Glossary Earthing Safety
A quick-reference companion to the dedicated Zs deep-dive, framed as a fast conceptual introduction.
The full mechanics of earth fault loop impedance (Zs) get a dedicated deep-dive elsewhere in this series. Here's the fast, plain-terms version.
When a live wire accidentally touches an earthed metal part, current needs a complete path back to the source to trip the protective breaker quickly. That complete path — from the fault, through the earth conductor, back through the supply, and through the live conductor to close the loop — is the "earth fault loop." Its total electrical resistance and reactance combined is the loop impedance, Zs.
Counterintuitively, a lower loop impedance is the safer outcome — it allows more fault current to flow, which is exactly what makes the breaker trip fast enough to clear the fault before it becomes a shock or fire hazard.
A poorly earthed installation, or one with an undersized earth conductor, can have a loop impedance too high for the installed protective device to clear the fault fast enough — meaning the breaker might not trip at all during a real fault, leaving a hazardous voltage present on exposed metal parts.
See the full mechanics and formula in the dedicated "Earth Fault Loop Impedance (Zs) Explained" article, or check your installation directly in the Earthing calculators.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks