Earthing Safety

Earth Fault Loop Impedance (Zs) Explained

A low earth fault loop impedance is what makes a circuit breaker actually protect you during a fault. Here is what Zs measures and why it matters.

A circuit breaker only protects you from a fault if it can actually detect that fault and trip fast enough. Earth fault loop impedance — usually written as Zs — is the number that determines whether that happens.

What Zs Actually Measures

When a live conductor faults to an earthed metal part, current flows through a specific path back to the source: through the fault point, through the circuit protective (earth) conductor, back through the supply transformer's earthing, and through the phase conductor to complete the loop. The total impedance of that entire loop is Zs.

Zs = Ze + R1 + R2
Ze = external earth loop impedance (transformer to the installation)
R1 = resistance of the phase/active conductor
R2 = resistance of the circuit protective (earth) conductor

Why a Lower Zs Is Better, Not Worse

Ohm's Law means fault current during a fault is roughly the system voltage divided by Zs. A lower Zs allows a higher fault current to flow — which sounds dangerous until you remember the point: that higher current is exactly what makes the breaker trip fast. Each protective device has a maximum permitted Zs value for a given disconnection time; exceed it, and the breaker may not trip fast enough (or at all) to clear the fault before it becomes a shock or fire hazard.

What Drives Zs Up in Practice

• Long cable runs — more conductor length means more R1 and R2
• Undersized earth conductors — a thin protective conductor has higher resistance per meter than the phase conductor it's paired with
• Poor or corroded connections anywhere along the loop — a bad joint adds resistance that doesn't show up on a simple visual inspection
• A high Ze at the supply end, outside the installer's control but still part of the total

The Practical Takeaway

Sizing the earth conductor correctly and keeping runs reasonable isn't just a code checkbox — it's the thing that determines whether your protective device does its actual job during a real fault. OhmWorks' Earthing calculators check Zs against your installed device's requirements directly, so you're not just trusting that "it's probably fine" — see it in the PEC and AS/NZS Earthing calculators.

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