Ring final circuit test calculator
Expected readings for the ring final cross-connection test, from the end-to-end resistances r1, rn and r2.
Every result has a reason.
Enter your values and calculate to see the result, formula and working here.
Reading at position a = [a·r1 + (1 − a)·r2] × [(1 − a)·r1 + a·r2] ÷ (r1 + r2); highest at the midpoint = (r1 + r2) ÷ 4A ring of uniform conductors with the ends cross-connected as in the standard test, and readings taken at points on the ring itself. Spurs add their own resistance. Test lead resistance is excluded. This does not assess continuity, polarity or compliance.
What the numbers mean.
Cross-connecting the ends makes every socket a meeting point of two parallel paths. When r1 = rn, every point on the ring reads (r1 + rn) ÷ 4; line and neutral of the same size normally measure close to equal. When the conductors differ, as with 2.5 mm² line and 1.5 mm² CPC, the reading peaks at (r1 + r2) ÷ 4 at the midpoint and is slightly lower towards the ends. The highest line–CPC reading is the circuit's R1 + R2.
Formula reference: IET Wiring Matters · Resistance readings for step 3 of the ring final circuit test ↗A worked example
r1 = 0.52 Ω and r2 = 0.86 Ω: the highest line–CPC reading, at the midpoint, is (0.52 + 0.86) ÷ 4 = 0.345 Ω. Next to the ends it falls to 0.52 × 0.86 ÷ 1.38 ≈ 0.32405797 Ω.
Validation, precision and limitations