Guidance only — always verify against current standards and manufacturer instructions.
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This tool calculates the earth fault loop impedance of a circuit from two figures every UK electrician records anyway: the external loop impedance Ze and the circuit conductor resistance R1+R2. It is the standard method for determining Zs by calculation, used at design stage and as a cross-check on live loop tests.
The calculation itself is simple addition — the total earth fault loop is the path outside the installation (Ze) plus the line conductor and CPC of the circuit (R1+R2). Enter both values in ohms and the tool returns Zs, which you can then compare against the maximum permitted value for the protective device.
Zs = Ze + (R1 + R2)
Zs is the total earth fault loop impedance in ohms. Ze is the external loop impedance — the part of the fault path outside the installation, measured at the origin with the earthing conductor disconnected or obtained from the distributor. R1 is the resistance of the circuit line conductor and R2 the resistance of the circuit protective conductor; they are normally measured together as R1+R2 during dead testing. Adding the two gives the impedance a line-to-earth fault at the end of the circuit would see.
Type in the external earth fault loop impedance in ohms, either measured at the origin or the distributor's declared maximum (typically used at design stage).
Enter the combined line and CPC resistance for the circuit in ohms, from your continuity test or from the R1+R2 calculator if you are working from cable size and length.
The tool returns Zs. Check it against the maximum Zs for the protective device — the Max Zs calculator gives that figure for MCBs and RCBOs, including the 80% rule for measured values.
Determining Zs as Ze + (R1+R2) is the method described in BS 7671 and the On-Site Guide, and it is often preferable to a live loop test on RCD-protected circuits where the test itself is awkward. It also lets you verify a design before anything is installed, using the distributor's declared Ze and calculated conductor resistances.
Be consistent about temperature. If your R1+R2 is a cold measured value, compare the resulting Zs against an 80%-adjusted device limit, or correct R1+R2 to operating temperature first — mixing cold measurements with hot limits gives a false margin.
Watch for parallel earth paths when measuring. Bonded services can make a measured Zs read lower than the true circuit loop, which is another reason the calculated method is a useful cross-check. Guidance only — verify the final figures against BS 7671 for the actual installation.
Ze is the earth fault loop impedance external to the installation, measured at the origin. Zs is the total loop impedance for a specific circuit, which adds the circuit's own line and protective conductor resistances (R1+R2) to Ze. Every circuit has its own Zs; the installation has one Ze.
Link the line conductor and CPC together at the distribution board, then measure the resistance between line and earth at each point on the circuit with a low-resistance ohmmeter. The highest reading, normally at the furthest point, is the circuit's R1+R2 and is recorded on the schedule of test results.
Yes — BS 7671 permits Zs to be determined by adding measured Ze and R1+R2, and it is common practice on RCD-protected circuits. You still need confidence in each input, so Ze should be measured or taken from the distributor and R1+R2 taken from your continuity tests.
Distributors commonly declare maximum values of around 0.35Ω for TN-C-S (PME) supplies and 0.8Ω for TN-S. Measured values are usually lower. TT systems rely on an earth electrode, so Ze is far higher and disconnection depends on RCDs. Always use the actual measured or declared value for your installation.
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