Part 6 of the short-circuit series. A 65 kA board looks right until the motor contribution is added: 68.8 kA r.m.s. and 175 kA peak. How to check breaking, making and short-time ratings.
| Supply | 500 MVA, 11 kV grid → 2.5 MVA, 11/0.4 kV Dyn transformer, uk = 6.25 %, Pk = 22 kW |
| Motors | 1,000 kW of running motors on the main LV switchboard |
| Candidate | Switchboard and ACB rated Icu = 65 kA |
| Settings | tmin = 0.1 s, Ith duration tk = 1 s |
From the transformer 57.75 kA and from the motors 11.38 kA.
Ik″ must not exceed the breaker's rated ultimate short-circuit breaking capacity. With the motor contribution kept in full, Ib = Ik″.
IEC 60947-2 Table 2: for Icu above 50 kA the minimum making capacity is Icm = 2.2 × Icu. A 65 kA breaker gives 143 kA < 175 kA; an 85 kA breaker gives 187 kA ≥ 175 kA.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Icu 65 kA vs Ik″ | ≥ 68.81 kA | 65 kA | ✗ FAIL |
| Icu 85 kA vs Ik″ | ≥ 68.81 kA | 85 kA | ✓ PASS |
| Icm = 2.2 × 85 kA vs ip | ≥ 175.2 kA | 187 kA | ✓ PASS |
| Icw (1 s) | ≥ 69.5 kA | Select ACB with Icw ≥ 70 kA, 1 s | ✓ PASS |
Key insight: Three different ratings protect against three different stresses: Icu for interrupting, Icm for the first asymmetrical peak, and Icw for carrying the fault while a downstream device clears it.
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Open IEC 60909 Short-Circuit Calculator calculator →When several sources feed the fault, IEC 60909 method B (1.15 κ) is a safe single-value approach. For purely radial sources the standard also allows summing the partial peak currents, which can give a somewhat lower ip.
Ics is the service breaking capacity, after which the breaker remains usable. Many specifications require Ics = 100 % Icu for main breakers.