Earth Fault Loop Impedance Calculator

What is earth fault loop impedance?

Earth fault loop impedance is the total resistance and reactance presented to electric current flowing through a complete fault loop during an active-to-earth short circuit. When insulation degrades or an active conductor contacts earthed metalwork, fault current travels along a defined path back to the distribution transformer secondary winding.

The complete loop consists of two distinct segments represented by the fundamental formula:

Where each term represents a critical component of the fault path:

• Ze (External Loop Impedance): The impedance of the supply network outside the installation. This includes the HV supply lines, step-down transformer secondary winding, service active conductor, service neutral, and the main earthing system back to the switchboard.
• R1 (Active Conductor Resistance): The resistance of the internal phase conductor running from the circuit protection device at the main switchboard out to the furthest point of the circuit.
• R2 (Earth Conductor Resistance): The resistance of the protective earth conductor running from the fault point back to the main earth bar and Multiple Earthed Neutral (MEN) point.

On standard Australian MEN installations supplied by network distributors like Ausgrid, Endeavour Energy, Essential Energy, Energex, Ergon Energy, or Western Power, $Z_e$ is measured directly at the main switchboard with the main switch isolated.

Why does disconnection time matter?

When an active conductor shorts to earth, touch voltage on exposed conductive parts jumps to dangerous levels (up to 230V relative to true earth). To prevent lethal cardiac ventricular fibrillation, protective devices must isolate power before ventricular shock thresholds are reached.

AS/NZS 3000 mandates two specific disconnection time limits based on circuit application:

• 0.4 Seconds (Final Sub-Circuits): Mandatory for all final sub-circuits supplying socket-outlets, lighting, or handheld appliances rated up to 63A where direct physical contact is likely.
• 5.0 Seconds (Distribution / Sub-Mains): Permitted for fixed distribution sub-mains feeding secondary switchboards where equipment is stationary and personal contact is improbable.

Circuit breakers rely on their electromagnetic trip mechanism for sub-second operation. To trip instantaneously, fault current ($I_f = 230 / Z_s$) must exceed the breaker's minimum magnetic pickup threshold:

• Type B MCB: Trips instantaneously between $3 \times I_n$ and $5 \times I_n$.
• Type C MCB: Trips instantaneously between $5 \times I_n$ and $10 \times I_n$.
• Type D MCB: Trips instantaneously between $10 \times I_n$ and $20 \times I_n$.

Because Type D MCBs require up to 20 times their rated current to trip magnetically, they demand drastically lower loop impedance than Type B breakers of equivalent current rating.

AS/NZS 3000 Table 8.1 — Maximum Zs limits

AS/NZS 3000 Table 8.1 establishes the absolute upper limit for total earth fault loop impedance ($Z_s$) at 230V nominal voltage to guarantee disconnection within 0.4 seconds. Measured or calculated $Z_s$ at the furthest point must never exceed these values.

Worked example — verifying Zs on a residential lighting circuit

Consider a newly installed outdoor floodlight circuit in a single-storey residence in Penrith, Western Sydney. The circuit is protected by a 16A Type B MCB. Wiring consists of 1.5 mm² flat TPS copper cable running 25 metres from the switchboard to the furthest luminaire. Supply type is PME with measured $Z_e = 0.35\ \Omega$ at the main switchboard.

What affects fault loop impedance on a job?

Several physical factors dictate whether a real-world circuit passes or fails loop impedance verification:

• Route Length: Resistance scales linearly with distance. Long single-phase sub-mains or final sub-circuits running to outbuildings, detached garages, or garden pumps accumulate high $R_1+R_2$ resistance rapidly.
• Conductor Cross-Section: Conductor resistance is inversely proportional to cross-sectional area in mm². Smaller 1.5 mm² lighting conductors have almost triple the resistance per metre of 4.0 mm² power cables.
• Reduced Earth Size: Standard Australian TPS cables often utilize reduced earth cores (e.g. 6.0 mm² active with a 2.5 mm² earth, or 10.0 mm² active with a 4.0 mm² earth). The smaller earth core increases $R_2$ significantly relative to $R_1$.
• Supply Authority Ze: External supply impedance varies by region and transformer proximity. Urban underground networks usually exhibit low $Z_e$ (0.15–0.30 Ω), whereas regional overhead spur lines across inland NSW, Queensland, or Western Australia frequently exceed 0.60 Ω.
• Operating Temperature: As copper conductors heat up under continuous load, atomic lattice vibration increases electrical resistance by approximately 0.393% per °C. A cable operating at 70°C exhibits ~20% higher resistance than during a cold ambient test at 20°C.

Common mistakes with fault loop impedance

• Testing cold without applying operating temperature correction: Cold verification tests measured at 20°C ambient may show pass results that fail when conductors heat up to 70°C under full load. If test results sit within 20% of Table 8.1 limits, multiply $R_1+R_2$ by 1.20 to confirm operating compliance.
• Selecting the wrong breaker curve in calculations: Assuming Type B limits apply when Type C or Type D breakers are installed. Type C MCBs halve the allowable $Z_s$ limit compared to Type B, while Type D cuts it by 75%.
• Assuming earth size equals active size: Assuming $R_1 = R_2$ on larger cables where the earth conductor is reduced (such as 6/2.5 mm² TPS).
• Guessing Ze instead of measuring: Entering an assumed 0.35 Ω $Z_e$ on a property with long overhead service mains where actual measured $Z_e$ is 0.75 Ω leads to false compliance calculations.

How to measure fault loop impedance

Physical verification on site must be conducted in accordance with AS/NZS 3017 testing standard procedures using a calibrated loop impedance tester:

1. External Impedance (Ze) Test: Disconnect the installation main neutral and main earthing conductor at the switchboard with the main switch OFF. Measure loop impedance between the incoming active line and the supply neutral bar. Reconnect main earthing immediately after measurement.
2. Total Loop Impedance (Zs) Test: Energise the circuit. Connect the test instrument between active, neutral, and earth terminals at the furthest socket-outlet or point of consumption. Use a non-trip loop test mode to prevent tripping upstream 30mA RCDs.
3. Documentation: Record measured $Z_s$ on the Certificate of Electrical Safety / Compliance Certificate for submission to state regulators and supply authorities.