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Finding reverse polarity on a ringWhich step finds a reversed socket?

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The most useful thing here, because it explains why there are three steps and not two. First, the definition, stated plainly:

A line conductor in a neutral terminal is reverse polarity

Now take one socket on the ring with its browns and blues swapped at its terminals — both blues in the line terminal, both browns in the neutral, the cpc correct — and run the tests:

Test	What happens	Detected?
Step 2 · line to neutral	That socket reads the same as every other — line and neutral are cross-connected together, so swapping them at the socket changes nothing	No
Step 3 · line to cpc	Open circuit at that socket — its line pin is sitting on the neutral, which is not connected in this test	Yes

Why step 3 catches it. Your plug-in adapter puts one lead on the line pin and one on the cpc pin. At the reversed socket the line pin is connected to the neutral conductors, and in step 3 the neutral is not cross-connected to anything — so that one socket reads open circuit while the others read normally.

A whole cable swapped is different. If one length of cable between two sockets has its brown and blue swapped, the line conductor is no longer continuous end to end — it shows up as an open circuit at step 1.

✋The one step 2 does catch. If the cpc has been put into a line or neutral terminal, step 2 gives you no reading at that socket — because the cpc is blocking the loop you are trying to measure. It is only a straight brown-and-blue swap, with the cpc correctly in the middle, that step 2 misses.

When you find one. A reverse polarity fault found while testing is identified and recorded — finding and recording it is the test's job.

Try it on a practice board. Worth trying: wire three sockets on a ring, deliberately reverse the browns and blues at one socket, then run steps 1, 2 and 3 and watch what each one does.

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