Maximum Ze for TN-S: Is 0.80 Ω the Limit?

The commonly quoted maximum Ze for a TN-S supply is 0.80 Ω. But calling it a hard pass or fail limit is too simple. It is a conventional distributor-quoted reference for ordinary UK supplies, commonly stated for supplies up to 100 A, while the actual Ze at an installation still needs to be established and recorded.

The number also is not the maximum Zs for a circuit. Ze is the external part of the earth fault loop. Zs includes Ze plus the resistance of the circuit conductors, so the protective device and required disconnection time still decide whether a circuit is satisfactory.

Here is the practical way to read the figure, deal with a higher result and record it properly.

The short answer: maximum Ze for TN-S

For a conventional UK reference point, use 0.80 Ω for TN-S.

That figure is best treated as a supply characteristic reference, not a universal statutory cap that automatically passes or fails every TN-S installation.

For comparison, commonly quoted UK reference figures are:

These figures are not interchangeable. They describe the expected external supply path. They do not replace the circuit-level maximum Zs value for a particular protective device.

What Ze means on a TN-S supply

Ze is the external earth fault loop impedance. It covers the part of the fault path outside the installation, including the supply network and the return path to the source. On a TN-S supply, that return path is typically associated with the metallic sheath or armour of the supply cable.

The exact arrangement varies, so do not treat the cable-sheath description as a guarantee about every TN-S service. The useful distinction is simpler:

• Ze is the external supply impedance, measured or established at the origin.
• R1 + R2 is the resistance of the line conductor and circuit protective conductor for the circuit being tested.
• Zs is the total earth fault loop impedance for that circuit.

The usual relationship is:

Zs = Ze + (R1 + R2)

That is why the supply reading matters to every circuit. A higher Ze leaves less impedance headroom for the circuit conductors before the circuit reaches its own permitted Zs value.

Why is the TN-S figure commonly 0.80 Ω?

The 0.80 Ω convention reflects expected supply-network characteristics and the assumptions used for ordinary metered supplies. It is not a number selected as a universal engineering limit for every individual TN-S service.

This is also why quick-reference pages can cause confusion. They often put “maximum” in the heading, then leave the scope and qualification in the small print. The useful wording is: 0.80 Ω is the commonly quoted TN-S external Ze reference, subject to the supply arrangement and applicable guidance.

Industry discussions about the history of the 0.80 Ω and 0.35 Ω values can help explain the background, but forum explanations should not be treated as the formal source of the rule. If a DNO or designer has provided a supply-specific value, keep that evidence with the design or installation records.

Is 0.80 Ω a legal or BS 7671 pass/fail limit?

Not by itself.

A Ze reading above 0.80 Ω is a reason to stop and verify the result, not a shortcut to an automatic EICR code. The conclusion depends on several things:

• whether the earthing arrangement has been identified correctly
• whether the test method and instrument setup were suitable
• whether the reading is repeatable
• what Zs values are present on the affected circuits
• what protective devices and disconnection times apply
• whether the supply-side earth appears defective or outside the expected arrangement

For a new supply, a DNO-quoted or design value may be relevant. On an existing installation, the measured result establishes the condition found at the installation. Those are related, but they are not the same piece of evidence.

Do not disturb sealed DNO equipment. If the result remains unexpectedly high after the electrician's checks, raise it with the DNO or relevant distribution network operator and record what was queried and why.

How Ze affects Zs and circuit compliance

The 0.80 Ω TN-S reference is not the number you compare against every circuit. The circuit check is based on Zs and the protective device's permitted value for the required disconnection time.

The IET explains that maximum permitted Zs depends on the protective device, its operating characteristics, the required disconnection time and, where available, manufacturer data. A 32 A Type B MCB, a Type C MCB, an RCBO and a fuse do not automatically share the same permitted Zs.

The practical effect of Ze is easiest to see with a simple comparison. If two otherwise similar installations have the same circuit resistance, a TN-S supply at 0.80 Ω starts with 0.45 Ω more external impedance than a TN-C-S supply at 0.35 Ω. That difference comes straight out of the circuit's available Zs headroom.

