Earth Fault Loop Impedance Calculator
Check that an MCB will disconnect an earth fault fast enough. The calculator adds the external loop impedance to the cable line and protective conductor resistance and compares the fault current with the breaker’s magnetic trip current.
- R1 + R2 = ρ70 × L × (1/A1 + 1/A2) = 0.8802 Ω
- Zs = Ze + R1 + R2 = 0.35 + 0.8802 = 1.23 Ω
- Zs max = 0.95 × U0 ÷ Ia = 0.95 × 230 ÷ 100 = 2.185 Ω
Earth Fault Loop Formulas
If = U0 ÷ Zs
Zsmax = Cmin × U0 ÷ Ia (Cmin = 0.95)
R1 and R2 are the resistances of the line and protective conductors, taken here at 70 °C operating temperature. Ia is the current that operates the breaker instantly (5, 10 or 20 times In for type B, C, D).
Worked example
A B20 circuit, Ze 0.35 Ω, 40 m of 2.5/1.5 mm² copper: R1 + R2 = 0.88 Ω (at 70 °C), Zs = 1.23 Ω, below the 2.19 Ω limit, so the breaker trips within 0.4 s.
Frequently Asked Questions
What is a typical Ze value?
Common design maxima are 0.35 Ω for TN-C-S (PME) and 0.8 Ω for TN-S supplies. Measure on site where possible.
Why is the cable resistance taken at 70 °C?
Conductors heat up under load. Using the hot resistance gives the worst case, highest Zs.
What if Zs is too high?
Increase the protective conductor size, reduce the cable length or breaker rating, or add an RCD for earth fault protection.
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