Two-tier protective device coordination for a 11 kV/0.4 kV substation transformer (1250 kVA, 6% Z) with LV main circuit breaker and downstream MCCB. Define selective coordination per IEC 60947-2: select instantaneous and short-time delay settings for the main breaker, verify coordination with LV upstream breaker, and confirm 50% selectivity margin for the downstream device.
| Transformer | 1250 kVA, 11/0.4 kV, Z% = 6% |
| LV main breaker | 1600 A air circuit breaker |
| Downstream MCCB | 630 A, electronic release |
| Isc LV side (Icc) | 15.8 kA (prospective) |
| Inom transformer | 1804 A (LV side) |
| Protection standard | IEC 60947-2 / IEC 60255-151 |
| Selectivity margin | 50% (per IEC 60364-4-47) |
| Check | Status | Util. | Value |
|---|---|---|---|
| Main breaker — instantaneous | PASS |
|
I_i = 6 × Inom = 9600 A (6–12× range) |
| Main breaker — short delay | PASS |
|
I_sd = 3 × Inom = 5412 A, t_sd = 0.3 s |
| MCCB — long time setting | PASS |
|
I_r = 560 A (0.9 × Inom_mccb) |
| MCCB — instantaneous | PASS |
|
I_i = 5 × I_r = 2800 A |
| Selectivity — main vs upstream | PASS |
|
ΔI_sel = 4200 A > 50% margin ✓ |
| Time-current margin | PASS |
|
t_margin = 180 ms ≥ 100 ms minimum |
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