circuit breaker are defined as follows:

Rated Ultimate Short-Circuit Breaking Capacity (Icu): The maximum short-circuit current the circuit breaker can interrupt without continuing to carry its rated current.

Rated Operating Short-Circuit Breaking Capacity (Ics): The maximum short-circuit current the breaker can interrupt while still being able to carry its rated current after the interruption.

Test procedures for these breaking capacities involve applying a short-circuit current and ensuring the circuit breaker can interrupt it successfully without sustaining damage.

03. Electrical Clearance and Creepage Distance of Circuit Breakers

Electrical clearance is determined based on the insulation coordination of the low-voltage system, which ensures that transient overvoltages are limited to specified levels. The insulation requirements include:

1.The rated insulation voltage should meet or exceed the system’s rated voltage.

2.The rated impulse withstand voltage should match or exceed the system’s impulse withstand voltage.

3.The transient overvoltage should not exceed the system’s rated impulse withstand voltage.

For example, a circuit breaker with an insulation rating of 660 V and a pollution level of 3 should have a creepage distance of at least 10 mm. This ensures the product can handle electrical overvoltages without breakdown.

04. Application of Four-Pole Circuit Breakers

Currently, there are no mandatory national standards in China for the use of four-pole circuit breakers. However, the choice of whether to use them should be based on safety and reliability considerations:

1.TN-C System: In this system, the N and PE lines are combined. For safety reasons, the PEN line cannot be disconnected, so four-pole circuit breakers are prohibited.

2.TT, TN-C-S, and TN-S Systems: These systems can use four-pole circuit breakers to ensure safety during maintenance. However, in TN-C-S and TN-S systems, the N pole of the circuit breaker should only be connected to the N line, not the PEN or PE line.

3.Dual Power Switching Systems: In systems with dual power sources, four-pole circuit breakers are necessary to ensure safe switching.

4.Single-Phase Main Switch: For residential applications, a two-pole circuit breaker with an N pole should be used for isolation during maintenance.

5.Residual Current Protectors: For systems where residual current protectors (RCDs) are used, especially in 380/220V systems, a four-pole or two-pole RCD should be selected to ensure proper operation.