What is the difference between TN-S and TN-C-S earthing?

In a TN-S system, the neutral conductor (N) and the protective earth conductor (PE) are separate throughout the entire network, from the transformer to the consumer's installation. The installation earth terminal connects to a dedicated PE conductor. In a TN-C-S system (commonly called PME — Protective Multiple Earthing), the neutral and protective earth are combined as a single PEN conductor in the distribution network. At the consumer's installation, the PEN conductor is split into separate N and PE conductors. TN-C-S is now the most common system for new domestic connections in the UK.

What is PME earthing and why does it have limitations?

PME (Protective Multiple Earthing) is the earthing arrangement used in TN-C-S systems. The distribution network's combined PEN conductor is connected to earth at multiple points (at the substation and at consumers' installations). The consumer's installation earth is taken from the DNO's PME terminal. The limitation of PME arises from the risk of a broken neutral: if the PEN conductor breaks upstream of the consumer, the consumer's earth terminal — and everything connected to it — can rise to line voltage potential relative to true earth. This is why PME earthing must not be extended to locations where people or livestock might simultaneously contact the PME earth and true earth (outbuildings, caravans, swimming pools, marinas).

When is a TT earthing system required?

A TT system (where the installation earth is provided by a local earth electrode rather than the DNO's earth terminal) is required in several situations: where the DNO cannot provide a PME terminal; for outbuildings that are separate structures; for caravans and static caravans; for marinas and caravan parks; for agricultural premises where livestock may contact earthed equipment; and for any installation where extending PME earthing would create an unacceptable risk. The installation of a TT system requires a suitable earth electrode (usually a copper earth rod) and RCD protection to ensure disconnection times are achieved given the typically higher earth loop impedance.

Why is earth loop impedance (Zs) different between TN-S and TT systems?

Earth loop impedance (Zs) is the impedance of the fault current loop from the source, through the protective conductors, through the fault, and back to the source. In TN-S and TN-C-S systems, the fault current returns via low-impedance metallic conductors, resulting in Zs values typically in the range of 0.1 to 1.0 ohms. In TT systems, the fault current returns via the soil between the installation earth electrode and the DNO's substation earth. Soil impedance is much higher — TT system Zs values are typically 20 to 200 ohms or more. This high impedance means overcurrent devices (MCBs/fuses) cannot reliably clear faults, which is why TT systems always require RCD protection for disconnection.

How do I identify which earthing system I have?

The most reliable method is to check the DNO's supply head (cut-out). A TN-S supply will have a separate earth terminal and neutral terminal at the cut-out, with the earth connection going to the PE conductor. A TN-C-S (PME) supply will have a PME earth terminal clearly marked, usually with a label. A TT supply will have no DNO-provided earth terminal — the installation will have its own earth electrode, typically an earth rod driven into the ground near the consumer unit. You can also contact the DNO to confirm the supply type for a specific address.

Can I extend PME earthing to my garden building?

No. PME earthing must not be extended to outbuildings, garden offices, sheds, garages (that are separate structures), or any building that is a separate structure from the main property. This is not merely best practice — it is a requirement of the Electricity Safety, Quality and Continuity Regulations 2002 and is reinforced by BS 7671. The outbuilding must be supplied via a TT system using its own earth electrode, or via an insulating separation supply, or via a separation supply. The supply cable between the main building and outbuilding must have no metallic armour or screen connected to the main building's earth.

What earthing arrangement is required for EV charge points?

EV charge point earthing depends on the supply earthing system. For TN-C-S (PME) supplies, BS 7671 18th Edition requires either: an earth electrode at the charge point location that provides a combined PME and electrode earthing arrangement (to limit touch voltage if the neutral is lost), or confirmation that the charge point has an integral protective measure against loss of earth (some devices provide this automatically). For TT supplies, the charge point must be on the TT earth system. The specific requirements are detailed in the EV charging regulations section of BS 7671 and the relevant IET Code of Practice for Electric Vehicle Charging Equipment Installation.

WIRING GUIDE

UK Earthing Systems Guide: TN-S, TN-C-S and TT Explained

A complete practical guide to earthing systems used in UK electrical installations — TN-S, TN-C-S (PME) and TT explained with BS 7671 requirements, PME limitations, EV charging earthing, outbuilding supplies, and earth loop impedance differences between systems.

