IEC 60909 Short Circuit Method — A 5-Minute Summary
The four fault current values you need from IEC 60909: Ik", Ip, Ib, Ith. When to use each one and why they matter for equipment specification.
February 26, 2026
The Standard in Brief
IEC 60909-0:2016 defines a calculation method for short-circuit currents in three-phase AC systems. It produces four distinct current values, each used for different equipment specification purposes.
The Four Values
1. I″k — Initial Symmetrical Short-Circuit Current
The RMS value of the AC symmetrical component of the fault current at the instant of fault initiation. This is the "headline number" most engineers reference.
Used for: Circuit breaker breaking capacity, protective device coordination, switchgear rating
Calculation basis: Uses the voltage factor c (1.05 for max, 0.95 for min on LV systems) applied to the nominal voltage, divided by the total impedance from the source to the fault point:
I″k = (c × Un) / (√3 × Zk)
2. Ip — Peak Short-Circuit Current
The maximum instantaneous value of the fault current, occurring approximately half a cycle after fault initiation. Includes the DC offset component.
Used for: Busbar mechanical bracing, switchgear making capacity, electrodynamic force calculations
Calculation: Ip = κ × √2 × I″k
Where κ depends on the R/X ratio: κ = 1.02 + 0.98 × e^(-3R/X). Ranges from ~1.0 (purely resistive) to ~1.8 (highly inductive near generators).
3. Ib — Symmetrical Breaking Current
The RMS value of the AC component at the instant of contact separation of the switching device. Accounts for DC component decay during the opening time.
Used for: Circuit breaker selection (breaking capacity must exceed Ib)
For circuits remote from generators (most distribution systems): Ib = I″k (no decay). For circuits near generators: Ib < I″k due to AC component decay.
4. Ith — Thermal Equivalent Short-Circuit Current
The RMS value that would produce the same thermal energy over the fault duration as the actual decaying fault current.
Used for: Cable thermal withstand (adiabatic equation), busbar thermal rating, transformer short-circuit withstand
Calculation: Ith = I″k × √(m + n)
Where m accounts for DC component heat and n accounts for AC component decay. For most distribution networks far from generators: m + n ≈ 1, so Ith ≈ I″k.
Which Value for Which Equipment?
| Equipment Specification | Use This Value |
|---|---|
| Circuit breaker breaking capacity | Ib (or I″k for remote faults) |
| Circuit breaker making capacity | Ip |
| Switchboard rated short-time current | I″k (RMS over rated duration) |
| Busbar mechanical bracing | Ip |
| Cable thermal withstand (k²S² ≥ I²t) | Ith |
| Protective device coordination curves | I″k |
The Voltage Factor c
IEC 60909 uses voltage factor c instead of the actual system voltage. This accounts for:
- Voltage variation (±5% on LV systems)
- Tap changer positions
- Load flow voltage profile
For maximum fault currents (equipment rating): c_max = 1.05 (LV) or 1.10 (MV/HV) For minimum fault currents (protection sensitivity): c_min = 0.95
Key Simplifications
IEC 60909 makes several conservative simplifications:
- All generators are at rated voltage (adjusted by c factor)
- Line capacitances and load currents are neglected
- Transformer tap changers are at main position
- Motors are represented by their subtransient reactance
These simplifications mean IEC 60909 gives slightly conservative (higher) fault currents than detailed simulation — which is the intended purpose for equipment specification.
Run the calculation: Use the Short Circuit Calculator for full IEC 60909 analysis.
Frequently Asked Questions
What standards govern cable sizing calculations?
The primary standards are AS/NZS 3008.1.1:2017 (Australia/NZ), BS 7671:2018 (UK), IEC 60364-5-52 (International), and NEC Article 310 (USA). Each has different assumptions for ambient temperature, installation methods, and derating factors.
Why do different standards give different cable ratings?
Standards differ in reference ambient temperature (AS/NZS uses 40°C, BS 7671 uses 30°C), test conditions, grouping factor calculations, and installation method classifications. A 50mm² XLPE cable can vary by 15% between standards.
Related Articles
- Short Circuit Calculator - Interactive calculator with standards compliance
Try It Yourself
Run the calculations from this article using our free calculators: