AS/NZS 3000:2018 CLAUSE 5.7

Fault Loop Impedance Calculator

Calculate total earth fault loop impedance (Zs = Ze + R1 + R2) and prospective fault current. Verify mandatory 0.4-second automatic disconnection times against AS/NZS 3000 Table 8.1 limits.

Circuit & Protective Device Parameters

Ω
Supply side loop impedance (transformer + mains)
m
Distance from switchboard to furthest point of circuit

AS/NZS 3000 Earth Fault Loop Impedance (Zs) & Disconnection Times

AS/NZS 3000:2018 Clause 5.7 mandates that total earth fault loop impedance (Zs) must be verified on every Australian electrical installation. If loop impedance exceeds Table 8.1 maximum thresholds, protective circuit breakers will fail to disconnect within the 0.4-second safety limit during an active-to-earth fault, creating a fatal shock hazard for building occupants.

AS/NZS 3000 Table 8.1 Maximum Allowable Zs Limits (230V Single-Phase)

Under AS/NZS 3000:2018 Clause 5.7.4 (Table 8.1), maximum earth fault loop impedance (Zs) in Ohms (Ω) ensures automatic disconnection within 0.4 seconds at 230V nominal voltage. Below is the official compliance lookup matrix for standard Australian Type B, C, and D circuit breakers and fuses:

Device Rating (In)MCB Type B (3–5× In)MCB Type C (5–10× In)MCB Type D (10–20× In)HRC Fuse BS88Rewirable Fuse
6 Amps7.67 Ω3.83 Ω1.92 Ω8.52 Ω5.35 Ω
10 Amps4.60 Ω2.30 Ω1.15 Ω5.11 Ω3.07 Ω
16 Amps (Standard GPO)2.87 Ω1.44 Ω0.72 Ω2.70 Ω1.77 Ω
20 Amps2.30 Ω1.15 Ω0.57 Ω1.77 Ω1.35 Ω
25 Amps1.84 Ω0.92 Ω0.46 Ω1.35 Ω1.04 Ω
32 Amps (Sub-mains)1.44 Ω0.72 Ω0.36 Ω1.04 Ω0.77 Ω
40 Amps1.15 Ω0.57 Ω0.29 Ω0.79 Ω0.59 Ω
50 Amps0.92 Ω0.46 Ω0.23 Ω0.59 Ω0.44 Ω
63 Amps0.73 Ω0.37 Ω0.18 Ω0.44 Ω0.34 Ω

AS/NZS 3000:2018 Clause 5.7 Automatic Disconnection of Supply (ADS)

Under the Australian Wiring Rules, Automatic Disconnection of Supply (ADS) is the primary defense against indirect contact shock. When an insulation fault occurs inside a luminaire, appliance, or motor housing, the earthing system must carry high fault current back to the switchboard to trigger instantaneous magnetic tripping.

Key Disconnection Time Requirements:

  • 0.4 Seconds (Final Sub-Circuits): Mandatory for all 230V final sub-circuits supplying socket-outlets (GPOs), portable appliances, handheld equipment, and residential lighting.
  • 5.0 Seconds (Sub-Mains & Fixed Equipment): Permitted for heavy sub-mains, distribution switchboards, and stationary equipment exceeding 32A where human contact risk is lower.
  • RCD Exemption Clause 5.7.1: Where a sub-circuit is protected by a 30mA RCD, automatic disconnection occurs in under 30 milliseconds (0.03s), permitting higher loop impedance values up to 766 Ω at 230V.

The Complete Earth Fault Loop Pathway Breakdown

When a phase conductor contacts grounded metalwork, fault current travels through a continuous loop comprising five distinct circuit legs:

  1. Transformer Secondary Winding: The local distribution transformer winding (step-down 11kV/415V to 230V phase-to-neutral).
  2. Service Mains Conductor: The distributor service line (underground or overhead drop) bringing power to the premises main switchboard.
  3. Active Circuit Conductor (R1): The phase wire running from the protective circuit breaker down the circuit route to the fault point.
  4. Protective Earthing Conductor (R2): The earth wire returning fault current from the appliance back to the main earth bar.
  5. MEN Neutral Link & Earth Return (Ze): The Multiple Earthed Neutral link inside the main switchboard connecting the main earth bar back to the neutral conductor and local distribution transformer.

