What is earth fault loop impedance (Zs)?

Glossary · Testing, inspection and certificates

How earth fault loop impedance works

If a live conductor in your home touches earth, for example through a damaged cable or a faulty appliance, current flows round a loop. It leaves the supply, travels along the line conductor to the fault, and comes back along the protective conductor to the source. The impedance of that whole path is the earth fault loop impedance, Zs.^{[2]Institution of Engineering and Technology (IET), Minimizing unnecessary live testing for initial verification (Wiring Matters, May 2025)}

The reason it matters is speed. NICEIC says the test verifies that, if a fault occurs, protective devices will trip quickly to cut off the supply safely.^{[1]NICEIC, Electrical Installation Condition Reports (EICRs)} A low impedance lets a large fault current flow, and a large current makes a breaker operate fast. Where the impedance is too high, the current may be too small for the device to react in time.

How it is checked

The IET explains that the installation must be designed so that a device disconnects in the time the standard requires. This is confirmed by making sure the measured Zs is less than or equal to the maximum Zs permitted for that device.^{[3]Institution of Engineering and Technology (IET), Determining the maximum earth fault loop impedance for protective devices (Wiring Matters, March 2023)} The maximum values vary with the type of device and between manufacturers, so wherever possible the manufacturer’s own data is used.^{[3]Institution of Engineering and Technology (IET), Determining the maximum earth fault loop impedance for protective devices (Wiring Matters, March 2023)}

Zs is made up of two parts:

The IET says Zs can be found by adding the measured R1 plus R2, from dead testing, to the Ze value. It also warns that testing Zs live on every circuit is often unnecessary and can be dangerous.^{[2]Institution of Engineering and Technology (IET), Minimizing unnecessary live testing for initial verification (Wiring Matters, May 2025)} Where the circuit is protected by an RCD, the loop impedance matters less, because RCDs are designed to operate within 300 milliseconds at the rated residual current. Our RCD trip test page covers that.^{[2]Institution of Engineering and Technology (IET), Minimizing unnecessary live testing for initial verification (Wiring Matters, May 2025)}

Typical Ze values

The IET lists the typical maximum external values that the network operator declares for supplies up to 100 amps. The right row depends on your earthing system.^{[2]Institution of Engineering and Technology (IET), Minimizing unnecessary live testing for initial verification (Wiring Matters, May 2025)}

The IET adds that higher values may apply where a home is fed from a small pole-mounted transformer or a long overhead line.^{[2]Institution of Engineering and Technology (IET), Minimizing unnecessary live testing for initial verification (Wiring Matters, May 2025)} The measured value for your home is recorded in the test results.

Example

Aisha has an EICR on her house in Moortown. For the kitchen circuit protected by a circuit breaker, the electrician records the measured Zs and compares it with the maximum permitted for that breaker, taken from the manufacturer’s data or from BS 7671.^{[2]Institution of Engineering and Technology (IET), Minimizing unnecessary live testing for initial verification (Wiring Matters, May 2025)} The reading is under the limit for that device, so the check passes for that circuit. A different type of breaker on another circuit has a different limit, so the same reading could be judged differently.^{[3]Institution of Engineering and Technology (IET), Determining the maximum earth fault loop impedance for protective devices (Wiring Matters, March 2023)}

What people get wrong

People often treat Zs as a single pass mark. It is not. The limit depends on the type and rating of the protective device, and the manufacturer’s data can differ from the general tables. A reading is judged against the maximum for that circuit’s device, with an allowance for temperature.

The IET says the maximum values need to be adjusted for temperature, because conductor resistance rises as cables warm up.^{[2]Institution of Engineering and Technology (IET), Minimizing unnecessary live testing for initial verification (Wiring Matters, May 2025)} That is one reason published tables differ and one reason the test is left to a competent electrician.

The bottom line

Earth fault loop impedance tells you whether a fault would make enough current flow to trip a breaker or fuse quickly. It is Ze plus R1 plus R2, and the limit depends on the device protecting the circuit. It is a standard part of an EICR, and you do not test it yourself. Our EICR testing page explains how we carry out the inspection.