EICR on a 1985 semi: interpreting ring readings and a sub-1 MΩ insulation result
You are carrying out an EICR on an owner-occupied three-bedroom semi built in 1985. The board is an 8-way unit with a single 30 mA RCD covering all circuits. Ring final continuity gives r1 = 0.40 Ω, rn = 0.40 Ω, r2 = 0.66 Ω. Cross-connected readings at the sockets are 0.26 Ω at six outlets and 0.41 Ω at the two socket outlets in the utility room. On the lighting circuit, with lamps removed and all switches closed, insulation resistance is L-N 120 MΩ, L-E 0.9 MΩ, N-E 0.8 MΩ. The lighting circuit feeds an outside bulkhead light via a spur. There is no heat damage at the board. The client wants the report by Friday and asks whether the low reading can just be recorded as a note.
1. Interpret the ring final results. What are the possible causes and what do you check before you code anything?
Read the worked answer
Step 1 - are the conductors intact?
r1 = rn = 0.40 Ω is what you expect for a healthy ring: 2.5 mm² is 7.41 mΩ/m, so 0.40 Ω corresponds to roughly 54 m of conductor, about a 27 m ring. r2 = 0.66 Ω is close to the expected 1.67 × r1 for a 2.5/1.5 ring, so the CPC is continuous too. A broken ring would normally show r1 at about double the expected value.
Step 2 - the cross-connected readings
Expected (r1 + r2)/4 = (0.40 + 0.66)/4 = 0.265 Ω. Six sockets read 0.26 Ω - on the ring and consistent. The two utility sockets read 0.41 Ω, i.e. 0.145 Ω above the ring value. At 19.5 mΩ/m for 2.5 line plus 1.5 CPC, that corresponds to about 7 m of extra cable.
Possible causes
- A spur off the ring feeding the utility sockets - most likely, given the extra resistance matches a plausible cable length.
- A break in the ring with the two legs joined by a long path - unlikely, because a break usually shows up as a doubling of r1 and as readings that climb steadily round the ring.
- An interconnection with a second ring - would normally give lower, not higher, readings.
What you check on site before coding
Open the utility accessories and count the cables. Two cables at each = ring, so the anomaly is elsewhere. One cable at each, both fed from the same point = a spur. Then ask how many outlets the spur feeds: one double socket on a non-fused spur is permitted; two double sockets on one non-fused spur is not, and that changes the code.
Judgement
Numbers suggest; inspection decides. Coding a spur as a defect without opening the box is how EICRs get challenged.
2. Interpret the insulation resistance results and set out your test sequence to find the cause.
Read the worked answer
What the pattern means
L-N is healthy at 120 MΩ, but L-E (0.9 MΩ) and N-E (0.8 MΩ) are both low and similar. A single line-to-earth fault would normally leave N-E high. Both live conductors sitting at a similar low resistance to the CPC points to one common path bridging line and neutral to earth - typically moisture tracking across a fitting, a crushed cable against a metal box, or water in an outside accessory.
An outside bulkhead spur is the prime suspect: water ingress gives exactly this signature, and it explains why both readings are depressed.
Sequence
- Before touching anything, confirm the reading is repeatable and note ambient conditions - a wet day and a dry day can differ by a factor of ten.
- Re-test at 250 V DC first if there is any connected electronics; 500 V DC otherwise. Table 64 requires at least 1 MΩ, so 0.8 MΩ fails the minimum either way.
- Disconnect the bulkhead spur at its FCU or joint and re-test the main lighting circuit. If it goes above 299 MΩ, the fault is in the spur or the fitting.
- If not, split the circuit at ceiling roses and test each half to bisect the fault.
- Inspect and, if necessary, dismantle the bulkhead. Look for the gasket, cable entry, and any damage where the cable passes through the wall.
What I would not do
Leave it as 'damp, will dry out'. A path of 0.8 MΩ that carries leakage to earth is a deteriorating condition, and drying out proves nothing about the state of the cable or the enclosure.
3. Code the observations, state the overall outcome, and answer the client.
Read the worked answer
Codes
- Ring final: no code if inspection confirms a single double socket on a non-fused spur (permitted). If it is two double sockets on one non-fused spur, that is a potential overload of the spur cable - code C2 if the load makes it credible, C3 if not. Record the verification either way.
- Lighting insulation resistance: once the cause is established as water ingress into the bulkhead, code C2, potentially dangerous, urgent remedial action. Deterioration that could give rise to danger is C2, not C3.
- If the cause cannot be established within the inspection, code FI (further investigation) and say so plainly on the report.
Overall outcome
Unsatisfactory, because a C2 exists. The report records the observed condition, the regulation or guidance reference, and the recommended action. An EICR is a statement of condition at a point in time, not a certificate of compliance you can tailor to a deadline.
Answer to the client
No, it cannot be 'just a note'. A reading below 1 MΩ is a departure from the minimum in Table 64 and has to be investigated and coded. Friday is achievable if the investigation is done today; if the bulkhead cannot be accessed before then, the report still goes out on Friday with the observation recorded and the outcome unsatisfactory - that is the honest version, and it is also the version that protects them.
Judgement
The exam is testing whether you will trade a code for a client's convenience. You will not, and you can explain why in one sentence.