The most important protection question of any electrical installation: does the breaker trip fast enough in the event of a fault? The Short-Circuit Current Calculator (Loop Impedance) answers it from the measured loop impedance.
The formula
Ik = c × U0 ÷ Zs – with voltage factor c = 0.95, nominal voltage U0 = 230 V and the measured loop impedance Zs. The calculated short-circuit current must exceed the tripping current Ia of the protective device: 5 times the rated current for a type B MCB, 10 times for type C, 20 times for type D.
Step by step
- Measure loop impedance: with the installation tester at the most distant socket of the circuit (Zs in Ω)
- Select protective device: characteristic (B/C/D) and rated current of the MCB, alternatively an RCD
- Check the result: the calculator shows Ik, the required Ia and whether the disconnection condition is met
Worked example
Measured: Zs = 0.45 Ω on a B16 breaker. Ik = 0.95 × 230 ÷ 0.45 = 486 A. Required: 5 × 16 = 80 A – condition met by a wide margin. This breaker would only become critical above roughly Zs > 2.73 Ω.
Practical tips
Long cable runs and sub-distribution boards push Zs up – in extensive installations always measure at the least favourable point. If a type C breaker fails the condition, switching to characteristic B or adding an RCD for fault protection often helps. None of this replaces the standard-compliant test by a qualified electrician.
