Earth Fault Loop Impedance Calculator (Zs)
By Santaji GadepatilinUpdated October 28, 2026
For an MCB or fuse to disconnect an earth fault quickly, the earth fault loop impedance (Zs) must be low enough to let a large fault current flow. Testing Zs is part of every installation inspection — see earth fault loop impedance (Zs) testing. This earth fault loop impedance calculator gives the fault current and the maximum Zs for common MCBs.
How to use the earth fault loop impedance calculator
Enter the voltage to earth, the external loop impedance Ze (measured at the origin or from the supplier), the circuit’s R1 + R2 (line plus protective conductor resistance), the MCB type and its rating. You get Zs, the prospective fault current, the maximum Zs for instantaneous tripping, and the margin.
Earth fault loop impedance formula
Zs = Ze + (R₁ + R₂) · If = Cmin × U₀ ÷ Zs
Zs,max = Cmin × U₀ ÷ Ia
with Cmin = 0.95 and Ia the instantaneous trip current of the device.
Maximum Zs for MCBs (230 V, 0.4 s)
| Rating | Type B | Type C | Type D |
|---|---|---|---|
| 6 A | 7.28 Ω | 3.64 Ω | 1.82 Ω |
| 10 A | 4.37 Ω | 2.19 Ω | 1.09 Ω |
| 16 A | 2.73 Ω | 1.37 Ω | 0.68 Ω |
| 20 A | 2.19 Ω | 1.09 Ω | 0.55 Ω |
| 25 A | 1.75 Ω | 0.87 Ω | 0.44 Ω |
| 32 A | 1.37 Ω | 0.68 Ω | 0.34 Ω |
| 40 A | 1.09 Ω | 0.55 Ω | 0.27 Ω |
| 50 A | 0.87 Ω | 0.44 Ω | 0.22 Ω |
| 63 A | 0.69 Ω | 0.35 Ω | 0.17 Ω |
For measured values at room temperature, compare against about 80% of these figures.
Reducing Zs
- Use larger line and protective conductors (lower R1 + R2).
- Shorten the circuit or move the distribution board closer.
- Use RCD protection where Zs cannot be reduced enough — in TT systems this is the usual solution.
Measuring R1 + R2
Link line and protective conductor at the distribution board and measure between them at each outlet with a low-resistance ohmmeter. Copper 2.5/1.5 mm² cable has about 19.5 mΩ/m for R1 + R2 at 20 °C.
Frequently asked questions
What is earth fault loop impedance?
The total impedance of the path a fault current takes — transformer winding, line conductor, the fault, the protective conductor and the return path. Zs = Ze + (R1 + R2).
How do I find the maximum Zs for an MCB?
Max Zs = Cmin × U0 ÷ Ia, where Ia is the current that trips the MCB instantly (5 × In for Type B) and Cmin = 0.95. A 32 A Type B gives 1.37 Ω.
Why apply a 0.8 factor when testing?
Tests are done at ambient temperature, but conductors run hotter in service with higher resistance. Measured Zs should be at most about 80% of the tabulated maximum.
What are typical Ze values?
In the UK, typical maximums are 0.8 Ω for TN-S and 0.35 Ω for TN-C-S (PME) supplies.