What is the difference between TN-S and TN-C-S earthing in an industrial context?

In a TN-S system, the protective earth (PE) conductor and the neutral (N) conductor are kept separate throughout the installation. The transformer neutral is earthed at the substation, and a separate PE conductor is run to all equipment. This is the safest arrangement for industrial premises as there are no stray currents on the PE conductor and no risk of the PE becoming live if the neutral conductor fails. In a TN-C-S (PME) system, the supply cable uses a combined PEN conductor (earth and neutral in one conductor). At the point of entry to the premises, PEN is separated into PE and N. The risk in TN-C-S is that if the PEN conductor fails upstream, the neutral terminal in the installation rises to line voltage — any metalwork connected to PE becomes live at line voltage. This is a particular concern in industrial premises with large unbalanced single-phase loads.

When is a TT earthing system required for an industrial installation?

A TT earthing system (using an earth electrode with no metallic connection to the supply earth) is used where the DNO cannot provide an earth terminal (common in rural areas), where a private HV/LV transformer is used and the designer chooses to create a separate TT earth, or where the installer wants to avoid the risks associated with TN-C-S PME earthing (e.g., in agricultural premises or marinas where the BS 7671 Appendix 11 prohibition of PME applies). TT systems require RCD protection on all circuits and a sufficiently low earth electrode resistance to ensure disconnection time requirements are met (Ra × IΔn ≤ 50 V).

What is supplementary equipotential bonding and when is it required?

Supplementary equipotential bonding (SEB) connects all simultaneously accessible extraneous conductive parts (metal pipework, structural steelwork, machinery frames, cable trays) to the main protective conductor, ensuring they are all at the same potential. BS 7671 Regulation 415.2 requires SEB where the automatic disconnection requirements cannot be met within the required time, or where touch voltage could exceed 50 V under fault conditions. In industrial process areas, SEB is commonly required in: switchrooms and motor control centres, compressor rooms, chemical process areas with conductive pipework, and anywhere that the earth fault loop impedance is high enough that disconnection time cannot be guaranteed within limits.

What is star-point earthing for electronic equipment and why does it matter?

Star-point (radial) earthing connects each item of electronic equipment back to a single common earth reference point using individual earth conductors that radiate like spokes from the central point. This prevents earth return currents from one piece of equipment flowing through the earth connections of adjacent equipment, which would create voltage differences and introduce noise into sensitive circuits. Star-point earthing is recommended for PLC systems, power quality analysers, instrumentation, and communications equipment. It must ultimately connect to the main earthing terminal of the installation — it cannot be a floating separate earth, which would be dangerous and non-compliant with BS 7671.

How does lightning protection integrate with the electrical earthing system?

BS EN 62305-3 (Lightning Protection — Physical damage to structures) requires the lightning protection system (LPS) earth termination network to be bonded to all other earthing systems in and around the structure, including the electrical installation main earth bar, structural steelwork, water and gas pipework entry points, and telecommunications cable screens. The purpose is to ensure that when a lightning strike current flows, all earthed metalwork in the building rises to the same potential simultaneously — preventing dangerous potential differences (side flash) between the LPS and nearby conductive parts. Surge protective devices (SPDs) at the main distribution board are also required to protect equipment from conducted lightning surges entering via the supply cables.

What earth electrode resistance values are required for industrial installations?

The required earth electrode resistance depends on the earthing system and the fault protection method. For TT systems: Ra × IΔn ≤ 50 V, where Ra is the electrode resistance and IΔn is the RCD operating current. With a 30 mA RCD, Ra must be ≤ 1667 Ω (in practice aim for less than 200 Ω for safety margin). For lightning protection: BS EN 62305-3 recommends less than 10 Ω for each earth termination electrode, with interconnection of all electrodes to achieve the lowest practicable overall resistance. For functional earthing of electronic equipment, the lower the impedance the better — typically aim for less than 1 Ω at power frequency. Earth electrode resistance is measured using a four-terminal earth resistance test set (fall-of-potential method).

