Earth Loop Impedance Test Values: What to Compare and Why

Zs is not just a lookup. The value you compare against depends on the protective device, required disconnection time, earthing arrangement, and whether youre using design values or measured comparison values.

Get that context wrong and you can fail a circuit that is acceptable, or pass one with weak justification in the report.

This is the practical version: what Zs means, what max Zs means in context, where the 80% convention fits, and how to record your basis clearly.

Quick summary

• Zs is used to confirm ADS by checking measured or calculated loop impedance against a maximum permitted value for the protective arrangement
• Max Zs is not one fixed number for every circuit, it changes with device type/rating, disconnection time, and system context
• Design-condition values and measured comparison values are not the same thing, temperature assumptions matter
• The 80% convention is useful in the right method context, not a universal standalone rule
• On TT systems, keep interpretation cautious and keep RCD strategy front and centre

What Zs means in practice

Zs is the earth fault loop impedance of the circuit, the total loop impedance seen by fault current on that path.

You use it to judge whether fault current should be high enough for the protective measure to disconnect within the required ADS time.

That is why it matters in both initial verification and EICR work.

Why maximum Zs values matter

Maximum Zs is about proving disconnection performance, not filling a table.

For TN systems, required times vary by circuit and application context, with common examples including 0.4 s and 5 s contexts. So the comparison basis must be context-led, not generic.

A number without context is just a number.

What changes the acceptable comparison value

Before comparing any reading, lock these in:

1. Protective device type and rating
2. Required disconnection time for that circuit context
3. Earthing arrangement (TN or TT)
4. Final circuit vs distribution circuit context
5. Whether manufacturer device data is available

Where manufacturer-specific values exist, use them as the governing reference for that protective device.

Calculated route: Zs = Ze + (R1 + R2)

The calculated route is often the cleanest starting point.

Use measured Ze and measured (R1 + R2), then calculate Zs. That can reduce the risk of parallel-path masking during direct live loop measurement.

Its especially useful before energising and during structured initial verification.

Live loop test vs calculated method

Both methods are useful, they answer slightly different questions.

• Calculated Zs gives a baseline from measured components
• Live loop testing can expose high-impedance issues at terminations or device paths that calculated checks may not show as clearly

So in practice: calculated baseline first, then live results where justified and safe by method.

Why design values and measured comparisons differ

A lot of bad calls come from mixing design-condition values with measured comparison values.

Design values and measured comparisons use different assumptions, including temperature and voltage factors. That is why values can differ across guidance sources while still coming from the same framework.

Cmin and the 80% convention: use it properly

On site youll hear, compare measured Zs to around 80% of tabulated max values.

There is a technical basis for measured-value adjustment methodology, including Cmin framing and temperature-related treatment. But it still needs context.

Use it as part of a method, not as a universal law that overrides device-specific data or circuit context.

If your report only says used 80% rule, with no comparison basis, thats weak evidence.

TT and RCD nuance

On TT systems, avoid forcing TN-style loop reasoning.

TT disconnection strategy is generally RCD-led, so your recorded basis should reflect that.

Keep wording explicit about the protective arrangement used.

Common mistakes that cause bad calls

• Using a raw table value without confirming circuit context
• Treating design and measured comparison values as interchangeable without explanation
• Writing pass or fail with no recorded comparison basis
• Treating 80% as a standalone universal rule
• Ignoring manufacturer data where available
• Applying TN-style reasoning to TT records without RCD context

Practical decision flow on site

Use this sequence:

1. Identify protective device and rating
2. Confirm required disconnection time for that circuit/application context
3. Confirm earthing arrangement and whether TT/RCD strategy changes interpretation
4. Choose comparison basis: manufacturer data, relevant table method, and measured-vs-design convention as applicable
5. Take and/or calculate readings with clear method notes
6. Record not just the result, but the basis used for judgement
7. If high, record likely cause and next action, not just unsatisfactory

EICR reporting: what good looks like

The gap between average and strong reports is usually traceability.

For each circuit, record:

• Measured or calculated Zs
• Comparison basis used, including whether measured-value adjustment convention was applied
• Protective device reference
• Any limitation, uncertainty, or follow-up action

If you want a faster comparison workflow with a visible basis trail, use the max Zs calculator

For full EICR workflow and cleaner schedules give TestFast a try on us

References

• IET Wiring Matters (2023), determining maximum earth fault loop impedance
• IET Wiring Matters (2024), why maximum earth fault loop impedance values differ
• Electrical Safety First master earth fault loop table PDF (reference only)