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Ring final practical, step 1: end to endLine and neutral read 0.06 and 0.15. Problem?

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Three separate readings, each one conductor from one end of the ring to the other. Clip onto each pair — far quicker and steadier than probing.

r₁ line to line 2.5 mm² 0.06 Ω rₙ neutral to neutral same size, same length 0.07 Ω r₂ CPC to CPC 1.5 mm² · smaller, so higher ≈ 1.67 × r₁
Example readings from a very short test ring. Line and neutral are the same size and length, so r1 and rn should agree within 0.05 Ω. The CPC is smaller, so r2 should be bigger.

Know what to expect before you press test

  • r1 and rn should agree within 0.05 Ω of each other. A neutral reading well above the line — 0.15 Ω against 0.06, say — points to a loose termination somewhere on the neutral, or a screw clamped on insulation instead of copper.
  • r2 comes from the ratio. Twin and earth is the only common cable with a smaller CPC: 2.5 mm² line with a 1.5 mm² CPC. 2.5 ÷ 1.5 = 1.67, and because the CPC is smaller its resistance is 1.67 times bigger.

The expected r2

r₁ = 0.06 Ω   (rₙ 0.07 Ω agrees with it)
r₂ ≈ 0.06 × 1.67 = 0.10 Ω
✋1.67 times bigger, not smaller. Less copper, more resistance. It’s an easy one to say backwards.
On a very short ring, the ratio won’t hold. A metre or two of cable is a few hundredths of an ohm, and connections dominate. On a real circuit the numbers line up; what matters on a short one is that nothing is wildly high.

The three readings go in the r1, rn and r2 columns of the schedule of test results — columns 18, 19 and 20.

Twin and earthCPC
1 mm²1 mm²
1.5 mm²1 mm²
2.5 mm²1.5 mm²
4 mm²1.5 mm²
6 mm²2.5 mm²
10 mm²4 mm²
16 mm²6 mm²

Guidance Note 3 · 2.6.6 step 1 · p.63 On-Site Guide · 10.3.2 step 1 · p.127 · the 0.05 Ω between r₁ and rₙ

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