Earth fault loop impedance
Maximum Zs checker
Pick the protective device and rating to get the maximum permitted earth fault loop impedance (Zs) for a 0.4 second disconnection time. Values are corrected to 0.95 (Cmin), ready to compare with your measured Zs.
Maximum Zs from BS 7671 Table 41.3, corrected by 0.95 for the 80% rule. Final circuits up to 32 A use 0.4 s.
What Zs is, and why the 80% rule exists
Earth fault loop impedance (Zs) is the total impedance of the loop a fault current travels: out along the line conductor and back through the earth path to the source. It is Ze (the external loop impedance at the origin) plus R1 + R2 (the circuit's own line and protective conductors). The lower it is, the larger the fault current, and the faster the device trips.
Disconnection times
On a TN system a final circuit up to 32 A must clear an earth fault in 0.4 seconds. Distribution circuits and final circuits above 32 A get 5 seconds. The maximum Zs in the tables is the value at which the device just meets that time. This checker shows the 0.4 s figures, which cover most domestic final circuits.
Why the values are corrected to 0.95
The published values assume conductors at full operating temperature and a nominal supply voltage. You usually measure Zs cold and the supply varies, so BS 7671 applies a 0.95 factor (Cmin), and many electricians use the 80% rule as well, so a circuit tested cold still disconnects when hot. If a measured Zs is too high, the usual fixes are RCD additional protection, a larger protective conductor to cut R2, or a more sensitive device. The fault current calculator works the same loop from the other side.
FAQs
What is the maximum Zs for a 32A Type B MCB?
1.37 ohms for a 0.4 second disconnection time, using the BS 7671 value corrected by 0.95 (Cmin). The uncorrected tabulated figure is 1.44 ohms.
Why are the values corrected to 0.95?
Published maximum Zs values assume conductors at operating temperature and a nominal voltage. The 0.95 factor, and the 80% rule for site measurements taken cold, give a margin so a circuit still disconnects in time once it warms up.
What if my measured Zs is too high?
Add RCD protection so disconnection no longer relies on the overcurrent device alone, increase the size of the circuit protective conductor to reduce R2, or use a more sensitive device. Re-test after any change.