Ze Values UK: External Earth Loop Impedance by Earthing Type

Ze stands for external earth fault loop impedance. It is the impedance of the earth fault loop path that is external to the electrical installation — the part you, as the electrician, cannot modify. It represents the impedance of the supply system from the transformer to the origin of the installation and back.

Ze comprises three main components: the impedance of the supply transformer winding, the impedance of the line conductor from the transformer to the installation, and the impedance of the earth return path from the installation back to the transformer. The earth return path is what varies most between earthing arrangements — and this is why Ze differs significantly between TN-S, TN-C-S, and TT systems.

The importance of Ze is straightforward: it sets the baseline for every Zs measurement in the installation. Since Zs = Ze + (R1+R2) — worked through in full in our earth fault loop impedance calculation guide — a high Ze means less headroom for the circuit's own impedance (R1+R2) within the maximum permitted Zs. If Ze is unusually high, it can push every circuit in the installation towards or beyond the maximum, causing widespread compliance failures.

A TN-S (terre neutre separate) earthing arrangement provides a separate earth conductor from the supply transformer to the installation. In practice, this earth return path is the metallic sheath or armour of the supply cable — typically lead sheath on older cables or steel wire armour on newer ones.

TN-S systems tend to have higher Ze values than TN-C-S because the cable sheath has a smaller effective cross-sectional area and higher resistance than a dedicated PEN conductor. Older installations with ageing lead-sheathed cables can have Ze values approaching or exceeding 0.80 ohms, especially if there are corroded joints in the cable sheath. If Ze exceeds 0.80 ohms on a TN-S supply, the cable sheath may have deteriorated and the DNO should be queried.

With a Ze of 0.60 to 0.80 ohms, the available R1+R2 headroom for a B32 circuit (maximum corrected Zs of 1.10 ohms) is only 0.30 to 0.50 ohms. This limits the permissible cable length and may require larger conductor sizes than would be needed on a lower-Ze supply. Always check Ze first when testing an older TN-S installation — if it is high, expect some circuits to fail Zs testing.

A TN-C-S (terre neutre combined-separate) system, commonly called PME (Protective Multiple Earthing), is the most common earthing arrangement for modern domestic installations in the UK. The earth return path is via the combined neutral and earth (PEN) conductor in the supply cable. The PEN conductor is earthed at multiple points along the DNO network and at the supply transformer.

The low Ze of a PME supply provides excellent headroom for Zs compliance. With a typical Ze of 0.20 ohms, a B32 circuit has 0.90 ohms of R1+R2 headroom (corrected maximum Zs of 1.10 ohms minus Ze of 0.20). This allows comfortably long cable runs even with smaller conductor sizes.

However, PME systems have a specific limitation: if the PEN conductor is lost (for example, due to a DNO cable fault), the installation's earth potential can rise to a dangerous level. This is why BS 7671 imposes restrictions on bonding, extraneous conductive parts, and the use of PME earthing in certain locations (bathrooms in caravan parks, marinas, construction sites). These restrictions are separate from the Ze value itself but are important considerations when assessing a PME installation.

A TT (terre terre) earthing arrangement has no metallic earth connection between the installation and the supply transformer. Instead, the installation has its own earth electrode (typically a driven rod, plate, or tape buried in the ground), and the earth fault current returns to the supply transformer via the general mass of earth. The transformer has its own separate earth electrode.

The very high Ze of a TT system means that the fault current during an earth fault is far too low to trip an MCB within the required disconnection time. For example, with a Ze of 50 ohms, the fault current at 230V is only 4.6A — not enough to trip even a 6A Type B MCB within 0.4 seconds. This is why TT systems absolutely require RCD protection on all circuits. The RCD trips on the residual current (30mA) rather than the fault current magnitude, bypassing the Zs limitation.

For TT systems, BS 7671 Regulation 411.5.3 requires that the product of the earth electrode resistance (RA) and the RCD operating current (I delta n) does not exceed 50V. For a 30mA RCD: RA must not exceed 50 / 0.030 = 1667 ohms. This is easily achieved with any reasonable earth electrode. However, the lower the electrode resistance, the better — aim for below 200 ohms as a practical target.

Maximum Ze Values UK: TN-S, TN-C-S & TT (BS 7671)

Typical maximum Ze: TN-S 0.80 ohms, TN-C-S (PME) 0.35 ohms, TT 21 ohms. How to measure external earth fault loop impedance and when to query the DNO.

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Ze is measured at the origin of the installation — typically at the consumer unit or main distribution board. The procedure is a live test and requires care.

Voice entry for Ze and all test results

Speak your Ze reading — 'Ze, 0.24 ohms' — and Elec-Mate records it on the EICR. Continue speaking Zs, R1+R2, insulation resistance…

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Ze is determined by factors that are entirely outside the electrician's control — they are characteristics of the supply network. Understanding what affects Ze helps explain why readings vary between installations and why some properties consistently have higher values.

Because Ze is outside the electrician's control, it must be measured and accepted as a given. The installation design must work within the Ze constraints — selecting appropriate MCB types, cable sizes, and circuit lengths to achieve compliant Zs values on every circuit.

If your measured Ze exceeds the distributor-quoted typical maximum value for the earthing arrangement, it is a significant finding that requires action. A Ze above that figure means the supply earth may be compromised, and every circuit in the installation is affected.

A high Ze on a TN-S installation is particularly concerning because it may indicate that the cable sheath is deteriorating. In the worst case, the earth path could become completely open, leaving the installation with no effective earth connection. This is a C1 (Danger Present) defect if discovered.

The Distribution Network Operator (DNO) owns and maintains the supply cable, including the earth terminal. If you measure a Ze that exceeds the expected maximum for the earthing arrangement, or if you observe visible damage or deterioration to the supply earthing, you should query the DNO.

The UK DNOs have different names depending on the region: UK Power Networks (UKPN) in London and the South East, Western Power Distribution (now part of National Grid) in the South West and Midlands, Northern Powergrid in the North East, SP Energy Networks in Central and Southern Scotland, and Scottish and Southern Electricity Networks (SSEN) in the North of Scotland and South of England. Contact details are on the electricity meter or on the DNO's website.

Record the DNO query on the EICR under "Observations" with a note of the measured Ze value and the action taken.

Record Ze, PFC, and all supply characteristics digitally

Elec-Mate captures the full supply characteristics section of the EICR — earthing arrangement, Ze, PFC, supply voltage…

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For new installations requiring a new electrical supply, the procedure for establishing Ze is different from measuring it at an existing installation. OSG Reg 1.3 requires that, before starting work, the installer shall establish with the local electricity distributor the typical maximum Ze of the earth fault path outside the consumer's installation. This is a distinct workflow from on-site Ze measurement — the value comes from the DNO, not from a tester.

This two-stage approach — obtain from DNO for design, measure at commissioning — ensures that circuits are designed to work within the worst-case supply impedance and that the as-built installation is verified against it. For periodic inspection of an existing installation, only the measured Ze is relevant; the DNO-quoted figure is a design tool, not a pass/fail threshold for condition reporting.