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Polarity, and why step 2 can't prove itWhich steps prove polarity on a ring?

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The question at the end: Which step proves polarity on a ring? The answer is in the notes below.

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Worth asking twice — most people say yes both times. The answer is no.

✋Step 2 on its own does not prove polarity. Step 2 finds a line or neutral swapped with the cpc: that socket reads open circuit. But line to neutral is a continuous loop round the ring, so swap the line and neutral at a socket and step 2 still gives a perfectly respectable reading. Step 3 finds that one.

What step 3 adds

The socket outlet adapter has the line, earth and neutral pins in fixed positions, and your test leads plug into it. So on step 3, when you test line to CPC:

⚡If the line conductor isn't in the line terminal — if a neutral has been put there instead — you get an open circuit. That is the swap step 2 misses. But a line and CPC swapped with each other still reads at step 3 — step 2 is the one that finds that.
3On a ring final circuit, polarity gets its tick once steps 2 and 3 are both done. Step 2 finds a line or neutral swapped with the CPC; step 3 finds a neutral in the line terminal. Neither proves it on its own.

GN3 · 2.6.12 · p.77 · polarity ★

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