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14 Jan 2023


Jonathan Carroll B.Eng (Electrical)
Director & Engineering Manager


AS3000:2018 Wiring Rules, Standards Australia "Fault Loop Impedance Explained", Electrical Safety First "Residual Current Devices (RCDs)", Electrical Safety Office (Queensland) "Fault Loop Impedance (FLI) Calculations", Electrical Engineering Portal

Fault Loop Impedance

Fault Loop Impedance (FLI) is a measure of the resistance and reactance in an electrical circuit that is used to calculate the fault current that flows through the circuit in the event of a fault.

AS3000, the Australian Standard for Wiring Rules, is the main reference for FLI calculations in Australia. It provides guidelines on how to calculate FLI, and the minimum values that are required to ensure the safe and efficient operation of an electrical system.

RCDs (Residual Current Devices) are often used to protect cables and other electrical equipment from faults. These devices work by detecting any imbalance in the current flowing through the circuit and quickly shutting off the power to prevent further damage.

Because RCDs have a low current sensitivity and quick operation, FLI calculations are not required for circuits that are protected by RCDs.

However, when a circuit breaker is used to protect a cable, FLI calculations become important. This is because the fault current must be high enough to quickly operate the circuit breaker and prevent further damage to the electrical system.

FLI calculations are important in ensuring the safe and efficient operation of an electrical system. They help to identify potential issues and ensure that the system is designed to handle the required fault current.

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