Maximum Zs calculator
Maximum permitted earth fault loop impedance for circuit breakers to BS EN 60898 and RCBOs to BS EN 61009-1, on a 230 V TN system.
Tabulated maximum
0.68 Ω
The BS 7671 table value, which assumes the conductors are at their normal operating temperature.
Rule of thumb (80%)
0.54 Ω
Compare a value measured at ambient temperature against this, per IET Guidance Note 3.
How this is worked out
For a circuit breaker, the fault current needed to operate it inside the required disconnection time is a fixed multiple of its rating: 5 × In for Type B, 10 × In for Type C and 20 × In for Type D. The maximum loop impedance is then the supply voltage divided by that current.
Zs = (Cmin × U0) ÷ Ia = (0.95 × 230) ÷ (multiplier × In)
Cmin is the 0.95 factor BS 7671:2018 applies to allow for permitted supply voltage variation, giving 218.5 V. Because the operating current of a circuit breaker is set magnetically, the same maximum applies whether the circuit needs to disconnect in 0.4 s or 5 s.
The 80% figure exists because the tabulated values assume conductors sitting at their operating temperature, whereas you measure a cold circuit. Multiplying the tabulated value by 0.8 is the standard on-site comparison.
Fuses
Fuses to BS 88-2, BS 88-3 and BS 3036 do not follow a simple multiplier — their maximum values come from BS 7671 Tables 41.2 and 41.4 and differ between 0.4 s and 5 s. Rather than publish figures we have not had verified, fuse support is being added once our reviewing engineer has signed the tables off. Use the printed tables in the meantime.
This calculator is a decision aid for competent persons. Always verify against the current edition of BS 7671 and the relevant IET Guidance Notes. Certably accepts no liability for work carried out on the basis of these results.
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