What test equipment is needed for industrial earthing system testing?

The principal instruments required are: an earth resistance tester (4-terminal Wenner method or 3-terminal fall-of-potential method) for measuring earth electrode resistance; a clamp-type earth resistance tester (non-disruptive method for testing electrodes that cannot be disconnected from the system); a loop impedance tester with a high current output (typically 20–25 A) for measuring earth fault loop impedance in industrial premises with low loop impedance; a phase rotation meter for confirming phase sequence; and a power quality analyser for measuring neutral-to-earth voltage (which indicates stray currents or earthing problems in TN-C-S systems). All instruments must be calibrated and used in accordance with BS EN 61557 (Electrical safety in low-voltage distribution systems).

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Industrial Earthing Systems UK: Factory Earthing Guide

Comprehensive guide to industrial earthing systems in the UK — TN-S, TN-C-S (PME), and TT earthing for factories and process plant, supplementary equipotential bonding, EMC star-point earthing for electronic equipment, lightning protection integration to BS EN 62305, and earthing measurement and testing methods.

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15 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

1TN-S (Protective Earth and Neutral separated throughout) is the preferred earthing system for large industrial premises because it provides a low-impedance earth path and avoids stray currents on the protective conductor.
2TN-C-S (PME) introduces multiple earth references along the combined PEN conductor — in industrial premises with significant single-phase loads this can create hazardous touch voltages on exposed metalwork under PEN conductor failure.
3TT systems (earth electrode only, no metallic connection to the supply earthing) require RCDs on all circuits and a low earth electrode resistance (typically less than 200 Ω for 30 mA RCD protection).
4Supplementary equipotential bonding in process areas (compressor rooms, switchrooms, chemical plant) connects all simultaneously accessible metal parts to the same potential, reducing touch voltage to safe levels even under fault conditions.
5EMC earthing (star-point earthing for electronic equipment, separate analogue and digital earth bars) must be integrated with the main protective earthing system — they cannot be independent floating references in most industrial installations.
6Lightning protection systems (LPS) designed to BS EN 62305 must be bonded to the main earth bar; the SPD (surge protective device) installation at the main distribution board is also part of the integrated earthing strategy.

01 · Industrial Guide

Earthing System Types in UK Industrial Premises

The earthing system defines how the supply source (transformer neutral or generator neutral) is connected to earth, and how exposed conductive parts of equipment are connected to earth. BS 7671 adopts the IEC system of designations using two-letter codes. Understanding which earthing system is in use is fundamental to designing fault protection, selecting protective devices, and ensuring safety in industrial premises.

TN-S — Terre-Neutre Séparé. The supply neutral is earthed at the source (transformer). A separate protective earth (PE) conductor, independent of the neutral, is provided throughout the installation. Preferred for large industrial premises with private HV/LV substation.
TN-C-S — Terre-Neutre Combiné-Séparé. The supply uses a combined PEN conductor (protective and neutral combined). At the point of entry to the premises (or at the main distribution board), PEN is separated into PE and N. Also known as PME (Protective Multiple Earthing) in the UK. The most common system for DNO-supplied commercial premises.
TT — Terre-Terre. The supply neutral is earthed at the source, but the installation earth uses a local earth electrode with no metallic connection to the supply earth. Used in agricultural premises, marinas, caravan sites, and rural installations where PME is prohibited or unavailable.
IT — Isolé-Terre. The supply is not connected to earth (isolated neutral). Used in medical locations (operating theatres, cardiac care) and some marine applications. A first fault does not cause disconnection, but an insulation monitoring device (IMD) must be fitted to detect the first fault.

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02 · Industrial Guide

TN-S Earthing for Industrial Premises

TN-S is the preferred earthing system for large industrial premises operating their own HV/LV substation. With a private transformer, the installer has control over the earthing arrangement and can provide a clean, low-impedance TN-S earth without the complications of PME. The transformer neutral is connected to an earth electrode at the substation, and a copper PE conductor of adequate size is run from the main earth bar to all distribution boards and equipment.

