Fault level
Fault current at the end of a cable or overhead line
- where
- source impedance, split into RQ + jXQ using the sending-end X/R (Ω)
- IEC 60909 voltage factor
- nominal voltage (kV)
- sending-end fault level (MVA)
- cable or line impedance (Ω, complex)
- resistance and reactance per km (Ω/km)
- route length (km)
- number of identical parallel circuits
- fault current at the far end (kA)
Explain this equation
Finds the three-phase fault current at the far end of an HV cable or overhead line fed from a busbar with a known fault level. Use it for remote-end protection checks and for the duty on downstream switchgear.
ZQ is the source impedance from the sending-end fault level, split into R and X using the sending-end X/R. ZL is the circuit impedance: per-kilometre resistance r and reactance x times the route length L, divided by n when several identical circuits run in parallel. The j marks the reactive part, so both are complex numbers.
The source and circuit impedances are in series, so they are added as complex numbers and the magnitude of the sum sets the current. This matters because a cable is relatively resistive while the source is mostly reactive; adding magnitudes would overstate the impedance.
Sense check: 2 km of 11 kV cable at about 0.32 Ω cuts a 250 MVA (13 kA) source to roughly 8.8 kA. Doubling the length roughly doubles ZL, but the current falls by less because ZQ stays the same.
Watch out: use conductor resistance at 20 °C for maximum faults and at end-of-fault temperature for minimum faults.
Adds a cable or overhead line to a known source fault level, using complex impedances, to find the fault current at the far end.
Enter your values to see the result.
Good to know
Use manufacturer positive-sequence r and x. For maximum faults IEC 60909 uses resistance at 20 °C; for minimum faults use the resistance at the end-of-fault temperature. Typical 11 kV 185 mm² Al XLPE triplex: r ≈ 0.164 Ω/km, x ≈ 0.1 Ω/km. Overhead lines have x ≈ 0.3–0.4 Ω/km.
Common mistake
Adding |ZQ| and |ZL| arithmetically. Their angles differ, so they must be added as complex numbers; scalar addition understates the fault current.
Limits
Three-phase balanced fault, radial feed, no infeed from the far end, no shunt capacitance (fine for distribution lengths). Sending-end fault level is assumed to include c in the IEC way.
Sources
For competent high-voltage engineers. Results are calculations from the published methods shown and depend entirely on the data you enter. They are not a substitute for a system study, manufacturer data, the applicable standards and Engineering Recommendations, or your organisation’s safety rules. Never use a result to justify working on or near live HV equipment.