Detailed examples of circuit breaker parameters and selection methods at all levels of the distribution system

1. Select motor protection circuit breaker Q3

Given that the rated current of the motor is 182.4A, a circuit breaker with a rated current of 200A is suitable. Considering the short-circuit current at the point of fault (Ik3 = 19.12 kA), a circuit breaker with a breaking capacity of at least 20 kA should be selected. The system voltage is 380V, so the circuit breaker must have a rated voltage of 380V. A 20kA or 50kA short-circuit breaking capacity would be appropriate for this application.

The setting values for the circuit breaker are as follows:

- Long-time delay current setting: 200A

- Instantaneous trip current: Iin = 12 × IN = 12 × 182.4 = 2189A

When a single-phase short circuit occurs at the end of the line, the short-circuit current is I'k4 = 4.9kA.

The ratio I'k4 / Iin = 4900 / 2189 ≈ 2.238 > 1.25, which ensures that the circuit breaker will operate correctly in case of a fault.

Since motor M1 is lightly loaded, when the long-time delay current reaches 6 times its setting value, the return time can be set to 5 seconds.

2. Select distribution circuit breaker Q2

With a load current of Icl = 600A, a circuit breaker with a rated current of 630A is suitable. Given that the short-circuit current Ik2 = 29.7kA, a circuit breaker with a short-circuit breaking capacity of at least 30kA is required.

After consulting the selection manual, it was found that standard 630A circuit breakers do not meet the necessary delay and short-circuit requirements. Therefore, the DW15-1000 circuit breaker is used, which has a short-circuit breaking capacity of 30kA and an overcurrent release rated at 630A.

As Q2 is the second stage in the power distribution system, the short-time delay is set to 0.2 seconds.

Short-time delay current setting:

Isd ≥ 1.1 × (Icl + 1.35 × Ks × IN) = 1.1 × (600 + 1.35 × 6.5 × 182.4) = 2421A

This can be set to 2800A, which is approximately 4.5 times the rated current of the trip unit.

The return time for the 3x long-time delay setting is 8 seconds.

The instantaneous setting current is calculated as Iin = 1.1 × 19.12 = 21.03kA. This can be set to 22kA to match the incoming line short-circuit current at Q3.

3. Select main circuit breaker Q1

Q1 serves as the main protection switch for the transformer. The secondary side rated current of the transformer is IN2 = 1445A, so a DW15-1600 circuit breaker is initially considered.

However, the short-circuit breaking capacity of the DW15-1600 is only 30kA, which is insufficient for Ik1 = 31.5kA. To meet the requirement, a DW15-2500 circuit breaker is needed, which has a short-circuit breaking capacity of 40kA.

Although the instantaneous setting current of the DW15-2500 is set to 1.1 × 29.7 = 32.67kA, this value is below the breaking capacity of the circuit breaker, which may prevent the instantaneous release from operating properly.

To save costs, the DW15-1600 can still be used, with the instantaneous setting current set to 30kA. When the short-circuit current exceeds 30kA, the circuit breaker will trip instantly.

The short-time delay is set to 0.4 seconds (0.2 + 0.2).

Short-time delay current setting:

Isd ≥ 1.1 × (1445 + 1.35 × 5.8 × 329) = 4423A

This can be set to 4500A.

The long-time delay setting is 1600A, and the return time for the 3x long-time delay setting is 15 seconds.

4. Select circuit breaker Q4

Q4 is installed close to the transformer and must handle a high short-circuit current of 31.5kA. Motor M2 is an air compressor that starts infrequently, making it more cost-effective to use a circuit breaker capable of direct starting and short-circuit protection instead of a fuse and contactor or a general circuit breaker with a contactor.

The DWX15-400 current-limiting circuit breaker is a suitable choice, with a short-circuit breaking capacity of 50kA. It features an electromagnetic operating mechanism, allowing it to be operated similarly to a contactor.

The long-time delay current setting can be adjusted to 0.85 times the rated current:

Ie = 400 × 0.85 = 340A

The instantaneous trip setting is 12 × 400 = 4800A. Since the air compressor is a light-load starter, the return time can be set to 5 seconds when the 6x long-time delay current is reached.

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