So the workflow is:

1. establish the earthing arrangement and actual Ze
2. identify the protective device and required disconnection time
3. measure or calculate the circuit Zs
4. compare it with the correct maximum permitted or manufacturer value
5. record the basis for the comparison

For the circuit-level lookup, use the Max Zs calculator or the maximum Zs values explained article. Do not use 0.80 Ω as a substitute for that check.

What to do if your TN-S Ze reading is above 0.80 Ω

A high reading needs a measured response, not a number-only verdict.

The exact testing procedure belongs in the current On-Site Guide, Guidance Note 3, your competent-person practice and the instrument manufacturer's instructions. This article is not a substitute for those procedures.

What a high Ze does not prove

A high Ze does not automatically prove that:

• every circuit fails
• the installation must receive a particular EICR code
• the DNO will replace equipment
• the test result is valid without verification

It does prove that the supply characteristic needs to be understood before the circuit results are judged. The safety consequence is found by checking the affected circuits and their protective arrangements.

How to record Ze in an electrical report

Record the supply information so that another competent person can understand what was measured and what it was used for.

At minimum, keep the following together:

• earthing arrangement, such as TN-S
• measured or established Ze
• the location and basis of the reading
• measured or calculated circuit Zs values
• protective device and required disconnection time
• the maximum permitted or manufacturer comparison value
• relevant observations, limitations and any DNO query

Avoid coding the Ze number alone. An EICR observation depends on the safety consequence and the applicable requirements, not simply on whether one reference figure has been exceeded.

This is where consistent recording earns its keep. Once the actual Ze is known, the important thing is preserving the relationship between the supply reading, circuit result, comparison basis and conclusion. TestFast can help keep those test values and report notes together without turning the supply reference into a false pass or fail rule.

TN-S, TN-C-S and TT: do not mix the assumptions

The three arrangements are often shown side by side in quick-reference material, but the values mean different things in practice.

• TN-S: commonly quoted as 0.80 Ω for the external Ze reference in ordinary UK supply contexts.
• TN-C-S / PME: commonly quoted as 0.35 Ω, with different supply and bonding considerations.
• TT: often shown as 21 Ω in reference tables, but the assessment depends on the electrode resistance, RCD operation and disconnection requirements. Do not assess TT by applying TN-S assumptions.

If the arrangement is unclear, identify that first. A neat-looking number in the wrong earthing-system column is still the wrong answer.

FAQ

What is the maximum Ze for TN-S?

The commonly quoted UK reference is 0.80 Ω for ordinary TN-S supplies, often stated for supplies up to 100 A. It is a conventional supply reference, not a universal statutory cap. The actual Ze still needs to be established and considered with the circuit results.

What is the difference between Ze and Zs?

Ze is the external earth fault loop impedance at the origin. Zs includes Ze plus the circuit's R1 + R2. A circuit can therefore have a satisfactory Ze but an excessive Zs if its conductors or connections add too much resistance.

What if my TN-S Ze reading is 1.0 Ω?

Verify the earthing arrangement, test method and repeatability first. Then check the affected circuits against the correct protective-device and disconnection-time values, record the result and query the DNO where the external value remains unexpectedly high. Do not declare an automatic failure from the Ze number alone.

Does a high Ze make every circuit fail?

No. It reduces the available headroom in the Zs calculation and may cause individual circuits to exceed their permitted values. The circuit-by-circuit results and protective arrangements decide the consequence.

The practical takeaway

Use 0.80 Ω as the commonly quoted TN-S Ze reference, but do not treat it as a standalone pass or fail limit.

The useful sequence is:

identify the system → establish Ze → verify any high result → check circuit Zs → record the comparison basis → escalate where necessary

For further reading, see the IET's guides on determining maximum earth fault loop impedance for protective devices and why maximum earth fault loop impedance values differ. For background on how the conventional Ze figures are discussed in relation to supply networks, see the IET EngX discussion on max Ze values from the DNO. The discussion is useful context, not a substitute for current formal guidance.