Initial, peak, breaking and thermal short-circuit currents at three buses fed by the utility, a standby generator and an induction motor — and why the peak current needs the 1.15 factor here.
| Utility supply | 11 kV, Sk″ = 250 MVA, R/X = 0.1 |
| Generator on the 11 kV board | 2.5 MVA, 11 kV, xd″ = 14%, cos φ = 0.8 |
| Transformer | 1.6 MVA, 11/0.4 kV Dyn, uk = 6%, Pk = 15 kW |
| Cable to the MCC | 50 m of 3-core 240 mm² Cu (0.0754 + j0.078 Ω/km) |
| Motor on the MCC | 160 kW, 0.4 kV, ILR/IrM = 7, η = 0.95, cos φ = 0.88 |
| Voltage factor | cmax = 1.10 at 11 kV, 1.05 at 0.4 kV (+6% tolerance) |
Each element is converted to per-unit on 100 MVA: the utility from its Sk″, the generator with its correction factor KG, the transformer with KT = 0.95·cmax/(1 + 0.6·xT), the cable from its length, and the motor from its locked-rotor current.
| Bus | Ik″ (kA) | Utility share | Generator share | Motor share |
|---|---|---|---|---|
| 11 kV switchboard | 14.20 | 13.12 | 1.01 | 0.07 |
| 0.4 kV LV board | 39.58 | 35.00 | 2.70 | 1.95 |
| Motor control centre | 23.49 | 19.92 | 1.54 | 2.03 |
Shares are magnitudes from the network solution, so they add up only approximately.
Several sources feed each fault, so IEC 60909-0 method B applies: κ from the R/X ratio at the fault, multiplied by 1.15 and limited to 2.0 (HV) or 1.8 (LV).
| Bus | R/X | κ(b) | ip (kA) |
|---|---|---|---|
| 11 kV switchboard | 0.099 | 2.00 (limit) | 40.17 |
| 0.4 kV LV board | 0.165 | 1.80 (limit) | 100.77 |
| Motor control centre | 0.458 | 1.458 | 48.44 |
At the 11 kV board the generator's contribution decays by tmin = 0.1 s, so the breaking current Ib is slightly lower than Ik″. Motor decay is not credited, which is conservative. The thermal equivalent current uses Tk = 1 s and n = 1.
| Bus | Rating | Largest fault (3-phase or earth) | Result |
|---|---|---|---|
| 11 kV switchboard | 25 kA | 14.50 kA (earth fault) | Adequate |
| 0.4 kV LV board | 50 kA | 40.34 kA (earth fault) | Adequate |
| Motor control centre | 36 kA | 23.49 kA | Adequate |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| 11 kV board short-circuit rating | ≥ 14.50 kA | 25 kA | ✓ PASS |
| LV board short-circuit rating | ≥ 40.34 kA | 50 kA | ✓ PASS |
| LV board making capacity (peak) | ≥ 100.8 kA | Check breaker Icm | ✓ PASS |
| MCC short-circuit rating | ≥ 23.49 kA | 36 kA | ✓ PASS |
Key insight: The motor adds about 2 kA at the MCC, nearly 9% of the fault current there, even though it is a modest 160 kW machine. Leaving motors out of a maximum short-circuit study underestimates both the breaking and the peak duty.
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Open IEC 60909 Short-Circuit Calculator calculator →The generator's neutral is solidly earthed and its zero-sequence reactance (7%) is half its subtransient reactance (14%), while the utility's zero-sequence impedance equals its positive-sequence impedance in this model. The zero-sequence impedance at the bus is therefore lower than the positive-sequence impedance, and with Z0 < Z1 the single-phase fault current exceeds the three-phase current. (The transformer's delta winding is on the 11 kV side, so it adds no zero-sequence path there.)
When a single source feeds the fault through a radial (non-meshed) path, κ from that path's R/X is used directly. With several sources in parallel, method B's 1.15 factor covers the different R/X ratios of the branches.