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What Ze provesYour circuits reach the MET. Does the MET reach the source?
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Automatic disconnection of supply needs a path for fault current all the way back to the star point of the supply transformer. The dead tests proved every circuit reaches the MET. Ze — the external earth fault loop impedance — proves the MET reaches the source.
It’s impedance, not resistance, because it’s measured on the live AC supply: the transformer windings and the cable add reactance to the resistance. Still measured in ohms.
Most UK installations are TN (TN-S or TN-C-S), which this covers.
Guidance Note 3 · 2.6.14 · protection by automatic disconnection of supply
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