Max Zs Calculator - BS 7671 Table 41.3

Calculate maximum permissible earth fault loop impedance (Zs) values for electrical circuits according to BS 7671:2018

Calculate Maximum Zs

Calculate maximum earth fault loop impedance values according to BS 7671

Magnetic trip at 3-5× rated current. Most common for lighting and socket circuits.

Rated current of the protective device

0.4s required for most socket and lighting circuits. 5s allowed for distribution circuits.

Understanding Maximum Zs Values

The maximum earth fault loop impedance (Zs) is a critical safety parameter in electrical installations. It ensures that protective devices disconnect the supply within the required time in the event of an earth fault, preventing electric shock and fire hazards.

What is Zs?

Earth fault loop impedance (Zs) is the impedance of the complete path from the point of fault, through the earth fault loop, back to the source. It includes Ze (external earth fault loop impedance) and (R₁ + R₂) (circuit impedances).

Formula

Zs(max) = (U₀ × Cmin) / Ia

Where U₀ = 230V, Cmin = 0.95, and Ia = current causing disconnection in required time

Important Notes

  • ⚠️ Testing: Use 80% of max Zs as testing limit to account for measurement tolerances
  • ⚠️ Temperature: Apply 0.8 correction factor to account for conductor temperature rise during faults
  • ⚠️ Verification: Always verify by testing - don't rely on calculation alone

Frequently Asked Questions