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13 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

1The earthing system type determines how fault current returns to the source and directly affects earth loop impedance, disconnection times, and the type of protective measures required.
2TN-S systems have separate neutral (N) and protective earth (PE) conductors throughout — typically found in older properties served by concentric wiring or properties with a separate earth terminal at the cut-out.
3TN-C-S (PME) systems combine N and PE in the network (PEN conductor) and split them at the consumer's installation — the most common system in the UK, supplied by the DNO's PME terminal.
4TT systems use a local earth electrode — required for outbuildings, caravans, marinas, and any installation where the PME earth terminal is not available or suitable.
5PME earthing must not be extended to outbuildings, caravans, static caravans, or locations accessible to livestock due to the risk of dangerous touch voltages under open-circuit neutral conditions.
6EV charge points require specific earthing considerations — TN-C-S supplies need a PME earth electrode system or PME earthing prohibition check; TT supplies may require an earth electrode at the charge point.

01 · Wiring Guide

What Is an Earthing System?

An earthing system is the arrangement by which the electrical installation is connected to earth (the general mass of the earth) to ensure that exposed metallic parts of equipment and the installation are maintained at earth potential under fault conditions. Earthing provides a low-impedance path for fault current to flow, enabling protective devices (MCBs, fuses, RCDs) to operate and disconnect the supply before dangerous voltages or temperatures develop.

Protection against electric shock — if a live conductor contacts exposed metalwork (a fault to earth), the earthing system ensures fault current flows via the protective conductor rather than through a person who touches the metalwork. The fault current is high enough to operate the protective device quickly.
Protection against fire — by ensuring rapid disconnection of earth faults, the earthing system limits the duration and magnitude of arcing that could ignite surrounding materials.
BS 7671 classification — BS 7671 18th Edition classifies earthing systems using a two or three-letter code based on IEC 60364 standards. The letters describe the relationship between the supply source and earth, and the relationship between the installation's exposed conductive parts and earth.

The three earthing system types used in UK public electricity supply installations are TN-S, TN-C-S, and TT. Each has different characteristics affecting earth loop impedance, the types of protective measures required, and restrictions on where the earthing arrangement can be extended.

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02 · Wiring Guide

TN-S — Separate Neutral and Protective Conductors

In a TN-S system, the supply source (transformer) has one point directly earthed, and the exposed conductive parts of the installation are connected to that earth point via a protective conductor (PE) that is entirely separate from the neutral conductor (N). The two conductors are separate throughout the entire network.

Where it is found — TN-S is increasingly rare in new UK installations. It is typically found in older properties served by concentric wiring (where the outer sheath of the service cable acts as the PE), in properties with an armoured service cable where the armouring is used as the PE, and in some industrial and commercial installations with dedicated earth conductors.
Identification — at the DNO cut-out, a TN-S supply will have separate earth and neutral terminals. The earth terminal is connected to the cable's metallic sheath or armour rather than the neutral conductor. The neutral and earth are not bonded together at the cut-out.
Earth loop impedance — TN-S systems typically have Zs values in the range of 0.1 to 0.8 ohms at the consumer's installation, depending on the cable length and cross-sectional area. This relatively low impedance means MCBs and fuses can reliably clear earth faults within the required disconnection times.
Degradation risk — in concentric wiring systems, the outer sheath (PE conductor) can corrode over time. This is an important inspection point during EICRs on older properties — a corroded sheath increases Zs and may compromise the effectiveness of overcurrent device tripping under fault conditions.

03 · Wiring Guide

TN-C-S — PME (Protective Multiple Earthing)

In a TN-C-S system, the neutral and protective earth are combined as a single PEN (Protective Earth and Neutral) conductor in the distribution network. At the consumer's installation, the PEN conductor is split into separate N and PE conductors. This arrangement is called Protective Multiple Earthing (PME) and is by far the most common earthing system for new domestic connections in the UK.

DNO PME terminal — the DNO provides a combined N/E terminal at the cut-out, usually labelled "PME Earth Terminal" or marked with the combined N/PE symbol. The installation's main earthing terminal and the neutral are both connected to this terminal and bonded together within the installation.
Low earth loop impedance — TN-C-S systems typically have Zs values of 0.1 to 0.6 ohms, similar to TN-S. This allows MCBs and fuses to clear earth faults reliably. RCDs are required for additional protection on specified circuits (socket outlets, etc.) but are not the primary means of earth-fault disconnection.
Open-circuit neutral risk — the critical limitation of PME is that if the PEN conductor breaks upstream of the consumer, the consumer's earth terminal can rise to line voltage. Everything connected to the installation's earth — exposed metalwork, bonded pipework — becomes live relative to true earth. This is the reason for the restrictions on extending PME to certain locations.