Zs = Ze + (R1 + R2)

MCB Tripping Characteristics — Type B vs Type C vs Type D

Choosing the correct circuit breaker curve rating directly impacts allowable cable run length under AS/NZS 3000 Table 8.1:

  • Type B MCBs (3× to 5× In Magnetic Trip): Designed for residential lighting and resistive heating loads with minimal inrush. Requires less fault current to trip magnetically, permitting up to 2.87 Ω Zs for a 16A breaker (double the length of Type C).
  • Type C MCBs (5× to 10× In Magnetic Trip): Standard Australian residential and commercial choice for general power GPOs, air conditioning, and fluorescent lighting. A 16A Type C breaker requires 160A fault current to trip instantaneously, capping allowable Zs at 1.44 Ω.
  • Type D MCBs (10× to 20× In Magnetic Trip): Used in industrial environments with heavy motor loads, X-ray machinery, and large transformers. Requires 320A fault current for a 16A breaker, limiting max allowable Zs to 0.72 Ω.

Conductor Temperature & Resistance Correction

As conductors carry electrical load, resistive heating causes their temperature to rise, increasing internal wire resistance. Under AS/NZS 3008.1.1, copper has a thermal resistance coefficient of α = 0.00393 per °C.

Testing at 20°C ambient room temperature gives lower loop impedance than when cables operate under full rated continuous load at 70°C (PVC V-90) or 90°C (XLPE X-90). Electricians must apply temperature adjustment multipliers during circuit design:

R_temp = R_20°C × [ 1 + α × (T_operating - 20) ]

For standard 70°C PVC insulated cables, resistance increases by 19.6% above 20°C test values. A 2.5mm² copper conductor measures 0.00741 Ω/m at 20°C, but increases to 0.00887 Ω/m at 70°C operating temperature.

Detailed Site Example — Commercial GPO Circuit Verification

An electrician is installing a 35-metre sub-circuit for commercial computer workstations using 2.5mm² copper TPS (with 2.5mm² earth), protected by a 16A Type C MCB on a standard PME supply (Ze = 0.35 Ω).

Step 1 — Calculate Conductor Resistance at 70°C Operating Temp

From AS/NZS 3008 Table 35, 2.5mm² copper resistance at 20°C = 0.00741 Ω/m. Applying the 70°C thermal multiplier (1.196):

R1 + R2 = (0.00887 + 0.00887) × 35m = 0.01774 × 35 = 0.621 Ω

Step 2 — Determine Total Loop Impedance (Zs)

Zs = Ze + (R1 + R2) = 0.35 + 0.621 = 0.971 Ω

Step 3 — Evaluate Prospective Earth Fault Current (If)

If = 230V / 0.971 Ω = 236.9 Amps

Step 4 — Verify AS/NZS 3000 Table 8.1 Compliance

For a 16A Type C MCB, instantaneous magnetic trip requires 10 × 16A = 160 Amps. Calculated fault current (236.9A) easily exceeds 160A. Total Zs (0.971 Ω) is well below the Table 8.1 limit of 1.44 Ω.

Result: PASS (Compliant for 0.4s Disconnection). Safety Headroom = +32.6%. Maximum compliant run length = 61.4 metres.

On-Site Fault Loop Impedance Testing Guide

When verifying new sub-circuits for electrical safety certificates (CCEW in NSW, EWR in VIC/QLD), sparkies perform live loop impedance testing using a Multifunction Installation Tester (MFT):

  • Non-Trip 3-Wire Testing (RCD Protected Circuits): Uses a low test current (typically 15mA) or DC bias pulse to measure Zs without blinding or tripping 30mA safety switches.
  • High-Current 2-Wire Testing (Non-RCD Circuits): Delivers a 10A to 25A short test pulse between active and earth at the furthest outlet to measure precise AC loop impedance.
  • Testing at Furthest Socket-Outlet: Always test at the physical end of the circuit run (furthest GPO or light fitting) where R1 + R2 resistance is highest.

Strategies for Fixing Non-Compliant High Zs Circuits

If site measurements or design calculations show Zs exceeding Table 8.1 limits, apply these trade-proven remediation steps:

  1. Upsize Protective Earth Conductor (R2): Replace reduced earths (e.g., 2.5mm² active / 1.5mm² earth) with equal-sized 2.5mm² earth conductors to reduce R2 loop resistance.
  2. Shorten Circuit Route Length: Divide long sub-circuits into smaller sub-mains feeding local sub-boards.
  3. Switch to Type B Circuit Breakers: Replace Type C MCBs with Type B units for lighting and GPO circuits without heavy motor inrush, instantly doubling allowable Zs headroom.
  4. Ensure 30mA RCD Protection: Under AS/NZS 3000 Clause 5.7.1, installing a 30mA RCBO or RCD provides guaranteed compliance even on high Zs runs.

Explore our complete suite of AS/NZS compliance calculators built specifically for Australian sparkies:

Frequently Asked Questions

Common questions about fault loop impedance (Zs), Ze supply values, and AS/NZS 3000 Table 8.1