Electrical testing explained
Earth Fault Loop Impedance Testing
An earth loop impedance test checks whether enough fault current can flow to operate a fuse, MCB or RCBO within the required disconnection time. It is a core safety measurement during an EICR and after new electrical work.
What the test measures
The tester measures the complete earth fault path. The resulting value is recorded in ohms and commonly labelled Zs on the schedule of test results.
Why the result matters
A lower impedance generally permits a larger fault current and faster automatic disconnection. The reading must be assessed against the circuit design and its protective device.
How an electrician assesses the result
- 1.Identify the earthing arrangement and protective device
- 2.Confirm test equipment is suitable and in calibration
- 3.Measure external earth fault loop impedance where appropriate
- 4.Measure or calculate the circuit Zs value safely
- 5.Compare the result with the design and current BS 7671 requirements
- 6.Record the result and investigate readings that do not comply
Earth loop readings are not universal pass marks
There is no single safe value for every circuit. Maximum values depend on the protective device type and rating, required disconnection time, conductor temperature and the edition of BS 7671 being applied. A reading should never be interpreted without the circuit details.
If a result is unexpectedly high, possible causes include loose or corroded connections, inadequate earthing, damaged conductors or an unsuitable protective device. Further testing is needed before deciding on corrective work.
Frequently asked questions
What is an earth fault loop impedance test?
It measures the impedance of the complete fault path from the circuit, through the earthing system and supply, back to the transformer. The result helps confirm whether a protective device can disconnect quickly enough during an earth fault.
What does Zs mean on an electrical certificate?
Zs is the measured earth fault loop impedance for a circuit. It is compared with the maximum value permitted for the protective device and the applicable BS 7671 requirements.
Can I test earth loop impedance myself?
No. The test requires a suitable calibrated multifunction tester, knowledge of the circuit and safe working procedures. Live testing can expose people to dangerous voltages and should be completed by a competent electrician.
What happens if the reading is too high?
A high result can mean the fault current may be too low to disconnect the protective device within the required time. The electrician will investigate earthing, bonding, connections, cable condition and protective device selection before recommending remedial work.
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