04 · Wiring Guide

TT — Earth Electrode System

In a TT system, the installation's earth is provided by a local earth electrode (typically a copper earth rod driven into the ground) that is entirely independent of the DNO's neutral. The supply source is earthed at the substation, but the consumer's installation has no metallic connection to that earth — fault current must return via the soil.

Where it is required — TT is required for outbuildings (separate structures), caravans, static caravans, caravan parks, marinas, agricultural premises, and any location where PME earthing is prohibited or the DNO cannot provide an earth terminal. Some rural properties on overhead supply lines may also have TT systems.
High earth loop impedance — because fault current returns via soil rather than a metallic conductor, Zs values on TT systems are typically 20 to 200 ohms or higher, depending on soil type, moisture content, and electrode design. At these impedances, overcurrent devices (MCBs) cannot operate within the required disconnection times — RCD protection is mandatory on TT systems for this reason.
Earth electrode testing — the earth electrode resistance must be measured during installation and at each EICR. The electrode resistance (Ra) in ohms multiplied by the RCD rating (IΔn) in amps must not exceed 50V, per BS 7671. For a 30mA RCD: Ra × 0.03 ≤ 50, meaning Ra must not exceed approximately 1,667 ohms — achievable with a well-installed electrode, though lower is always better.
No bonding to main building's earth — where a TT outbuilding is fed from a TN-C-S main building, the supply cable must have no metallic armour or screen connected to the main building's earth. An SWA cable's armouring must be isolated at the outbuilding end, or an XLPE/LSF cable in plastic conduit used.

05 · Wiring Guide

PME Conditions and Limitations

The Electricity Safety, Quality and Continuity Regulations 2002 (ESQCR) and BS 7671 both place restrictions on where PME earthing can and cannot be extended. These limitations exist because of the open-circuit neutral risk inherent in TN-C-S systems.

Outbuildings — PME earthing must not be extended to a structure that is a separate building from the main installation. Even a detached garage requires a TT supply or an alternative protective measure. This is one of the most commonly misunderstood requirements in domestic electrical work.
Caravans and mobile homes — PME earthing is prohibited for caravan supplies. Static caravans and touring caravans require TT supplies with RCD protection. The caravan industry standard (BS EN 60309 / BS 7671 Part 7) reflects this requirement.
Livestock locations — agricultural premises where livestock have access to earthed metalwork must not use PME earthing. Animals are more susceptible to electric shock than humans (lower body resistance, multiple contact points) and may receive dangerous touch voltages under PME open-neutral conditions.
Marinas and boatyards — shore supply to boats must use TT or isolation transformer supplies. Extending PME to marina pontoons creates stray current corrosion and touch-voltage risks.

06 · Wiring Guide

Earthing Systems and EV Charging

EV charge points have specific earthing requirements that depend on the installation's earthing system. BS 7671 18th Edition and the IET Code of Practice for Electric Vehicle Charging Equipment Installation both address these requirements in detail.

TN-C-S (PME) supplies — an earth electrode must be installed at the charge point location, bonded to the charge point's earth terminal, to provide a combined PME/electrode earthing arrangement. This limits the touch voltage to a safe level if the neutral is lost. Alternatively, charge points with integral protective measures against loss-of-earth (including automatic disconnection) may be used without an additional electrode, subject to the device's documentation.
TT supplies — the charge point must be on the TT earth system. A Type A or Type B RCD must protect the charge point circuit. The earth electrode resistance must be verified to ensure the RCD trip condition (Ra × IΔn ≤ 50V) is met.
RCD type for EV circuits — EV charge point circuits require a Type A or Type B RCD (not Type AC) because the charging electronics can produce pulsating DC residual currents that Type AC RCDs cannot detect reliably.

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07 · Wiring Guide

Outbuildings, Caravans and TT Earthing

Supplying a separate outbuilding (garden office, garage, workshop, studio) is one of the most common domestic electrical jobs that requires a TT earthing system. Getting the earthing arrangement wrong is one of the most common and dangerous errors in this type of work.

