The earthing system determines fault protection, Zs values, RCD requirements, and PME risks. This guide explains TN-S, TN-C-S (PME), and TT systems, the Regulation 8 ESQCR 2002 requirements, and how to identify each system on site.
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Every electrical installation in the UK operates within one of three earthing systems: TN-S, TN-C-S (commonly called PME), or TT. The earthing system determines how fault current returns to the source, what earth fault loop impedance (Zs) values can be achieved, what protective devices are required, and what additional risks must be managed.
Identifying the earthing system before commencing any electrical work is a fundamental requirement. BS 7671:2018+A4:2026 Chapter 41 sets out the disconnection time requirements for each system, and the choice of earthing system directly affects whether those requirements can be met. The statutory framework is provided by the Electricity Safety, Quality and Continuity Regulations 2002 (ESQCR 2002), particularly Regulation 8 for PME supplies.
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In a TN-S (Terra Neutral Separate) system, the neutral and protective earth conductors are separate throughout the entire system — from the transformer star point to the installation. The earth connection is made at the transformer, and a dedicated earth conductor (often the metal sheath of the supply cable) runs to the installation.
TN-S supplies are becoming less common as ageing lead-sheathed cable networks are replaced. Where the lead sheath is the earth conductor, deterioration of the sheath can increase Ze. Always measure Ze at the supply intake before commencing installation work, and record it on the EIC or EICR.
TN-C-S (Terra Neutral Combined-Separated) is the standard earthing system for most new UK domestic and commercial installations. It is universally known as PME (Protective Multiple Earthing). In the supply network, the neutral and earth are combined into a single PEN (Protective Earth and Neutral) conductor. At the supply cut-out or meter position, the PEN conductor is split into separate neutral (N) and protective earth (PE) conductors for the installation.
The combined neutral and earth in the supply network is why PME provides lower Ze than TN-S — the multiple earth connections along the distribution network create additional parallel fault current paths, reducing the total impedance.
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In a TT (Terra Terra) system, the installation earth is provided entirely by a local earth electrode — there is no metallic connection to the supply network earth. Fault current must flow through the soil from the installation earth electrode back to the transformer earth. This high-resistance path results in much higher earth fault loop impedance than TN systems.
TT System: RCD Protection is Mandatory
BS 7671 Regulation 411.5.3 requires that in a TT system, a residual current device must be provided for circuits where the earth fault loop impedance is too high for an overcurrent device to disconnect within the required time. The RCD must have a rated residual operating current such that Ra × IΔn ≤ 50V, where Ra is the resistance of the earth electrode and associated earth conductor.
The earth fault loop impedance (Zs) is the total impedance of the fault current path: from the source, through the line conductor, through the fault, and back through the earth conductor to the source. A lower Zs means more fault current flows, enabling faster disconnection by the protective device.
| System | Typical Ze | Protection Required |
|---|---|---|
| TN-C-S (PME) | 0.20–0.35Ω | MCB + optional RCD |
| TN-S | 0.35–0.80Ω | MCB + optional RCD |
| TT | 20–200Ω+ | RCD mandatory |
BS 7671 Reg 411.4.4 sets maximum Zs using Zs × Ia ≤ Uo × Cmin (Cmin = 0.95). For a 32A Type B MCB with 0.4s disconnection (residential circuits), maximum Zs is 1.37Ω. Any TT installation will have Zs far exceeding this limit, confirming that RCD protection is a legal requirement, not an option.
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Start free trialPME (TN-C-S) carries a specific risk that does not exist in TN-S or TT systems: if the PEN conductor breaks between the transformer and the installation, the neutral at the installation is lost but the earth terminal remains connected to all metal parts. When the neutral is lost, the return current must flow through the earth conductor — and the full line voltage can appear on metal enclosures.
Regulation 8 of the Electricity Safety, Quality and Continuity Regulations 2002 requires that a DNO must not connect a PME earth terminal to an installation unless the owner or occupier has given written consent, and prohibits PME connections to certain categories of installation including those supplied by overhead line to agricultural premises.
Before starting any electrical work, identify the earthing system. The method combines visual inspection at the supply intake with measurement:
Record the earthing system type and measured Ze on the EIC or EICR. This information is required on the certificate and is essential for anyone carrying out future work on the installation.
Earthing system identification and Zs measurement are core competencies tested in C&G 2391 and examined in detail during periodic inspection work. Misidentifying the earthing system can lead to incorrect protective device selection, inadequate fault protection, and dangerous installations.
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