Calculate the prospective fault current Ik from the measured loop impedance Zs and verify the disconnection condition per IEC 60364-4-41 for MCBs and RCDs.
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Calculate the fault current from loop impedance
Every initial verification includes measuring the earth fault loop impedance Zs. The prospective fault current follows from Ik = c × U₀ / Zs, where c = 0.95 accounts for the permitted downward voltage tolerance. This calculator evaluates your measurement and instantly checks the automatic disconnection condition per IEC 60364-4-41.
Disconnection condition
The fault current must trip the protective device within the required time (0.4 s for final circuits up to 63 A). For MCBs per EN 60898-1 the upper magnetic trip limits are B = 5 × In, C = 10 × In, D = 20 × In. A B16 breaker therefore needs at least 80 A of fault current, a C16 already 160 A.
Worked example
Measured: Zs = 0.45 Ω on a socket circuit protected by a B16. Ik = 0.95 × 230 V / 0.45 Ω = 486 A ≥ 80 A – condition met with a wide margin. With a C16 at the end of a long cable and Zs = 1.6 Ω, Ik = 137 A < 160 A – the magnetic trip is not guaranteed. Remedies: larger cross-section, type B characteristic, or an RCD.
Maximum permissible Zs
The calculator also outputs Zs,max = c × U₀ / Ia for direct comparison with your meter reading: 2.73 Ω for a B16, only 1.37 Ω for a C16 (at c = 0.95, 230 V). For gG fuses the trip currents depend on the manufacturer's time-current curve.
