The earthing system decides your Zs limit, your disconnection time and whether an MCB or an RCD provides fault protection. This guide compares TN-S, TN-C-S (PME) and TT against BS 7671:2018+A4:2026, covers the PME open-PEN restrictions, and shows how to identify each system on site.
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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.
The short answer
TN-S keeps the neutral and protective conductors separate throughout the supply. TN-C-S combines them as a PEN conductor in the network and splits them at the cut-out — the arrangement known as PME. TT has no supply earth: the installation earths itself through a local electrode, so fault protection is provided by an RCD.
Typical Ze is 0.20–0.35Ω on TN-C-S, 0.35–0.80Ω on TN-S and 20–200Ω on TT. BS 7671 Table 41.1 allows 0.4s to disconnect a 230V AC final circuit on a TN system, but only 0.2s on TT.
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01 · Earthing Systems Guide
Every electrical installation in the UK sits within one of three earthing systems: TN-S, TN-C-S (commonly called PME), or TT. The system determines how fault current returns to the source, what earth fault loop impedance can be achieved, which protective device can provide fault protection, and what additional risks have to be managed.
| System | Earth provided by | Typical Ze | Max disconnection time | Fault protection |
|---|---|---|---|---|
| TN-C-S (PME) | Distributor's PEN conductor, split at the cut-out | 0.20–0.35Ω | 0.4s | Overcurrent device (MCB or fuse) |
| TN-S | Distributor's separate protective conductor or cable sheath | 0.35–0.80Ω | 0.4s | Overcurrent device (MCB or fuse) |
| TT | The installation's own earth electrode | 20–200Ω | 0.2s | RCD preferred (Reg 411.5.2) |
Disconnection times are the 230V AC figures from BS 7671 Table 41.1. Regulation 411.3.2.2 applies them to final circuits rated up to 63A with one or more socket-outlets, and up to 32A supplying only fixed connected current-using equipment. Ze figures are typical supply values, not limits set by BS 7671 — always measure.
Identifying the system before commencing work is fundamental. BS 7671:2018+A4:2026 Chapter 41 sets the disconnection times in Table 41.1 and the maximum Zs formulae in Regulations 411.4.4 and 411.5.4, and the earthing system decides whether those figures can be met with an overcurrent device or whether an RCD is needed. The statutory framework sits alongside it in the Electricity Safety, Quality and Continuity Regulations 2002 (ESQCR), particularly Regulation 9 for PME.
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02 · Earthing Systems Guide
BS 7671 defines a TN-S system as one having separate neutral and protective conductors throughout the system — from the transformer star point to the installation. The earth connection is made at the transformer, and a dedicated protective 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 protective conductor, deterioration of the sheath can increase Ze — which is why a measured Ze at the origin, recorded on the EIC or EICR, matters more than the value the distributor declares.
03 · Earthing Systems Guide
TN-C-S is the standard earthing system for most new UK domestic and commercial installations. BS 7671 defines it as a system in which neutral and protective functions are combined in a single conductor in part of the system, earthed at multiple points. That arrangement is PME — Protective Multiple Earthing. In the supply network the neutral and protective functions share one PEN conductor; at the cut-out or meter position the PEN conductor is split into separate neutral (N) and protective (PE) conductors for the installation.
The multiple earth connections along the distribution network create parallel return paths for fault current, which is why PME generally gives a lower Ze than TN-S. It is also the reason the open-PEN risk exists at all: the same conductor is carrying load current and serving as the earth reference.
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04 · Earthing Systems Guide
In a TT system the source is earthed but the exposed-conductive-parts of the installation are connected to earth electrodes electrically independent of the source earth. There is no metallic earth path back to the transformer, so fault current has to return through the general mass of earth. That gives a far higher earth fault loop impedance than either TN system.
Reg 411.5.3 sets two conditions where an RCD provides fault protection on a TT system: disconnection within the time required by Reg 411.3.2.2 or 411.3.2.4, and Ra x IΔn no greater than 50V, where Ra is the sum of the resistances of the earth electrode and the protective conductor connecting it to the exposed-conductive-parts. BS 7671 adds that where Ra is not known it may be replaced by Zs. Rather than calculate, you can read the limit straight off Table 41.5.
| Rated residual operating current (IΔn) | Maximum earth fault loop impedance Zs |
|---|---|
| 30mA | 1667Ω |
| 100mA | 500Ω |
| 300mA | 167Ω |
| 500mA | 100Ω |
BS 7671 Table 41.5, for Uo of 230V, RCDs to BS EN 61008-1 and BS EN 61009-1. Disconnection must still be within the times stated in Table 41.1. BS 7671 notes that the installation earth electrode resistance should be as low as practicable and that a value exceeding 200Ω may not be stable — see Reg 542.2.4.
One qualification worth knowing: the footnote to Table 41.1 permits the TN disconnection times to be used on a TT system where disconnection is achieved by an overcurrent protective device and the protective equipotential bonding is connected to all extraneous-conductive-parts within the installation in accordance with Reg 411.3.1.2.
05 · Earthing Systems Guide
Earth fault loop impedance is the total impedance of the fault current path: the source, the line conductor up to the point of the fault, and the protective conductor between the fault and the source. A lower Zs means more fault current, and more fault current means faster disconnection.