Earth conductor sizing — BS 7671 Regulation 543.1 requires the protective conductor to be sized in accordance with Table 54.7 (adiabatic equation) or Table 54.2 (simplified method). For industrial supply cables, the PE conductor is typically 50–300 mm² copper depending on the phase conductor size and the expected fault current.
Main earth bar (MEB) — all earthing conductors, bonding conductors, and protective conductors must terminate at a single main earthing terminal (MET). The MET is connected to the transformer neutral earth via the main protective conductor. The MET must be accessible for testing and clearly labelled.
Main protective bonding — all extraneous conductive parts entering the building (water pipes, gas pipes, structural steelwork, air conditioning pipework, compressed air lines) must be connected to the MET with main bonding conductors. Size per BS 7671 Table 54.8 — typically 10 mm² copper minimum for domestic, 25 mm² or more for industrial.
Electrode design — the substation earth electrode must achieve a resistance low enough to ensure fault current is sufficient for protective device operation. For TN-S, the electrode resistance contributes to the earth fault loop impedance; this must not be so high that disconnection times exceed BS 7671 requirements.

03 · Industrial Guide

TN-C-S (PME) Considerations in Industrial Premises

TN-C-S (PME) is the DNO's standard earthing arrangement for most UK commercial and industrial premises supplied at LV. The DNO connects the cable PEN conductor to earth at multiple points along the distribution network and provides an earthing terminal (the DNO earth) at the cut-out. PME is convenient but introduces specific risks in industrial premises that the designer must address.

PEN conductor failure risk — if the PEN conductor fails between the last earth connection and the premises, the installation neutral floats to line voltage. All metalwork connected to the PME earth becomes live at line voltage. Unbalanced single-phase loads in industrial premises can cause significant neutral current — increasing the risk of PEN conductor failure through overloading or corrosion.
Where PME is prohibited — BS 7671 Appendix 11 prohibits the use of PME earthing in agricultural premises, caravan parks, marinas, and locations accessible to livestock. In these locations, a TT earthing system with local earth electrode must be used.
Mitigation measures — where PME is used in industrial premises with large unbalanced loads, consider: monitoring the neutral-to-earth voltage (alarm if it exceeds 10 V, which indicates stray neutral current on the PE conductor); ensuring all three-phase loads are balanced; limiting the proportion of single-phase load; and considering a private earth electrode as a supplementary earth in addition to the DNO PME earth.

04 · Industrial Guide

TT Systems in Industrial Premises

TT systems use a local earth electrode to provide the installation earth with no metallic connection back to the supply transformer neutral. This avoids the risk of PME PEN conductor failure, but introduces the challenge of achieving a sufficiently low earth electrode resistance and the requirement for RCD protection on all circuits.

RCD requirement — BS 7671 Regulation 411.5.2 requires that in TT systems, the product Ra × IΔn ≤ 50 V, where Ra is the sum of resistances of the earth electrode and protective conductor, and IΔn is the residual operating current of the RCD. In practice, RCDs must be fitted on all circuits in a TT system to ensure the 50 V safety criterion is met.
Earth electrode installation — for TT systems in industrial premises, deep driven rod electrodes (typically 1.2–2.4 m copper-clad steel rods, or sectional rods driven to 6 m or more in high-resistivity soil) or ring electrodes are used. In high-resistivity ground (chalk, rock), chemical earth electrodes or deep bore electrodes may be required to achieve adequate resistance.
Electrode resistance testing — earth electrode resistance must be measured at installation and periodically thereafter using the three-terminal fall-of-potential method (BS EN 61557-5). The test must be carried out with any test spikes at a sufficient distance from the electrode to avoid interference. Results should be recorded and trended over time.

