🔋 HV Short-Circuit Current Calculator
Convert an upstream short-circuit capacity (MVA) into the primary short-circuit current and verify the required HV switchgear breaking capacity — KYN28A for 10 kV and KYN61 for 35 kV.
⚙️ System
📊 Result
For engineers specifying medium-voltage switchgear and its breaking capacity.
How it is calculated
· Short-circuit current: Isc = Ssc ÷ (√3 × U).
· Peak current: ip ≈ 2.5 × Isc.
· Selection: required breaking capacity ≥ Isc; rated current = next standard frame above load.
Referenced standards
| Standard | Scope |
|---|---|
| IEC 62271-100 | High-voltage switchgear and controlgear — AC circuit-breakers |
| IEEE C37 | High-voltage circuit-breaker ratings (ANSI) |
Worked example — 500 MVA at 10 kV → 28.9 kA
1. Ssc = 500 MVA, U = 10 kV → Isc = 500 ÷ (√3 × 10) = 28.9 kA.
2. Required breaking capacity = 31.5 kA; load current 1000 A → 1250 A frame.
3. Recommended model: KYN28A-12-1250-31.5.
FAQ
How do I calculate high-voltage short-circuit current from upstream capacity?
Convert upstream short-circuit capacity Ssc to current: I = Ssc / (sqrt(3) x Un). A 10 kV bus with 500 MVA upstream gives I = 500 / (sqrt3 x 10) = 28.9 kA. Add Transformer and cable impedance for downstream points. The calculator checks this against KYN28A/KYN61 Switchgear ratings (25/31.5/40 kA).
What short-circuit rating does 10 kV switchgear need?
KYN28A 10 kV switchgear is commonly rated 25 kA, 31.5 kA or 40 kA. Most urban 10 kV networks (300-500 MVA upstream) need 25-31.5 kA; industrial plants close to a large source may need 40 kA. The calculator verifies the fault level against the panel rating and flags undersizing.
How is the peak making current related to the RMS breaking current?
The peak (making) current is the maximum instantaneous Fault Current, about 2.5 times the RMS breaking current for a typical X/R Ratio of ~14. A 31.5 kA breaker has a ~80 kA peak making rating. Switchgear must withstand both; the calculator reports the breaking level for selection.
What is the difference between KYN28A and KYN61 switchgear?
KYN28A is metal-clad, removable (withdrawable) 12 kV switchgear with a vacuum circuit breaker, rated up to 40 kA, the mainstream indoor 10 kV switchgear. KYN61-40.5 is the 35 kV equivalent, rated up to 31.5-40 kA. The calculator selects the appropriate class by voltage and fault level.
How do I derate switchgear for high altitude?
Above 1000 m, reduced air density lowers dielectric strength and cooling, so switchgear is derated about 1% per 100 m (IEC 62271-1). At 2000 m a 31.5 kA panel derates to roughly 28-29 kA. The calculator applies altitude derating when checking the panel rating against the fault level.
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