All toolsCalculatorsElectricalEarth Fault Loop Impedance (Zs)

Earth Fault Loop Impedance (Zs)

Verify earth-fault loop impedance Zs against the BS 7671 maximum for the protective device.

External loop impedance (Ze)

0.35 Ω TN-C-S (PME) max; 0.8 Ω TN-S max; or a measured value

(R1+R2) per metre @ 20 °C

mΩ/m

Combined line + cpc resistance per metre

Conductor preset (On-Site Guide I1)

Copper T&E line/cpc pairs at 20 °C. Verify against actual cable data.

One-way circuit length

Operating-temp factor

Max permitted Zs (Table 41.3)

Tabulated value already includes Cmin = 0.95

Device preset (Table 41.3, 0.4 s)

BS EN 60898 / 61009 MCBs at 230 V, 0.4 s. Verify the disconnection time required.

Cold-measured rule-of-thumb

× max

0.8 (80 %) rule — a COLD site-measured Zs should be below this

Enter Ze, (R1+R2)/m and length to calculate