05 · Industrial Guide

Supplementary Equipotential Bonding in Process Areas

In industrial process areas, multiple items of plant and pipework are simultaneously accessible. Under earth fault conditions, touch voltages between different items of plant can exceed safe limits unless supplementary equipotential bonding is provided. BS 7671 Regulation 415.2 permits supplementary bonding as an alternative to achieving automatic disconnection within the required time.

What to bond — all simultaneously accessible extraneous conductive parts: process pipework (steam, water, chemical), structural steelwork, vessel frames, conveyor structures, cable tray/ladder, and exposed conductive parts of equipment. Bonding must be to the protective conductor of the circuit supplying the equipment (not to a remote earth point).
Conductor sizing — supplementary bonding conductors must meet the requirement of BS 7671 Regulation 415.2.2: the resistance R of the bonding conductor must satisfy R ≤ 50 V / Ia, where Ia is the operating current of the protective device. In practice, supplementary bonding conductors in industrial premises are typically 4–16 mm² copper, mechanically protected (clipped or in conduit) and labelled "Safety Electrical Connection — Do Not Remove".
Insulating flanges — process pipework sometimes incorporates insulating flanges (dielectric unions) to prevent galvanic corrosion. If insulating flanges are present, the electrical bonding strategy must be reviewed — bonding across insulating flanges may defeat their purpose, and the earthing of pipework on each side of the flange must be considered separately.

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06 · Industrial Guide

EMC Earthing for Electronic Equipment

Electronic equipment (PLCs, VFDs, power analysers, SCADA systems, instrumentation) requires a low-impedance, low-noise earth reference for correct operation. Standard safety earthing (which may carry noise currents from contactors, VFDs, and other equipment) is often not suitable as a signal reference without additional measures. BS EN 61000-5-2 provides guidance on earthing and cabling in installations for EMC.

Star-point earthing — each item of electronic equipment has its own earth conductor running directly back to a central earth reference bar (star point). The star point connects to the main earth bar via a single low-impedance conductor. No earth currents from one piece of equipment can flow through the earth of another. This is the preferred topology for sensitive analogue instrumentation and PLC systems.
Separate analogue earth bar — in panels containing both power electronics and precision analogue instrumentation, use a separate analogue earth bar connected to the main earth bar at a single point. VFD earth currents and contactor switching transients on the main earth bar do not then flow through the analogue earth reference.
Cable screen earthing — screen conductors of signal cables (4–20 mA, 0–10 V, fieldbus) must be terminated at the panel end using 360° EMC cable glands bonded to the panel EMC earth bar. This drains high-frequency induced noise from the cable screen to earth. Screens must remain continuous and must not be pigtailed (pigtail connections have high inductance at RF and are ineffective above a few MHz).
Not a floating earth — EMC star-point earthing must ultimately connect to the main earthing terminal of the installation. A completely separate "clean earth" that is not bonded to the safety earth is dangerous (it can rise to line voltage under fault conditions) and non-compliant with BS 7671 — it is NOT an acceptable arrangement regardless of what some equipment manufacturers specify.

07 · Industrial Guide

Lightning Protection Integration (BS EN 62305)

Lightning protection systems (LPS) must be bonded to all other earthing systems in the structure. BS EN 62305-3 (Lightning Protection — Physical damage to structures and life hazard) is the UK standard for structural lightning protection. BS EN 62305-4 covers electrical and electronic systems within structures. The integrated earthing strategy must satisfy both standards.