Earth rod installation — the earth rod should be driven vertically into the ground to a sufficient depth to achieve adequate electrode resistance (typically 1.2m or more, with multiple rods or horizontal electrodes where soil resistance is high). The rod must be accessible for testing.
Supply cable selection — use a cable with no metallic sheath or armour connected to the main building's earth terminal. An unarmoured cable in a protective conduit, or an SWA cable with the armouring isolated at the outbuilding end, are both acceptable. Never connect SWA armouring to both the main building's PME earth and the outbuilding's TT earth.
RCD protection mandatory — all circuits in TT outbuildings must be protected by RCDs because the high earth loop impedance prevents MCBs from clearing earth faults within required times. An RCCB or RCBOs at the outbuilding distribution board is essential.
Certification — an outbuilding supply installation is notifiable work requiring an Electrical Installation Certificate (EIC). The test results must include electrode resistance measurement, Zs for each circuit, and RCD test results.

08 · Wiring Guide

Earth Loop Impedance Differences Between Systems

Earth loop impedance (Zs) is the total impedance of the fault current loop and is fundamental to determining whether protective devices will operate within the required disconnection time under fault conditions. The earthing system has a major influence on Zs.

TN-S and TN-C-S — Zs is typically 0.1 to 1.0 ohm for final circuits. Low Zs means high fault current, which reliably trips MCBs within 0.4 seconds (for circuits up to 32A) or 5 seconds (for distribution circuits). RCDs are not needed as the primary means of disconnection, but are required for additional protection on specified circuit types.
TT systems — Zs is typically 20 to 200 ohms or more. At these impedances, a phase-to-earth fault would produce only 1.2 to 12 amperes of fault current — far below the trip threshold of any MCB or fuse. RCDs are therefore the primary means of disconnection in TT systems.
Impact on EICR assessment — measured Zs values must be compared against the maximum Zs values in BS 7671 Appendix 3 for the specific protective device. A TT system's Zs values will always appear extremely high in absolute terms but are acceptable because RCD protection is confirmed to operate correctly at those impedances.

09 · Wiring Guide

Identifying the Earthing System on Site

Correctly identifying the earthing system is an essential step before carrying out any electrical installation work or EICR. Misidentifying the earthing system can lead to incorrect protective measures being specified.

Check the cut-out and meter — the supply head (cut-out) and meter cabinet usually show the earthing arrangement. A clearly labelled PME terminal indicates TN-C-S. A separate earth terminal connected to cable sheath or armour indicates TN-S. No DNO earth terminal indicates TT.
Contact the DNO — for certainty, contact the Distribution Network Operator (Western Power Distribution, UK Power Networks, Northern Powergrid, etc.) and request confirmation of the earthing system for the specific address. This is the most reliable method and is particularly important for commercial installations.
Check previous certificates — the earthing system should be recorded on previous EICRs and EICs for the property. However, always verify physically — the system may have changed or been recorded incorrectly.

10 · Wiring Guide

For Electricians: Earthing System Documentation

The earthing system must be correctly identified and documented on the EICR and EIC. Errors in earthing system identification are among the most consequential mistakes on electrical certificates, as they affect the interpretation of all Zs measurements and the assessment of protective device adequacy.

Record Earthing System on Certificate

The earthing system type (TN-S, TN-C-S, TT) must be recorded on the EIC or EICR, along with the method of earthing (DNO terminal, earth electrode, etc.) and the measured earth electrode resistance where applicable. Use the Elec-Mate app to complete all sections correctly with built-in guidance.

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Frequently Asked Questions About UK Earthing Systems

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UK Earthing Systems Guide: TN-S, TN-C-S and TT Explained

Key Takeaways

What Is an Earthing System?

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TN-S — Separate Neutral and Protective Conductors

TN-C-S — PME (Protective Multiple Earthing)

TT — Earth Electrode System

PME Conditions and Limitations

Earthing Systems and EV Charging

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Outbuildings, Caravans and TT Earthing

Earth Loop Impedance Differences Between Systems

Identifying the Earthing System on Site

For Electricians: Earthing System Documentation

Record Earthing System on Certificate

Complete EICRs and EICs with correct earthing system

Frequently Asked Questions About UK Earthing Systems

What is the difference between TN-S and TN-C-S earthing?

What is PME earthing and why does it have limitations?

When is a TT earthing system required?

Why is earth loop impedance (Zs) different between TN-S and TT systems?

How do I identify which earthing system I have?

Can I extend PME earthing to my garden building?

What earthing arrangement is required for EV charge points?

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