Free Zs Values Lookup (BS 7671 Earth Fault Loop)

Zs is the earth fault loop impedance: the total impedance of the fault current path from the point of a fault back through the supply transformer and out again. A low enough Zs guarantees enough fault current flows to operate the protective device within the required disconnection time (0.4s for most final circuits, 5s for distribution circuits).

The tabulated maximum Zs values in BS 7671 assume conductors at their normal operating temperature. On site you usually test a cold circuit, so measured impedance is lower than under fault. The '0.8 rule' multiplies the tabulated value by 0.8 to give a practical measured limit your test reading should stay below. The values on this page apply that correction (and the current Cmin 0.95 factor) for you.

Use a loop impedance tester at the point furthest from the origin on the circuit (the point of highest Zs). Most testers offer a low-current 'no-trip' mode so you can measure on RCD-protected circuits without tripping them. Record the reading and compare it against the maximum measured Zs for the circuit's protective device and rating.

They come from BS 7671 Tables 41.2 (BS 3036 and BS 1361 fuses) and 41.3 (Type B/C/D circuit-breakers). Each value is derived from the device's disconnection characteristic (a Type B breaker trips instantly at 5×In, a Type C at 10×In and a Type D at 20×In) and the design voltage with the Cmin factor applied.

A Type C circuit-breaker needs a higher fault current to trip instantly (10×In versus 5×In for a Type B). To push that larger current through the fault loop, the loop impedance has to be lower, so the maximum permissible Zs for a Type C is roughly half that of the same-rated Type B, and a Type D lower still.

If measured Zs exceeds the maximum for the device, the protective device may not disconnect within the required time, leaving a dangerous touch voltage present during a fault. On an EICR this is typically coded C2 (potentially dangerous). Causes include long cable runs, undersized CPCs, or a poor supply earth. Each needs investigating rather than simply uprating the device.