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Ring Circuit Test Calculator
Validate ring final circuit continuity test readings per BS 7671. Check end-to-end resistances, cross-connected r1+r2, and identify potential faults such as interconnections or spurs.
Line conductor end-to-end at CU
Neutral conductor end-to-end at CU
Earth (CPC) conductor end-to-end at CU
Highest reading from step-3 cross-connected test (midpoint)
Indicative band for the ratio checks (default 5%). The nominal CSA ratio runs ~2% above the true resistance ratio, so 2% is the practical floor.
Correct cold r1+r2 UP to this for Zs (GN3): 70°C PVC, 90°C thermosetting
Safety notice
Electrical work in dwellings can be notifiable under Part P of the Building Regulations. Treat these figures as planning guidance only: circuits must be designed, installed and certified to BS 7671 by a competent person, normally a registered electrician.
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How We Calculate This
This calculator validates ring final circuit test readings by comparing measured values against expected values derived from the end-to-end resistance measurements.
Key Formulas
Expected r1+r2 = (R1 end-to-end + R2 end-to-end) ÷ 4
This is the theoretical r1+r2 reading at the electrical midpoint of the ring when line and CPC are cross-connected at the consumer unit.
For unequal line/CPC cross-sectional areas, the step-3 readings are not constant — they rise to this maximum at the midpoint, so enter the highest reading you measured.
Validation Checks
- Rn/R1 ratio: Should be ~1.0, because line and neutral are always the same CSA in twin-and-earth
- R2/R1 ratio: Should match your cable’s nominal CSA ratio (line CSA ÷ CPC CSA) — 2.5/1.5mm² = 1.67, 4.0/1.5mm² = 2.67, 6.0/2.5mm² = 2.40. Pick your cable from the selector so the right ratio is checked. The true resistance-per-metre ratio (IEC 60228) sits a little lower — 12.10÷7.41 = 1.63 for 2.5/1.5mm² — so keep the tolerance band at 2% or above
- Cross-connected deviation: Measured highest r1+r2 vs (R1+R2)÷4. The ±5% band is an indicative consistency rule of thumb, not a BS 7671 limit; differences under 0.05 Ω are treated as measurement scatter and ignored
- Interconnection detection: Abnormally low end-to-end = suspect
Note: temperature does not affect the consistency checks above — all readings are taken at the same ambient, so any factor cancels. The separately-labelled corrected r1+r2 is the cold-measured value scaled UP to the conductor’s operating temperature for downstream Zs verification (IET GN3: F = 1 + 0.004 × (top− 20), so 70°C PVC ×1.20, 90°C thermosetting ×1.28). Enter the operating temperature of the cable, not the test-day ambient.
Frequently Asked Questions
Stage 1: Measure end-to-end resistance of each conductor (L-L, N-N, E-E) with the ring broken at the consumer unit. Stage 2: Cross-connect line and neutral at the consumer unit, then measure resistance at each socket — readings should be substantially consistent around the ring with the highest reading approximately (R1+Rn)/4. Stage 3: Cross-connect line and CPC, measure at each socket — this gives r1+r2 at each point, which should equal approximately (R1+R2)/4.
For standard 6242Y twin and earth cable (2.5mm² line and neutral, 1.5mm² CPC), line and neutral are the same CSA so Rn/R1 should be approximately 1.0. The ratio R2/R1 (CPC vs line) should be approximately 1.67 (2.5 ÷ 1.5 = 1.67). If Rn is significantly higher than R1, suspect a different cable type, a joint or a fault on the neutral. For 4.0mm²/1.5mm² cable, the R2/R1 ratio would be approximately 2.67.
An interconnection (two cables joined at a point other than the consumer unit or socket) will show as abnormally low end-to-end readings and inconsistent cross-connected readings. If the end-to-end resistance is significantly lower than expected for the cable length, or if cross-connected readings vary widely, suspect an interconnection. A figure-of-eight pattern in the cross-connected readings confirms a healthy ring.
Typical end-to-end values depend on cable length. For 2.5mm² copper at 20°C: resistance is 7.41 mΩ/m. A 50m ring would give R1 = 0.37Ω. For 1.5mm² CPC: 12.10 mΩ/m, so R2 = 0.605Ω for 50m. Cross-connected r1+r2 at the midpoint should be approximately (R1+R2)/4 = (0.37+0.605)/4 = 0.244Ω. Values should be consistent at each socket within ±5%. Use our Zs Calculator to verify the total earth fault loop impedance meets BS 7671 requirements.
Yes. Unfused spurs from a ring final circuit are permitted, but the total number of unfused spurs should not exceed the number of points (socket outlets or items of fixed equipment) connected directly in the ring (IET On-Site Guide, ring final circuit appendix). Each unfused spur may feed only one single or one twin socket outlet, or one item of fixed current-using equipment. A spur shows a higher-than-average cross-connected reading at the spur socket compared with sockets on the ring; the end-to-end readings of the ring itself are unaffected by spurs.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.