Maximum Zs calculator
Pick the breaker type and rating to get its maximum earth fault loop impedance. Add your measured reading to check it.
Your test reading, taken at normal room temperature.
Maximum Zs (Table 41.3)
Values are for circuit-breakers to BS EN 60898 and RCBOs to BS EN 61009 at 0.4 s, Uo 230 V and Cmin 0.95, as in BS 7671 Table 41.3. Check against your copy of BS 7671 or the On-Site Guide before relying on a result.
How it's worked out
Zs = (Uo × Cmin) ÷ Ia
Uo is the nominal voltage to earth (230 V) and Cmin is 0.95, which allows for the supply voltage being lower than nominal. Ia is the current that makes the breaker trip instantly: 5 × In for Type B, 10 × In for Type C and 20 × In for Type D.
Worked example
A 32 A Type B breaker: Ia = 5 × 32 = 160 A, so Zs = (230 × 0.95) ÷ 160 = 1.37 Ω.
Why 80 %?
Table values apply with conductors at their full operating temperature. A cold circuit measured during testing has lower resistance, so a common rule of thumb is that a reading at room temperature should be no more than 80 % of the table value: 1.37 × 0.8 = 1.10 Ω for that 32 A Type B.
Circuits protected by an RCD can also meet the requirement through the RCD's own limits. Check BS 7671 for those cases.