BS 7671 design tool

Zs / disconnection time checker

Look up maximum permissible earth fault loop impedance (Zs) values for MCB, RCBO and fuse types per BS 7671 Tables 41.2–41.4. Enter your measured Zs to check compliance.

Circuit inputs

Results

Maximum Zs2.70 Ω

MCB Type B (BS EN 60898) — 32 A
Source: BS 7671 Table 41.3
0.4 s disconnection, 230 V TN system

Enter measured Zs above to check compliance

Use a loop impedance tester on site and enter the reading above. The measured value must not exceed 2.70 Ω for this device.

Disconnection time0.4 s (TN socket/lighting circuits ≤ 32 A)

These values are for 0.4 s disconnection per BS 7671 Table 41.1, applicable to 230 V socket outlet circuits and other circuits ≤ 32 A in TN systems. For final circuits supplying only fixed equipment (> 32 A), or submains, 5 s disconnection applies — Zs limits will be higher. Refer to BS 7671 Tables 41.2–41.4 directly for 5 s values.

Assumptions and limitations

What this tool covers

  • Maximum Zs at 0.4 s disconnection for 230 V TN systems
  • MCB Type B, C and D to BS EN 60898
  • BS 88-2/88-3 industrial cartridge fuses
  • BS 3036 semi-enclosed (rewirable) fuses

Known limitations

  • 0.4 s only. 5 s disconnection values (fixed equipment, submains) are not yet tabulated here — check BS 7671 Tables 41.2–41.4 directly
  • TT systems: Zs alone is insufficient — RCD protection is required (BS 7671 Reg 411.5)
  • RCBOs: Overcurrent trip characteristics follow the MCB type (B/C/D) — select accordingly
  • Temperature correction: Measured Zs should be corrected for conductor temperature if required — multiply by 1.24 for a cold PVC conductor correction per BS 7671 Appendix 14
  • This tool covers single-phase 230 V circuits only

⚠️ Important notice

This is a preliminary design aid only. Zs measurements on site must be verified against the current edition of BS 7671 Tables 41.2–41.4 by a qualified electrician before certification. Do not rely solely on this tool for certification decisions.

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