Earth Fault Loop Impedance

When an active conductor faults to earth, current has to complete a full loop to flow: out along the active conductor to the fault, back through the earthing system and the supply transformer’s earth connection, then through the transformer winding and back down the active conductor to the point of fault. Earth fault loop impedance — Zs — is the total impedance of that entire path.

Zs matters because it sets how much fault current flows. Ohm’s law does the rest: fault current equals supply voltage divided by Zs. A low Zs means a high fault current, which trips an MCB or blows a fuse fast. A high Zs starves the fault of current, and the protection device may not trip in time — or at all, in a severe case — leaving a live fault on exposed metalwork.

That is why AS/NZS 3000 sets maximum disconnection times for automatic protection: 0.4 seconds for final sub-circuits up to 32A on a TN system, and 5 seconds for distribution circuits and sub-mains. Every combination of protection device and rating has a corresponding maximum Zs — keep the loop impedance under that figure and the device is guaranteed to trip inside the time limit.

Maximum Zs is not one number — it depends on the protective device type, its curve, and its rating. A Type B MCB trips instantaneously between 3–5 times its rated current, a Type C between 5–10 times, and a Type D between 10–20 times. The higher the instantaneous trip multiplier, the more fault current is needed, and the lower the maximum Zs has to be to guarantee it. HRC fuses follow their own published time-current curves rather than a fixed multiplier.

In practice there are two ways to get a maximum Zs figure for a specific device and rating. Manufacturers publish trip curves showing the current needed to open the device within a given time — read the current at the 0.4s or 5s mark, then work back to impedance using I = V / Zs. Alternatively, AS/NZS 3000 Table 8.1 (live test, total loop impedance) and Table 8.2 (dead test, subcircuit resistance) publish the maximum values directly for common MCB types and HRC fuses, so you do not have to read a curve on site. This calculator uses those published maximum values — for the specific case of a 20A Type B MCB, that is 2.9 ohms live / 2.7 ohms dead. Every device type and rating combination has its own figure; always confirm the exact value for your device against the current edition of the standard or the manufacturer’s data rather than assuming it is the same as a similar-looking breaker.

Take a 20A Type B MCB protecting a power circuit run in 2.5mm² copper, 30m from the board to the furthest outlet — a common GPO circuit. Cable resistance (R1+R2) for 2.5mm² copper works out to about 0.0177 ohms per metre.

433A is well past the 60–100A a 20A Type B MCB needs to see to trip instantaneously, so this circuit clears the 0.4-second requirement with plenty of margin. If the run were 150m instead of 30m, R1+R2 would climb to 2.66 ohms — still technically under the 2.7 ohm maximum, but with almost no margin left for joint resistance, temperature rise, or a slightly undersized earth conductor. That is the kind of borderline result worth increasing cable size or shortening the run for, rather than relying on a razor-thin pass.

A calculated Zs — like the worked example above — is a design-stage estimate. It uses standard cable resistance values at a reference temperature and assumes sound connections throughout the loop. It is what you use before the cable is even pulled, to confirm a proposed cable size and protection device combination will comply.

A fault loop impedance test is the on-site verification step, done with a calibrated loop impedance tester after the circuit is installed and energised. It measures the actual loop, including joint resistance, connector contact resistance, and the real supply impedance at that point in the network — all the things a calculation cannot fully account for. This is the figure that goes on the test certificate, because it reflects what is actually installed rather than what was designed.

Use the calculator above at design stage to check a cable and device combination before you commit to it. Run the actual fault loop impedance test once the circuit is complete, and compare that reading against the same maximum Zs — if it is close to the calculated figure, the installation matches the design. If it is significantly higher, something in the real loop — a loose connection, an undersized earth, a longer run than planned — is adding impedance the calculation did not predict.

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