Ring final R1+R2 predictor
Work out what your end-to-end readings and cross-connected R1+R2 should be before you pick up the meter, so you know straight away when something is wrong.
Expected readings
- End-to-end line r1
- 0.30 Ω
- End-to-end neutral rn
- 0.30 Ω
- End-to-end CPC r2
- 0.48 Ω
- R1+R2 at any socket
- 0.20 Ω
r1 and rn are equal here because the line and neutral conductors of a ring final are the same size. On a correctly cross-connected ring, every socket on the ring should read within about 0.05 Ω of the R1+R2 figure — a socket reading noticeably higher is usually spurred, and one reading close to the full end-to-end value points to a broken ring.
How this is worked out
Each conductor's resistance is its length multiplied by the resistance per metre for its cross-sectional area, taken from the IET On-Site Guide table of copper conductor values at 20 °C. Because it is a ring, the full loop length is used for the end-to-end reading.
R1+R2 = (r1 + r2) ÷ 4
Once line and CPC are cross-connected at the board, a socket sees two parallel paths in each direction around the ring, which puts a quarter of the combined end-to-end resistance in circuit — hence the divide by four.
Copper resistance rises by roughly 0.4 % per degree, so the ambient temperature figure applies a correction of 1 + 0.004 × (t − 20) to the tabulated 20 °C values. On a cold day in an unheated loft this is worth having.
Predicted values assume plain copper conductors and an accurate stated length. They are a sanity check, not a substitute for testing. Always verify against BS 7671 and the IET On-Site Guide.
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