Transformer Fault Current Calculator — IEC 60909

Transformer Data

Transformer Type Affects zero-sequence impedance ratio

Winding Connection Delta primary blocks zero-sequence; earthed star secondary provides return path

Rating (kVA) Transformer nameplate rating

Secondary Voltage (V) Line-to-line voltage rating

Impedance Z (%) Typically 4–6% (distribution), 8–12% (power)

X/R Ratio Typical: 3–8 (distribution), 10–30 (power)

Supply Source (Optional)

Supply Fault Level Infinite bus = no source impedance (conservative)

Note: This calculator assumes the transformer is the sole impedance source (or includes supply impedance if specified). Results are prospective fault currents — actual fault currents may be lower due to cable impedance, arc resistance, and other factors. For protection grading, use detailed network analysis.

Fault Currents (Symmetrical RMS)

3-Phase fault I_3p—

Phase-to-phase I_pp—

Phase-to-neutral I_pn—

Phase-to-earth I_pe—

Peak & Asymmetrical

Peak current I_p—

Peak factor κ—

RMS asymmetrical (1st half-cycle)—

Transformer Data

Rated current I_r—

Base impedance Z_base—

Z₁ (positive seq, p.u.)—

Z₂ (negative seq, p.u.)—

Z₀ (zero seq, p.u.)—

Z₀/Z₁ ratio—

Calculation Methodology

Step	Parameter	Formula / Source	Value

Zero-Sequence Impedance by Winding Configuration

Winding	Type	Z₀/Z₁	Zero-Seq. Path	Earth Fault
Dyn (Delta-Star-Earth)	Core	0.85	Delta winding provides closed path; flux in core limbs	Supported
Dyn (Delta-Star-Earth)	Shell	0.85	Same as core — delta provides return path	Supported
YNyn (Star-Star-Earth)	Core	0.40 – 1.00	Zero-seq flux via core yoke and tank; highly variable	Supported (with caution)
YNyn (Star-Star-Earth)	Shell	5.0 – 10.0	Very high Z₀; shell geometry blocks zero-seq flux	Limited
Dd (Delta-Delta)	Any	∞	No neutral point; no zero-sequence current to earth	Not possible
Dy (Delta-Star unearthed)	Any	∞	Star not earthed; no zero-sequence return path	Not possible
Yyn (Star-Star, sec. earthed)	Core	0.40 – 1.00	Similar to YNyn core; depends on tank/core construction	Supported (limited)

Reference: IEC 60076-8, IEC 60909-0:2016 Clause 6.3. The jcalc.net calculator uses Z₀ = 0.85 × Z₁ for core-type Dyn connections.