This Question of the Day, concerning a ring main wired in mineral insulated cable, is answered by the NICEIC (Theme – Cables, Wiring & Wiring Accessories):
Question: Please could you inform me as to how I should carry out an R1 + R2 reading on a ring main wired in mineral insulated cable?
It is self explanatory as to doing R1 and Rn, but there is no way of doing R2, and hence R1 + R2 either.
Answer: R1 + R2 readings can only be reliably proven on an all insulated installation (PVC/PVC wiring). Any installation having metallic sheaths (SWA and MI) and metallic construction (conduit / trunking) can introduce parallel paths.
Therefore, a visual inspection for soundness of the circuit should be carried out, and then either Test Method 1 or 2 carried out thereafter.
Please refer to the On Site Guide (page 78) for further guidance.
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