Installation design
Prospective fault current estimate
- where
- fault current (A)
- voltage (V)
- complete fault-loop impedance (Ω)
Explain this equation
Gives a quick estimate of prospective fault current from the voltage and the impedance of the fault loop. Use it to sense-check a measured PFC, or to see how much current is available to operate a device.
U is the voltage driving the fault, such as 230 V to earth for a line-earth fault. Z_loop is the impedance of the complete fault path: supply transformer, supply cables, the circuit's line conductor and the return path. I_f is the fault current in amperes.
It is Ohm's law applied to the whole loop. In a short circuit the load is bypassed, so only the loop impedance limits the current.
230 V over 0.2 Ω gives 1150 A. Close to a large transformer Z_loop can be well under 0.1 Ω, giving several kA; at the end of a long circuit it may exceed 1 Ω, giving only a couple of hundred amps. Halve the impedance and the current doubles.
Watch out: use the whole loop, not just the faulty section. A line-neutral fault has a different loop from a line-earth fault. This is a simple estimate, not a breaking-capacity assessment.
Fault current is limited by the entire supply-and-return loop. In this simplified model divide the fault-driving voltage by that loop impedance.
For a line-to-earth fault use voltage to earth and the earth loop. A line-neutral fault uses its own return path; its impedance need not equal the earth loop.
Enter your values to see the result.
Common mistake
Using only the resistance of the damaged section as the complete loop.
Limits
Simplified steady RMS estimate, not a breaking-capacity assessment. Maximum and minimum fault studies need appropriate voltage factors, sources and impedance data.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.