Maximum Ze Values for UK Earthing Systems
Ze is a measured supply characteristic, not a target to force the installation to meet; use it to assess the circuit protective path honestly.
External earth fault loop impedance is measured at the origin and reflects the distributor supply arrangement. It provides a starting point for circuit Zs assessment and can expose a supply or bonding issue before individual circuit results are considered.
This is practical guidance for competent electricians. Assess the actual installation, follow the current edition of BS 7671 and manufacturer instructions, and stop where the work cannot be completed safely.
Key takeaways
- Agree the scope and make the installation safe before starting work.
- Use a controlled sequence rather than relying on a single reading or visual assumption.
- Record the actual result, limitation or action taken while it is still traceable.
- Check the result against the design, protective measures and equipment instructions.
- Issue the appropriate certificate or report when the work is complete.
Scope the work first
External earth fault loop impedance is measured at the origin and reflects the distributor supply arrangement. It provides a starting point for circuit Zs assessment and can expose a supply or bonding issue before individual circuit results are considered.
Method
Identify the declared earthing arrangement, inspect the main earthing terminal and bonding, then take the measurement with an appropriate method. Compare the result with the value applicable to the supply type and with the installation design evidence.
Doing this on a job? Zs from Ze + R1+R2 calculator — part of TradePlanr. Free to start, no card needed.
Checks and common pitfalls
Do not assume a supply type from an old label or use typical figures to replace a measurement. A high or inconsistent value may need investigation with the distributor, and poor bonding or test setup can create misleading results.
Record and hand over
Enter the measured Ze, supply earthing arrangement, test method and any limitation on the certificate or report. Use the value transparently when assessing each circuit's Zs and protective device performance.
Keep the evidence with the job
Record readings, limitations and remedial actions as you work. A clear certificate or report protects the client and makes the next visit easier to manage.
From guidance to action
Frequently asked questions
Is a typical Ze value a substitute for testing?
No. Typical values are useful context only. Record the actual measured or verified design value and investigate a result that does not make sense for the installation.
Can I rely on a standard rule of thumb?
No. The installation method, supply characteristics, equipment instructions and the current requirements all affect the correct decision. Use the guide as a workflow, then assess the actual job.
What should be recorded?
Record the scope, results, limitations, observations, remedial work and the certificate or report appropriate to the work carried out.
Related guides
Earthing Systems: TN-S, TN-C-S and TT Explained
How TN-S, TN-C-S (PME) and TT earthing arrangements work, and how to identify each on site.
Max Zs Values Table (BS 7671 18th Edition)
Full maximum Zs tables for Type B/C/D MCBs and RCBOs, the Cmin formula and the 80% rule.
Earth Fault Loop Impedance Explained (Ze, Zs & Zdb)
What Ze, Zs and Zdb mean, how the fault loop works, and how to measure and verify it against BS 7671 limits.
