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Zs: the whole loopMeasure Zs, or calculate it?

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The question at the end: Measure Zs, or calculate it? The answer is in the notes below.

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Zs is the complete loop — the external part plus your circuit — and it is taken at the end of line, the point of utilisation. A socket, a lighting point, the furthest accessory.

Zs = Ze + (R1 + R2)
Capital R, not little r. Little r belongs to ring final end-to-end readings. The Zs formula takes the capital values.

You can get Zs two ways: measure it directly at the far point, or calculate it by adding your measured R1 + R2 to Ze. Both are legitimate, and the calculation is the one that saves you when you cannot get a reading. Either way, nothing is disconnected — you want the parallel paths.

If you have a Zdb instead of a Ze. Guidance Note 3 · p.85 gives the second form: swap Ze out and use Zdb + (R1 + R2). That is what you use for circuits fed from a sub-main board — a garage, workshop or shed. Highlight both formulas.
supply transformer and the DNO’s earth consumer unit end of line furthest point Zₑ R₁ + R₂ Zₛ = Zₑ + (R₁ + R₂) capital R — little r is ring end-to-end
Measured at the point of utilisation — a socket, a lighting point, the furthest accessory. Either measure it directly, or add your measured R₁ + R₂ to Zₑ.

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