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Ring final circuits, step 1: end to endWhat should r1 and rn agree within?

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A ring final circuit gets its own three-step continuity procedure, and it takes longer than everything else on the board put together. Step 1 is three end-to-end tests.

TestBetween
r1Line to line — the two legs of the ring
rnNeutral to neutral
r2CPC to CPC

Disconnect the two legs from the protective device, the neutral bar and the earth bar, and clip onto each pair — far quicker than probing. Your leads are already nulled from the last test, so long as you haven't changed function.

≡This is exactly an R2 test, done three times — two leads, one conductor, both ends. That's why "end to end" and "R2" are used interchangeably.

What the readings should look like

±Line and neutral are the same size and the same length, so r1 and rn should agree within 0.05 Ω of each other. That figure is in the On-Site Guide, 10.3.2 step 1 (p.127). Guidance Note 3's ring test (2.6.6, p.63) does not give it, and the On-Site Guide is not one of the exam books, so learn the number.

And r2, from the 1.67 ratio

2.5 mm² / 1.5 mm² twin & earth → ratio 1.67

r1 = 0.01 Ω
Expected r2 = 0.01 × 1.67 = 0.017 Ω
A real fault, worked through. Say a ring gives r1 = 0.01 Ω but rn = 0.52 Ω — nowhere near the 0.05 Ω tolerance. Not an open circuit; there's still continuity. But a loose connection or a high-resistance joint somewhere on that neutral leg. This is exactly what step 1 exists to catch, and it threw every calculation that followed.
And say r2 measures 0.02 Ω against an expected 0.017 Ω. That's as close as the meter can show — the CPC is sound. The neutral is the only leg with a problem.

GN3 · 2.6.6 · p.63 · ring final circuit continuity, step 1 ★ OSG · 10.3.2 · p.127 · ring final step 1 · the 0.05 Ω tolerance

consumer unit legs disconnected L → L = r₁ 0.01 Ω N → N = rₙ 0.52 Ω ⚠ 52× r₁ cpc → cpc = r₂ 0.02 Ω expected 0.017 Ω L and N are the same size and length, so r₁ and rₙ must agree within 0.05 Ω. cpc is 1.5 mm² against 2.5 mm², so r₂ ≈ 2.5 ÷ 1.5 = 1.67 × r₁
The worked example’s readings. rₙ of 0.52 Ω against an r₁ of 0.01 Ω is not a tolerance problem — it is a genuine fault on the neutral leg.

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