Earth termination network bonding — the LPS earth electrode ring (or system of rods) must be connected to the electrical installation main earth bar, structural steelwork foundation earth, and all other earthing systems at a single point (or at multiple points via the ring conductor). BS EN 62305-3 requires earth electrodes to be interconnected where practicable to reduce the overall earth resistance.
Equipotential bonding at service entry points — all metallic services entering the structure (power cables, water pipes, gas pipes, data cables, CCTV cables) must be bonded to the LPS at their point of entry into the structure. This prevents step voltages inside the building when a lightning current flows through the earth. SPDs (Surge Protective Devices) are used for conductors that cannot be directly bonded (live power conductors, data cables without earthed screen).
SPD installation at MDB — BS EN 62305-4 and BS 7671 Section 534 require SPDs at the main distribution board (Type 1 or combined Type 1+2) where the building has a lightning protection system or is in an exposed location. SPDs must be connected between each line conductor and earth with the shortest possible lead lengths (ideally less than 0.5 m total). SPD earth connections must be to the main earth bar, not to a local earth electrode.
LPS design and risk assessment — BS EN 62305-2 provides the risk assessment methodology for determining whether an LPS is required and what level of protection (LPL I to LPL IV) is needed. LPS design and installation is specialist work — all LPS installations should be designed and inspected by a competent lightning protection engineer (ATLAS registered or equivalent).

08 · Industrial Guide

Measurement and Testing of Industrial Earthing Systems

The Electricity at Work Regulations 1989 require that earthing systems are maintained in a condition to prevent danger. This requires periodic measurement and testing. BS EN 61557 series covers electrical safety measurement methods for low-voltage distribution systems, including earthing measurements. Test records must be retained.

Earth fault loop impedance testing — measured using a loop impedance tester at each distribution board and at selected final circuits. Verify that the measured Zs allows the protective device to operate within the required disconnection time (0.4 s for socket outlets, 5 s for fixed equipment in TN systems). Record Zs values and compare with calculated maximum permissible Zs.
Earth electrode resistance measurement — four-terminal fall-of- potential method (BS EN 61557-5). Current and voltage spikes driven into the ground at specified distances from the electrode under test. Results sensitive to season (higher in dry summer), so test in dry conditions and record soil conditions. Alternatively use a clamp meter method for in-service testing.
Insulation resistance measurement — phase-to-earth IR testing (500 VDC for LV circuits) verifies insulation integrity. Deteriorating IR values indicate moisture ingress or insulation degradation. Trend results over successive tests. Very low IR values indicate an earth leakage path that may cause nuisance RCD tripping or fire risk.
Neutral-to-earth voltage monitoring — in TN-C-S (PME) installations with large unbalanced loads, measure the neutral-to-earth voltage at the main distribution board under maximum loading conditions. Voltage above 10 V indicates significant neutral current flowing on the PE conductor — this should be investigated and corrected to prevent nuisance tripping and potential hazard.

09 · Industrial Guide

For Electricians: Industrial Earthing Certification

Industrial earthing system installation and testing is high-value work that requires specialist knowledge. All new earthing installations and major modifications require an Electrical Installation Certificate. Periodic inspections of industrial electrical installations (typically every 3–5 years) require an Electrical Installation Condition Report (EICR), which must include observation codes for any earthing deficiencies found.

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Industrial Earthing Systems UK: Factory Earthing Guide

Key Takeaways

Earthing System Types in UK Industrial Premises

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TN-S Earthing for Industrial Premises

TN-C-S (PME) Considerations in Industrial Premises

TT Systems in Industrial Premises

Supplementary Equipotential Bonding in Process Areas

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EMC Earthing for Electronic Equipment

Lightning Protection Integration (BS EN 62305)

Measurement and Testing of Industrial Earthing Systems

For Electricians: Industrial Earthing Certification

Complete EICs and EICRs on Site

Industrial earthing system certification with Elec-Mate

Frequently Asked Questions About Industrial Earthing Systems

What is the difference between TN-S and TN-C-S earthing in an industrial context?

When is a TT earthing system required for an industrial installation?

What is supplementary equipotential bonding and when is it required?

What is star-point earthing for electronic equipment and why does it matter?

How does lightning protection integrate with the electrical earthing system?

What earth electrode resistance values are required for industrial installations?

What test equipment is needed for industrial earthing system testing?

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