A Guide to Fault Loop Impedance and Testing

Fault loop impedance plays a critical role in electrical safety. It ensures systems operate effectively and reduces risks like electric shock and fire. This guide provides an overview of the Earth fault loop impedance, its importance, testing methods, and the formula used to calculate it accurately.

Earth fault loop impedance, often referred to as EFLI or simply FLI, measures the resistance in the fault loop of an electrical system. This loop includes the power source, live conductors, protective earthing, and devices like RCDs or RCBOs.

The fault loop impedance test verifies that this resistance is within acceptable limits. As such, it plays a key role in ensuring the system’s safety and compliance. The test measures the total resistance in the fault loop, which directly affects how quickly safety devices respond. A low impedance allows protective devices to operate smoothly, while a high impedance may delay disconnection, increasing the risk of overheating, electric shock, or fire.

In addition, the Earth fault loop impedance test is essential for confirming that electrical installations can handle fault conditions safely. It is typically performed during new installations, inspections, or after system modifications.

Components of the Fault Loop

The fault loop is composed of various elements that contribute to its overall impedance:

• Source Impedance: This includes the internal resistance of the power supply and transformers.
• Line Impedance: Resistance from the live conductors that carry the current.
• Protective Earthing System: Resistance of the earthing conductors and connections, which ensures the safe fault current flow to earth.

Why Fault Loop Impedance Must Be Managed

Managing fault loop impedance is critical for maintaining the safety of electrical systems. It ensures that protective devices such as RCDs, RCBOs, and type A circuit breakers can disconnect circuits quickly during a fault. Without proper management, the fault current may not flow sufficiently to activate these devices, putting the system and its users at risk.

Moreover, circuits left live for extended periods not only pose a serious safety hazard but can also cause damage to equipment and infrastructure.

The AS/NZS 3000:2018 Wiring Rules set mandatory standards for electrical safety across Australia and New Zealand, ensuring all electrical circuits comply with minimum design and installation requirements. Referenced in legislation throughout both regions, the rules aim to protect people and property by minimising electrical hazards.

A key requirement of AS/NZS 3000 is the protection against ‘indirect contact’ with live parts. This protection is achieved through automatic disconnection of the power supply when a fault occurs. For this disconnection to happen, the total earth fault loop impedance (Zs) must be low enough to allow sufficient fault current to flow, enabling protective devices to disconnect circuits within specified timeframes. To ensure compliance, the Standard also defines the maximum allowable fault loop impedance (Zmax).

Common Scenarios for Installations

Earth fault loop impedance testing upholds safety across all types of installations. Here's how it applies in various settings:

• Residential: Regular fault loop impedance testing is essential for varied earthing systems in residential properties, ensuring fault currents flow efficiently to activate protective devices like RCDs and circuit breakers. This reduces risks of electric shock, overheating, and fire while protecting appliances and wiring from long-term damage.
• Commercial: With complex systems and higher load demands, commercial installations require testing to maintain safe fault loop impedance. This ensures protective devices function effectively during faults, minimising safety risks and preventing disruptions in high-traffic environments.
• Industrial: Industrial environments demand stringent testing due to heavy machinery and high fault currents. Maintaining low impedance is crucial to ensure protective devices respond quickly and, ultimately, promote electrical safety in industrial environments at all times.

Fault Loop Impedance Formula

Fault loop impedance is determined by summing the impedance of all components within the fault loop path. The total earth fault loop impedance (Zs) is expressed as:

Zs = Zint + Zext

Where:

• Internal Impedance (Zint): This includes the combined impedance of active conductors and earth conductors within the circuit. It accounts for the resistance and reactance values of the cables.
• External Impedance (Zext): According to AS/NZS 3000, at least 80% of the nominal phase voltage must be available at the position of the protective device to determine external impedance accurately.

Specialised calculations or tools, such as resistance and reactance data from standards like AS/NZS 3008.1, are often used to ensure accuracy when determining these values.

