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

1. Select motor protection circuit breaker Q3

Since the rated current of Motor 1 is 182.4A, a circuit breaker with a rated current of 200A can be chosen. Considering the short-circuit current at the location (Ik3 = 19.12 kA), a circuit breaker with a breaking capacity of at least 20 kA is required. The system voltage is 380V, so the circuit breaker should be rated for 380V. A 20kA or 50kA breaking capacity model would be suitable.

The settings for the circuit breaker are as follows:

- Long-time trip setting: 200A

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

When a short circuit occurs at the end of the line, the short-circuit current is I'k4 = 4.9kA. Calculating the ratio: I'k4 / Iin = 4900 / 2189 ≈ 2.238 > 1.25, which confirms that the circuit breaker Q3 will provide effective protection.

As Motor 1 is lightly loaded, the return time for the long-time trip setting at 6 times the rated current can be set to 5 seconds.

2. Select distribution circuit breaker Q2

With a load current of 600A, a 630A-rated circuit breaker is appropriate. Given the short-circuit current of Ik2 = 29.7kA, the circuit breaker must have a breaking capacity of at least 30kA.

However, standard 630A circuit breakers may not offer sufficient delay and short-circuit protection. Therefore, the DW15-1000 model is used in this case, with a 630A overcurrent release unit. This model has a short-circuit breaking capacity of 30kA.

Q2 is the second stage in the power distribution system, so it is set with a short-delay time of 0.2 seconds.

Short-delay trip setting: Isd ≥ 1.1 × (Icl + 1.35 × Ks × IN) = 1.1 × (600 + 1.35 × 6.5 × 182.4) = 2421A. It can be set to 2800A, approximately 4.5 times the trip unit’s rated value.

The long-time trip setting at 3 times the rated current has a return time of 8 seconds.

The instantaneous trip setting is calculated as Iin = 1.1 × Ik3 = 1.1 × 19.12 = 21.03kA. To ensure proper operation, it can be set at 22kA.

3. Select main circuit breaker Q1

As Q1 is the main switch for the transformer, its secondary side rated current is IN2 = 1445A. The DW15-1600 model was initially considered, but its short-circuit breaking capacity of 30kA is insufficient for the system's Ik1 = 31.5kA.

To meet the requirement, the DW15-2500 model is selected, which has a short-circuit breaking capacity of 40kA. Its instantaneous trip setting is set at 1.1 × Ik2 = 1.1 × 29.7 = 32.67kA. However, this value exceeds the circuit breaker’s delay short-circuit breaking capacity, meaning the instantaneous release may not function properly.

Despite this, the DW15-1600 model can still be used if the instantaneous trip is set at 30kA. In cases where the short-circuit current exceeds this, the circuit breaker will trip instantaneously.

The short-delay time is set to 0.4 seconds (0.2s + 0.2s). The short-delay trip setting is calculated as Isd ≥ 1.1 × (IN2 + 1.35 × Ks × IN) = 1.1 × (1445 + 1.35 × 5.8 × 329) = 4423A. It can be adjusted to 4500A.

The long-time trip setting is set at 1600A, with a return time of 15 seconds at 3 times the rated current.

4. Select circuit breaker Q4

Q4 is installed close to the transformer and must handle a high short-circuit current of 31.5kA. For Motor M2, which is an air compressor with infrequent starts, using a direct-start circuit breaker with short-circuit protection is more cost-effective than a fuse and contactor setup.

The DWX15-400 current-limiting circuit breaker is a good choice. It has a short-circuit breaking capacity of 50kA and uses an electromagnetic operating mechanism, making it similar to a contactor in operation.

The long-time trip setting for the trip unit can be adjusted to 0.85 times the rated current: Ie = 400 × 0.85 = 340A.

The instantaneous trip setting is set at 12 times the rated current: 12 × 400 = 4800A. Since Motor M2 is light-loaded, the return time for the 6-times long-time trip setting can be set to 5 seconds.

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