← Back to Tools Directory Field Reference Hub →

Maximum Zs Calculator

Calculate the maximum earth-fault loop impedance Zs_max = U0 / Iafrom first principles. This is a reference computation - always verify the result against the published BS 7671 tables before use.

Results

Maximum Zs -

Zs_max (table, 70°C) -

Zs_max (max measured, 10°C) -

Disconnection time used -

Ia used -

Formula with values substituted -

This is a calculated value. Always verify against the published BS 7671 tables (Table 41.2 for fuses, Table 41.3 for MCBs).

The corrected value applies a 0.8 temperature factor (BS 7671 Table 41.2 values assume conductors at operating temperature). Compare the corrected value against published BS 7671 tables.

Worked example

MCB Type B, 32 A, 0.4 s disconnection.

Ia = 5 × 32 = 160 A

Zs_max (table, 70°C) = 230 / 160 = 1.44 Ω

Zs_max (max measured, 10°C) = 1.44 × 0.8 = 1.15 Ω

Compare against BS 7671 Table 41.3 (corrected value should match table).

⚠️ Educational design aid. Verify all results against the current BS 7671 edition. Not a compliance tool - does not replace professional engineering judgement or BS 7671 table lookups.

Regulation reference: Per BS 7671 Regulation 411.3.2 (disconnection times) and 411.4.4 (Zs values). The formulaZs = U0 / Ia is from first principles. The 5×/10×/20× multiplying factors for MCB types are from BS 7671 Appendix 3.

Applicability: This calculation applies to TN systems (TN-S, TN-C-S) per BS 7671 Regulation 411.4. For TT systems, see Regulation 411.5 (earth electrode resistance limits apply instead of Zs tables).