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Ring final circuits: the three stepsWhy can't you stop at step two?

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A ring is tested in three steps, in order, and each step proves something the one before it could not. The drawing for step 2 is at Guidance Note 3 · p.64.

  1. Step 1 — end to end. Measure r1, rn and r2 separately, each conductor end to end, before anything is cross-connected. Both leads on one conductor each time.
  2. Step 2 — line to neutral. Cross-connect the legs and measure at every socket.
  3. Step 3 — line to cpc. Cross-connect the other way and measure again at every socket. This is the reading you record as R1 + R2.
Expected step 2 = (r1 + rn) ÷ 4  ·  expected step 3 = (r1 + r2) ÷ 4

Why divide by four? With the cross-connection made, the ring becomes one loop of resistance r1 + rn (or r1 + r2 at step 3). Wherever you plug in, you are measuring two halves of that loop in parallel: each half is (r1 + rn) ÷ 2, and two equal halves in parallel give half of that again — (r1 + rn) ÷ 4.

Worked example

r1 measured  = 0.3 Ω
rn           = 0.3 Ω   (same csa, same length, same cable)

Expected R1 + Rn = (0.3 + 0.3) ÷ 4
                  = 0.15 Ω
rn should match r1 within 0.05 Ω — line and neutral are the same size and the same length. That figure is in the On-Site Guide · 10.3.2 step 1 · p.127 (“within 0.05 Ω if the conductors are all of the same size”), not in Guidance Note 3 — and the On-Site Guide is not an exam book.
Cross-connecting in practice. Connector blocks, or thin crocodile test leads (about £10 a set). Whatever extra leads you use, they must be nulled like any other.
You do not record R1 + Rn. Step 2 is a check, not a recorded value — it is step 3 that goes on the schedule. If you want it on record, it can go in the remarks.
✋Step 2 alone is not enough to prove polarity. Step 1 only shows the conductors are present. Step 2 shows up a line or neutral swapped with the cpc at an outlet; a line and neutral swapped only shows itself at step 3. You need both.

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