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Short Circuit & Fault Calculator

Symmetrical fault currents · Impedance · Short-circuit MVA · Radial & Ring systems

IEC 60909

ℹ Calculates max/min three-phase symmetrical fault current at the fault point using IEC 60909 voltage factor c. Includes source, transformer and feeder impedances in series.

System Parameters

Nominal Voltage U_n

Voltage Factor c (IEC 60909)

System Frequency

Network / Source

Initial Symmetrical Short-Circuit Power S″_kQ

MVA

Source X/R Ratio

Transformer

Rated Power S_n

MVA

Short-circuit Voltage u_kr

Resistive Component u_Rr

Correction Factor K_T (IEC 60909 §3.3.3)

Feeder Cable / Line

Resistance r'

Ω/km

Reactance x'

Ω/km

Length

Fault Results

Fault Currents

I″_k3 — Three-phase fault

—

Peak current i_p

—

kA peak

I_th — Thermal equiv.

—

kA (1 s)

S″_k — SC MVA

—

MVA

Total Impedance at Fault Point

Z_k — Total

—

R_k

—

X_k

—

X/R ratio

—

Impedance Composition

Z_source—

Z_transformer—

Z_cable/line—

Impedance Breakdown

Component	R (Ω)	X (Ω)	Z (Ω)
— Enter values and calculate —

Applied Formula

// IEC 60909 Eq. (29) — Three-phase fault
I″_k3 = c · U_n / (√3 · |Z_k|)

// Peak current factor κ (IEC 60909 §4.3.1)
κ = 1.02 + 0.98 · e^−3R/X
i_p = κ · √2 · I″_k3

ℹ Ring/loop distribution: fault current supplied from both ends simultaneously. Total I″_k = I_A + I_B. Each path computed as radial equivalent per IEC 60909.

Common Parameters

Nominal Voltage U_n

Voltage Factor c

Fault Location f (from bus A)

Bus A — Source & Feeder

S″_kA Network short-circuit power

MVA

X/R ratio (source A)

Ring cable resistance r'_A

Ω/km

Ring cable reactance x'_A

Ω/km

Total ring length (A→fault)

Bus B — Source & Feeder

S″_kB Network short-circuit power

MVA

X/R ratio (source B)

Ring cable resistance r'_B

Ω/km

Ring cable reactance x'_B

Ω/km

Total ring length (B→fault)

Ring Fault Results

Path Contributions

Path A

—

Path B

—

Total at Fault Point

I″_k total

—

Peak i_p

—

kA peak

S″_k

—

MVA

Thevenin Z_eq

—

System Diagram

Impedance per Path

Path	Z_src (Ω)	Z_cable (Ω)	Z_total (Ω)	I (kA)
— Enter values —

Notes

// Path impedances in parallel (Thevenin equivalent)
Z_eq = Z_A ‖ Z_B = Z_A·Z_B / (Z_A+Z_B)

I_A = c·U_n / (√3·Z_A)
I_B = c·U_n / (√3·Z_B)
I_total = I_A + I_B

ℹ Typical impedance values for common cables, transformers and overhead lines. Values referenced to the system voltage shown. Use these to populate the calculator inputs.

Power Transformers — u_kr & u_Rr

S_n	u_kr %	u_Rr %	X/R
100 kVA	4.0	1.8	2.2
250 kVA	4.0	1.4	2.8
630 kVA	4.0	1.1	3.6
1 MVA	5.0	1.0	5.0
2.5 MVA	5.5	0.8	6.9
10 MVA	6.0	0.5	12
40 MVA	8.0	0.3	27
100 MVA	12.0	0.2	60
250 MVA	14.0	0.1	140

Overhead Lines (Al conductor)

Size (mm²)	r' (Ω/km)	x' (Ω/km)	I_max (A)
50 Al	0.641	0.391	170
95 Al	0.306	0.371	250
150 Al	0.206	0.356	320
240 Al	0.125	0.340	420
400 Al	0.080	0.328	550

Underground Cables (XLPE)

Size (mm²)	r' (Ω/km)	x' (Ω/km)	I_max (A)
35 Cu	0.524	0.113	165
70 Cu	0.268	0.101	230
120 Cu	0.153	0.094	295
185 Cu	0.099	0.087	375
300 Cu	0.060	0.082	470
95 Al	0.320	0.099	195
150 Al	0.206	0.091	250
240 Al	0.125	0.086	320
400 Al	0.077	0.079	420

Voltage Factor c — IEC 60909 Table 1

Voltage Level	c_max	c_min
LV: 100V–1kV (±6%)	1.05	0.95
LV: 100V–1kV (±10%)	1.10	0.95
MV: 1kV–35kV	1.10	1.00
HV: 35kV–230kV	1.10	1.00
EHV: >230kV	1.10	1.00

IEC 60909 Key Equations

// Three-phase symmetrical fault current
I″_k3 = c·U_n / (√3·|Z_k|)

// Peak short-circuit current
i_p = κ·√2·I″_k3
κ = 1.02 + 0.98·e^−3R/X

// Thermal equivalent current (t_k = 1s)
I_th = I″_k·√(m+n)

// Short-circuit MVA
S″_k = √3·U_n·I″_k3

// Network impedance from fault level
Z_Q = c·U_n² / S″_kQ

// Transformer impedance (referred to LV)
Z_T = K_T·(u_kr/100)·(U_n²/S_n)
R_T = K_T·(u_Rr/100)·(U_n²/S_n)
X_T = √(Z_T²−R_T²)

Kappa Factor κ vs X/R

Fault Type Multipliers

Fault Type	Multiplier	Typical I/I_k3
3-phase (L-L-L)	1.00	100%
3-phase-to-earth (L-L-L-E)	≈1.00	~100%
2-phase (L-L)	√3/2 ≈ 0.866	~87%
1-phase-to-earth (L-E)	Depends Z₀	60–120%

κ Factor Quick Reference

X/R ratio	κ	i_p/I″_k·√2
1	1.128	1.128
2	1.247	1.247
5	1.468	1.468
10	1.642	1.642
20	1.760	1.760
50	1.840	1.840
∞	2.000	2.000

Correction Factors

// Transformer correction factor K_T
K_T = 0.95·c_max / (1+0.6·x_T)
where x_T = u_kr/100 (pu reactance)

// Generator correction K_G
K_G = U_n·c_max / (U_rG·(1+x″_d·sin φ_rG))