Dead testing: continuity, ring final circuits, polarity and insulation resistance
Dead testing
Dead tests are carried out with the supply isolated, in a set order, because later tests depend on earlier ones.
Continuity of protective conductors. Either the R1+R2 method (temporary link between line and CPC at the board, measure at each point) or a long wander lead. Readings should be consistent with expected conductor resistance and circuit length.
Ring final circuit continuity. Measure end-to-end r1, rn and r2 at the board. Cross-connect the line of one leg to the neutral of the other and measure at each socket; readings should be roughly equal, around (r1+rn)/4. Repeat with line to CPC, expecting about (r1+r2)/4. A reading much higher than its neighbours indicates a spur; no reading indicates a break.
Polarity. Confirm line to the centre contact of ES lampholders, line to the protective device and not the neutral, correct socket polarity, and that all single-pole switches are in the line.
Insulation resistance. With the main switch off, lamps removed and sensitive equipment and SPDs disconnected, test at 500 V DC between line/neutral, line/earth and neutral/earth. The minimum for a 500 V test is 1 M ohm; a healthy installation typically reads hundreds of megohms.
Traps. Testing through connected equipment, RCDs, neon indicators or surge protection gives misleadingly low readings. Never infer polarity from cable colours alone. R1+R2 is not the same as end-to-end r1+r2.
Rule of thumb. Any insulation resistance below 1 M ohm is a failure and must be investigated, not recorded and forgotten.