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Method 2 (R2) in practice: bonding conductorsWhy disconnect bonding one at a time?
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Done at the main earthing terminal, and there's one thing you must do first.
✋Disconnect the bonding conductor from the MET — and only one at a time. Leave them connected and you create parallel paths: if there's supplementary bonding between two pipes, current can run down one bonding conductor, across the supplementary bond and back up the other. You'd be measuring both paths at once, and the reading would be meaningless.
If the bonding conductor runs into the earth block inside the consumer unit rather than a separate MET, you can disconnect it at the consumer unit end instead.
- Null your leads. Set the instrument to continuity, put the two leads together and zero them. If the instrument won't null, note what the leads read and subtract it later — if the leads read 1.50 Ω and the test reads 1.51 Ω, the conductor is 0.01 Ω.
- Clip onto the disconnected bonding conductor — on clean copper. Clipping is easier than probing at that end.
- Probe the bonding clamp — the screw or a clean piece of the clamp.
- Then probe the pipe itself, on clean copper. This proves the clamp is actually making a connection to the pipe, not just sitting on it.
- Put the bonding conductor back. Never leave it hanging.
0.00 Ω on the display? Good news — so little resistance the meter can't read it. But you can't write 0.00 on the certificate. Record 0.01 Ω.
↔A wander lead is the long lead in the Guidance Note 3 figure on p.62, and it solves the obvious problem: your MET and your pipe may be a long way apart. It lets you do an end-to-end test over roughly 50 to 100 metres. And yes — you still null it before you use it.
✋An open circuit on the pipe usually means you're on paint, corrosion or dirt rather than clean copper. Move to a clean spot and test again before you start writing up a fault.
GN3 · Section 2 · test method 2 · the wander lead figure · p.62 ★
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