⚡ Công cụ tính dòng ngắn mạch

Use IEC 60909: short-circuit current = voltage / (sqrt(3) x total impedance). For a 0.4 kV system, the Transformer impedance dominates. A 1000 kVA, 6% impedance transformer contributes about 24 kA at the LV bus (1000 / (sqrt3 x 0.4 x 0.06) ~ 24 kA). The calculator computes three-phase and single-phase fault currents.

The breaker breaking capacity must exceed the prospective Fault Current at its location. For 24 kA at the bus, select a 36 kA breaker (standard IEC 60947 ratings are 16/25/36/50 kA). Undersizing risks a failure to interrupt a fault, so use a 1.25-1.5 safety margin. The calculator matches the breaker to the fault level.

Icu is the ultimate breaking capacity — the breaker can interrupt it once but may need replacement. Ics is the service breaking capacity — the breaker can interrupt it repeatedly and remain usable. Ics is typically 50-75% of Icu. Select breakers whose Icu exceeds the fault level; the calculator checks against standard ratings.

Long cables add impedance that reduces fault current downstream. A 50 m run of 35 mm2 copper cable can cut a 24 kA fault to roughly 15-18 kA at the far end. This is why downstream breakers can be lower-rated. The calculator accounts for cable impedance when computing the fault level at the load.

Symmetrical fault current is the steady AC component; asymmetrical adds a decaying DC offset, peaking in the first half-cycle. The peak (making) current is 1.5-2.5 times the RMS symmetrical value, so Switchgear has both a breaking (Isc) and a making (peak) rating. IEC 60909 computes both; the calculator reports the symmetrical level for breaker selection.