Network Fault Level: Converting Fault Current, MVA and Impedance

How to convert network fault level between fault current, apparent power and external impedance for three-phase AC, single-phase AC and DC systems.

Updated May 27, 2026

The network fault level, sometimes called the external fault level, is the fault contribution available at the source side of a cable from all upstream network elements. It may be specified as apparent power, impedance, or fault current with a power factor.

These quantities are different ways of describing the same upstream source strength. Converting between them is useful when entering source data for fault current calculations, IEC 60909 calculations or earth fault loop impedance checks.

Three-phase AC system

Quantity required	Using apparent power	Using impedance
Fault current	$I_{k} = \frac{VA}{\sqrt{3} U_{n}}$	$I_{k} = \frac{U_{n}}{\sqrt{3} Z_{e}}$
Apparent power	$VA = \sqrt{3} I_{k} U_{n}$	$VA = \frac{{U_{n}}^{2}}{Z_{e}}$
External impedance	$Z_{e} = \frac{U_{n}}{\sqrt{3} I_{k}}$	$Z_{e} = \frac{{U_{n}}^{2}}{VA}$

Single-phase AC system

Quantity required	Using line-line voltage	Using line-neutral voltage	Using impedance
Fault current	$I_{k} = \frac{\sqrt{3} VA}{U_{n}}$	$I_{k} = \frac{VA}{U_{LN}}$	$I_{k} = \frac{U_{n}}{\sqrt{3} Z_{e}}$
Apparent power	$VA = \frac{I_{k} U_{n}}{\sqrt{3}}$	$VA = I_{k} U_{LN}$	$VA = \frac{{U_{n}}^{2}}{3 Z_{e}}$
External impedance	$Z_{e} = \frac{U_{n}}{\sqrt{3} I_{k}}$	$Z_{e} = \frac{U_{LN}}{I_{k}}$	$Z_{e} = \frac{{U_{n}}^{2}}{3 VA}$

DC system

Quantity required	Using apparent power or current	Using impedance
Fault current	$I_{k} = \frac{VA}{U_{n}}$	$I_{k} = \frac{U_{n}}{Z_{e}}$
Apparent power	$VA = I_{k} U_{n}$	$VA = \frac{{U_{n}}^{2}}{Z_{e}}$
External impedance	$Z_{e} = \frac{U_{n}}{I_{k}}$	$Z_{e} = \frac{{U_{n}}^{2}}{VA}$

For AC systems, current, voltage and impedance are phasor quantities. Using only absolute values can introduce error, particularly where the source fault power factor is important. The most robust approach is to carry out the calculations in complex form so resistance and reactance are both represented.

Symbols

I_k	Network or external fault current, A
VA	Network or external fault apparent power, VA or MVA. Multiply MVA by 10⁶ to convert to VA.
pf	Network or external fault power factor
U_LN	Nominal line-neutral voltage for a single-phase circuit, V
U_n	Nominal line-line voltage, or absolute voltage for d.c. systems, V
Z_e	Network or external fault impedance, ohm

For transformer-derived source fault data, see Transformer Secondary Fault Level.

Network Fault Level: Converting Fault Current, MVA and Impedance

Three-phase AC system

Single-phase AC system

DC system

Symbols

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