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Ze: the external impedance, and what you are not allowed to touchCan you put your leads in the meter tails?

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Ze is everything outside your installation — the supply transformer, the service cable and the earth return path. You are testing the DNO's supply, not your own work. Guidance Note 3 · p.84.

✋Ze is a LIVE test. You disconnect the earthing conductor, but you are measuring across the incoming (supply) terminals of the main switch — usually the top on a single-phase consumer unit; check the board in front of you, on the incoming supply. This one catches people out.

Which raises the question of where you are allowed to put your leads:

PointMay you touch it?
DNO cutout / service headNo — illegal
The kWh meter and its tailsNo — the DNO's
The electricity company's isolatorYou may lock off there, but not disconnect inside it
Incoming terminals of your main switch (usually the top)Yes — the closest legal point to the incoming tails
Incoming terminals of a separate isolator, if fittedYes — you can take the measurement there instead
No parallel paths for Ze. You disconnect the earthing conductor at the MET. In principle you could instead remove every cpc in the board — nobody does, because one conductor is quicker than twenty.
✋While that earthing conductor is out, the installation is not protected. Metal casings and extraneous conductive parts — gas and water pipes — have lost their connection to earth. If a fault developed on one while you were brushing past it, you would be in serious trouble. Reconnect it the moment the test is done, before you move on to anything else.

Say the reading comes back at Ze = 1.09 Ω on a TN-C-S supply, which is very high for that system type — which is exactly the point of measuring it rather than assuming it.

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