Earth fault loop impedance and earth electrode resistance testing
Loop impedance
Ze is measured at the origin with the supply isolated and the main earthing conductor disconnected from the MET, so that parallel paths do not mask a poor earth. Zs is measured live at the furthest point of each circuit, or calculated as Ze + R1+R2 (parallel paths usually make the measured value lower). Zdb is measured at distribution boards where they exist. Prospective fault current is measured at the origin and at each board, and the highest value recorded.
Comparing with the standard
Compare Zs with the maximum in BS 7671 Table 41.3 (or the RCD tables for TT). Tabulated values assume conductors at their operating temperature; a cold measurement should not exceed 80% of the tabulated maximum. For a 32 A Type B MCB the tabulated maximum is 1.44 Ω, so the measured value should be no more than about 1.15 Ω.
Earth electrodes
For a TT system, RA × IΔn must not exceed 50 V. A 30 mA RCD therefore permits up to 1667 Ω, though a practical target is 100 Ω or less. Measure electrode resistance with a fall-of-potential tester or a clamp, with the electrode disconnected.
Trap: measuring Ze with the main earthing conductor still connected.
Trap: using a standard loop tester on an RCD-protected circuit and tripping it - use the no-trip setting.
Rule of thumb: Zs measured ≤ 0.8 × Zs tabulated.