Maximum Fault Loop Impedance

The maximum allowable fault loop impedance (Zmax) ensures that protective devices will trip within the specified time during a fault. It is calculated using the following formula:

Zmax=Uo/Ia

Where:

• Zmax : Maximum earth fault loop impedance
• Uo : Nominal phase voltage
• Ia : Current required for automatic operation of the protective device

For circuit breakers, Ia is based on the typical rated current multiplied by a factor specific to the breaker type:

• Type B: 4 x rated current
• Type C: 7.5 x rated current
• Type D: 12.5 x rated current

For various fuse types, the values are taken from AS 60269.1.

The actual earth fault loop impedance (Zs) must always be lower than Zmax to ensure the protective device trips as intended during a fault.

The fault loop impedance test is conducted to verify that the impedance of an electrical circuit remains within safe and acceptable limits. This ensures that protective devices can activate promptly in the event of a fault.

Here's everything you need to know about conducting a fault loop impedance test:

Key Elements of the Test

The Earth fault loop impedance test involves measuring the resistance across the fault loop, which includes the active conductors, protective earthing system, and all connections within the circuit. Accurate measurement is crucial to confirm that the impedance is within permissible limits.

In addition, the fault loop impedance test verifies that protective devices such as circuit breakers and RCDs will operate correctly under fault conditions to ensure timely disconnection of the faulty circuit.

Tools and Equipment

Although standard multimeters can handle basic measurements, they lack the accuracy and precision required for fault loop impedance testing. For this reason, specialised instruments such as multifunction testers or dedicated earth loop impedance testers are necessary.

RS Australia is home to some of the most trusted brands, such as Megger, Martindale, andGossen Metrawatt, that offer high-quality fault loop impedance testers. These tools are designed to provide the precise measurements needed to ensure safety compliance. Regular calibration is equally important to maintain their accuracy over time.

When to Perform the Test

The Earth fault loop impedance test is carried out at critical stages throughout an electrical system’s lifecycle. During initial installations, the test is done to ensure that circuits meet safety standards before being energised. 

After that, periodic inspections and maintenance are also critical in verifying that the system remains compliant and safe over time. Any modifications or upgrades—such as adding new circuits—require retesting to confirm that the changes have not compromised fault loop impedance or overall system safety.

Safety Precautions

Safety is a top priority when performing a fault loop impedance test, especially when working on live circuits. To minimise risks, it’s essential to ensure the testing environment is well-prepared. This includes clearing the area of unnecessary personnel and inspecting all tools and equipment to confirm they are in good condition. 

In addition, wearing Personal Protective Equipment (PPE) such as insulated gloves, safety goggles, and flame-resistant clothing goes a long way in ensuring workplace safety and protecting against potential hazards like electric shock or burns. Live testing poses inherent risks, so electricians must follow strict safety protocols to protect themselves and others.

Common Mistakes to Avoid

Fault loop impedance testing is a critical process that demands accuracy, as even small mistakes can result in unreliable readings or safety risks.

One of the most common errors is incorrect test setup or connections, which can lead to inaccurate results or hazards. This often occurs when testing devices are not properly configured or when connections to the circuit are loose or incorrectly placed, which compromises both the validity of the test and the safety of the technician.

Aside from that, misinterpretation of results is another frequent issue during fault loop impedance testing. This often arises due to external factors such as fluctuating environmental conditions, incorrect circuit behaviour assumptions, or poorly calibrated equipment. Misreading values can lead to incorrect conclusions about a system's safety, potentially leaving faults undetected.

Test Results

Maintaining detailed and accurate records is crucial for audits and inspections, as they provide evidence of adherence to safety standards and support the smooth handover of maintenance responsibilities.

As such, electricians must record all fault loop impedance test outcomes, including details of test instruments used, in a clear and consistent format to verify that installations meet safety requirements and adhere to the AS/NZS 3000:2018 standards. Corrective actions must be taken if non-compliance is identified, followed by retesting to confirm the issue has been resolved.

DISCLAIMER: Fault loop impedance testing, along with any other electrical work, must be performed exclusively by licensed and qualified professionals. Attempting such tasks without the proper expertise can pose serious safety risks and may not comply with regulatory requirements.

Want to know more about Earth fault loop impedance testing and the right tools for you? Contact our customer support team at RS Australia here.