Question 1

What is earth fault loop impedance?

Accepted Answer

Earth fault loop impedance (Zs) is the total impedance of the fault current path from the active conductor through the fault to the earth return and back to the supply transformer. It determines whether enough fault current flows to trip the protection device within the required time.

Question 2

What is the maximum disconnection time for fault protection in Australia?

Accepted Answer

AS/NZS 3000 requires 0.4 seconds for final sub-circuits up to 32A and 5 seconds for distribution circuits. The fault loop impedance must be low enough to generate sufficient current to trip the breaker within these times.

Question 3

How do I measure earth fault loop impedance on site?

Accepted Answer

Use a loop impedance tester connected between active and earth at the furthest point of the circuit. Compare the reading against the maximum Zs values for your protection device. This calculator helps you estimate expected values before testing.

Question 4

What is the maximum Zs for a 20A Type B MCB?

Accepted Answer

The maximum earth fault loop impedance (Zs) for a 20A Type B MCB is 2.9 ohms for 0.4-second disconnection per AS/NZS 3000 Table 8.1. For a dead test (R1+R2), the maximum subcircuit resistance is 2.7 ohms per Table 8.2. This calculator supports both live and dead test methods with the correct comparison values.

Question 5

What is the difference between live and dead earth fault loop testing?

Accepted Answer

A dead test (AS/NZS 3000 Clause 8.3.9.2.2) measures the resistance of the earth fault loop with the supply disconnected, using a low-current ohmmeter. A live test (Clause 8.3.9.2.3) measures actual loop impedance with the circuit energised, including the supply transformer impedance. The live test gives a more accurate real-world value but carries shock risk. Both methods are accepted for verification. This calculator estimates the expected loop impedance to compare against your site measurements.

Question 6

What does a high earth fault loop impedance reading mean?

Accepted Answer

A high Zs reading means the fault current path has too much resistance for the protection device to trip within the required 0.4 seconds. Common causes include long cable runs, undersized earth conductors, loose connections, or corroded joints. If Zs exceeds the maximum for your MCB or RCD, the circuit is non-compliant with AS/NZS 3000 and poses a shock risk. You may need to increase the earth conductor size, shorten the run, or install an RCD which trips at only 30mA regardless of loop impedance. Use this calculator to identify which part of the loop contributes the most impedance.

Earth Fault Loop Impedance

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