Calculates balanced three-phase bolted short-circuit current (Isc), fault apparent power (MVA), and peak making current (ip). Explicitly compares simplified transformer-limited calculation against combined upstream grid + transformer + cable impedance modeling to prevent over-specification of switchgear.
Governing Formulas & Standards
Standards Basis: IEC 60909 / IEEE 141 (Red Book)
I_{\text{sc}} = \frac{c \cdot V_{\text{LL}}}{\sqrt{3} \cdot Z_{\text{total}}}, \quad i_p = \kappa \cdot \sqrt{2} \cdot I_{\text{sc}}, \quad \kappa = 1.02 + 0.98 e^{-3/(X/R)}
Calculates initial symmetrical short-circuit current per IEC 60909 with voltage factor c and applies peak making factor kappa based on the network X/R ratio.
Worked Engineering Example: Fault Level at 415V Secondary of 2.5 MVA, 6.5% Transformer Fed from 500 MVA Grid
- Transformer Secondary Impedance: Z_tx = 0.065 × (0.415)² / 2.5 → 0.004478 Ω
- Grid Thevenin Impedance at 415V: Z_grid = 1.05 × (0.415)² / 500 → 0.000362 Ω
- Combined Thevenin Fault Current: I_sc = (1.05 × 415) / (√3 × 0.00484) → 52.0 kA RMS
Final Solution: Symmetrical Isc: 52.0 kA | Peak Make ip: 114.2 kA | Fault MVA: 37.4 MVA
Frequently Asked Questions
- When should I use the infinite bus model vs the source+transformer model?
- Use the infinite bus model when upstream utility fault level is unknown or for conservative preliminary design. Use the combined source+transformer model for detailed substation design and breaker sizing to avoid paying for over-rated switchgear.
Interactive calculation engine and real-time CAD solver available online at https://amithvijayan.in/tools/three-phase-fault-calculator.