The ring final that looks fine until you read the spur
During a periodic inspection of a 1990s three-bedroom house you test the ring final circuit supplying the kitchen and utility room. It is protected by a 32 A Type B MCB and wired in 2.5 mm² T&E with a 1.5 mm² CPC. End-to-end continuity gives r1 = 0.40 Ω, rn = 0.40 Ω and r2 = 0.68 Ω. With the conductors cross-connected, socket readings around the ring range from 0.25 Ω to 0.29 Ω. The conservatory socket is fed from a single 3 m spur taken from the back of a kitchen socket and gives R1+R2 = 0.62 Ω. There is no RCD on this circuit. The client mentions the conservatory socket 'sometimes works'.
1. Do the three end-to-end readings and the socket readings indicate a sound ring? Show your checks.
Read the worked answer
Three cross-checks, and all three have to agree:
- r1 should equal rn. 0.40 Ω against 0.40 Ω - consistent.
- r2/r1 should match the resistance-per-metre ratio of the conductors: 12.1/7.41 = 1.63 for a 1.5 mm² CPC against 2.5 mm² line. Measured 0.68/0.40 = 1.70 - within the tolerance you expect from joints, temperature and meter resolution. A ratio near 2.4 would suggest a 1.0 mm² CPC or a partly broken CPC.
- Ring length and the socket values. r1/7.41 mΩ per metre = 0.40/0.00741 ≈ 54 m - long, but plausible for this house. Predicted value at the sockets is (r1+r2)/4 = (0.40+0.68)/4 = 0.27 Ω, and the measured range is 0.25–0.29 Ω, which is exactly the spread you get depending on where around the ring you test.
Conclusion: line, neutral and CPC are continuous around both legs and the ring is sound. A broken ring would show r2 high or open and socket readings well below (r1+r2)/4.
2. Interpret the 0.62 Ω at the conservatory spur and say what you would do about it.
Read the worked answer
Expected value: the ring value at the point of connection (about 0.27 Ω) plus the spur's own R1+R2: 3 m × (7.41 + 12.1) mΩ/m ≈ 0.06 Ω, so roughly 0.33 Ω.
Measured 0.62 Ω is about 0.29 Ω high. That is not a long spur - 0.29 Ω is equivalent to roughly 15 m of 2.5/1.5 that does not exist. Together with 'sometimes works' this points to an intermittent high-resistance joint: a loose terminal, a twisted joint, a damaged cable, or a badly made connection at the spur origin.
Action: isolate, inspect the spur end to end, test R1+R2 along it, remake the connection, re-measure and record. A reading you cannot reconcile with the circuit is a symptom, not a result - writing it on the schedule as it stands would hide the defect.
3. Classify (a) the spur defect and (b) the absence of 30 mA RCD protection on the socket circuit of this existing installation.
Read the worked answer
(a) Spur defect: with no evidence of overheating or arcing, an intermittent high-resistance path is potentially dangerous - C2. If you find burning, arcing or an accessible live conductor at the accessory it becomes C1, and you must isolate that circuit immediately and notify the client in writing.
(b) No 30 mA RCD: the installation met the standard of its day, so this is not a C2 by default - C3 (improvement recommended), with a recommendation to add RCBO protection. It becomes C2 if a socket is liable to supply equipment used outdoors (for example garden tools from a rear socket), because the risk from a lost CPC in contact with earth is materially higher.
Note also what is not a defect: a single unbranched spur from a ring final is a permitted arrangement, so the spur's existence attracts no code - only its measured resistance does.