For a TN system, Reg 411.4.4 requires Zs x Ia ≤ Uo x Cmin. For a TT system protected by an overcurrent device, Reg 411.5.4 states the same requirement. Uo is the nominal line to earth voltage and Cmin is the minimum voltage factor, given the value 0.95 for a low voltage supply provided in accordance with the ESQCR. Ia is the current causing automatic operation of the device within the time required by Reg 411.3.2.2 or 411.3.2.3.
Worked through for a 32A Type B MCB, that gives a maximum Zs of 1.37Ω — the figure printed in Table 41.3. A TN-C-S installation with a Ze of 0.30Ω leaves ample headroom for the circuit conductors. A TT installation with a Ze of 50Ω is nowhere near it, which is why fault protection on TT is provided by an RCD reading against Table 41.5 instead.
Table 41.1 gives 0.4s for a 230V AC final circuit on a TN system and 0.2s for the same circuit on TT. For distribution circuits and circuits outside the scope of Reg 411.3.2.2, Reg 411.3.2.3 permits up to 5s on TN and Reg 411.3.2.4 permits up to 1s on TT. Quoting the TN figure on a TT installation is a common slip on certificates and in exams.
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06 · Earthing Systems Guide
PME carries a specific risk that does not exist on TN-S or TT: if the PEN conductor goes open circuit between the transformer and the installation, the installation loses its neutral but its earth terminal remains connected to all the metalwork. Load current then seeks a return path through the earthing arrangement, and a voltage can appear between that metalwork and true Earth.
BS 7671 does not leave that to judgement — it restricts PME earthing in the specific locations where someone is most likely to be in contact with true earth potential at the same time as they touch the metalwork.
Caravan and camping parks — Reg 708.553.1.14
Socket-outlet protective conductors shall not be connected to a PME earthing facility. This does not stop PME being used elsewhere on the site, for example for the installations of permanent buildings.
Marinas — Reg 709.553.1.14 and 709.411.4
The same prohibition applies to berth socket-outlets, and Reg 709.411.4 records the statutory prohibition in ESQCR Reg 9(4) on connecting a PME earthing facility to any metalwork in a boat.
EV charging equipment — Reg 722.411.4.1 and 722.312.2.1
A PME earthing facility shall not be used as the means of earthing for the protective conductor contact of a charging point located outdoors, or one that might reasonably be expected to be used to charge a vehicle outdoors, unless one of the listed alternatives is used — an installation earth electrode holding the MET below 70V RMS under an open-circuit PEN fault, or a device that disconnects the vehicle within 5s on detecting 70V RMS, or on the utilisation voltage leaving the 207V to 253V band, or an equivalent alternative. A circuit supplying EV charging equipment shall not include a PEN conductor. This is a BS 7671 requirement, not a manufacturer preference.
Agricultural and horticultural premises — Reg 705.411.4
A PEN conductor shall not be used within the installation. NOTE 2 to that regulation adds that, unless a metal grid is laid in the floor, using a PME earthing facility as the means of earthing is not recommended.
Protective multiple earthing is dealt with by Regulation 9 of the Electricity Safety, Quality and Continuity Regulations 2002. Regulation 9(2) requires the distributor to connect the supply neutral conductor with earth at multiple points, including such points as are necessary to prevent, so far as is reasonably practicable, danger arising from the supply neutral conductor becoming open circuit. Regulation 9(4) prohibits the distributor connecting the combined neutral and protective conductor to any metalwork in a caravan or boat.
Regulation 8 is the general requirement for connection with earth. The paragraph electricians meet most often is 8(4): a consumer shall not combine the neutral and protective functions in a single conductor in the consumer's installation. That is the statutory reason the PEN conductor stops at the origin, and BS 7671 cites it in the notes to Regulations 543.4.1, 444.4.3.1 and 710.312.2.
07 · Earthing Systems Guide
Identify the earthing system before starting work. The method combines visual inspection at the supply intake with a Ze measurement — neither on its own is conclusive.
TN-C-S (PME)
A conductor links the cut-out neutral terminal to the main earthing terminal. The meter neutral and the earthing conductor originate from the same cut-out termination. Confirmation from the distributor is best practice.
TN-S
The cut-out has a separate earth terminal, distinct from the neutral, typically connected to the lead sheath of the supply cable or to a separate protective conductor. A measured Ze below about 0.8Ω supports TN-S.
TT
No distributor earth terminal at the cut-out. The installation has its own earth electrode — rod, tape or plate — connected to the MET. A measured Ze in the tens or hundreds of ohms supports TT. Check that fault protection is provided by RCDs.
Record the earthing system type and the measured Ze on the EIC or EICR. On a TT installation, record Ra as well — it is the value the next person needs in order to check Ra x IΔn ≤ 50V without re-testing the electrode.
08 · Earthing Systems Guide
Earthing system identification, Ze measurement and the Zs limits that follow from them are core competencies tested in C&G 2391 and examined in detail during periodic inspection work. Misidentifying the system leads to the wrong protective device, the wrong Zs limit and the wrong disconnection time on the certificate.
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