Amendment 4 split the previous single TN-C-S option on the BS 7671 model forms into two distinct earthing arrangements: TN-C-S (PME) and TN-C-S (PNB). This guide explains the difference, when each applies, and what the new Section I tick-box choice means for design and inspection decisions.
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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.
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The Electrical Installation Condition Report (EICR), Electrical Installation Certificate (EIC) and Minor Electrical Installation Works Certificate (MEIWC) model forms in BS 7671:2018+A4:2026 published 15 April 2026 separate the previously single TN-C-S option into two distinct tick-boxes:
PME and PNB look very similar on paper — both have a combined PEN conductor at some point. The location of that PEN/N split changes which party (distributor or installation owner) is responsible for its integrity, and triggers different bonding and protection rules in BS 7671. A4 made the distinction visible on the certificate so the responsible person and any subsequent inspector know which set of rules applies.
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TN-C-S (PME) is the most common UK supply arrangement for domestic and small commercial premises. The distributor provides a combined protective-and-neutral (PEN) conductor from the substation to the cut-out at each property. Inside the property, the PEN conductor is split into separate N and PE conductors at the supply terminals — typically at the meter or main switch.
Main protective bonding conductor sizes for PME installations are governed by Regulation 544.1.1 and Table 54.8. Where the PME supply PEN conductor is 35 mm² or less (the most common domestic and small commercial case), the minimum copper main protective bonding conductor is 10 mm². Larger PEN conductors require proportionally larger bonding conductors — consult Table 54.8 for the full schedule. This reflects the higher fault current that can flow through the PEN under broken-neutral conditions in a PME network.
TN-C-S (PNB) is typically used in larger commercial, industrial and institutional sites where the supply originates at a privately-owned HV/LV transformer (often a substation on the customer's land). Between the transformer secondary and the main LV distribution board there is a combined PEN conductor — and downstream of that distribution board, the conductor splits into separate N and PE for the rest of the installation.
Large hospitals, university campuses, manufacturing plants, data centres, large retail parks with their own substation, water treatment works, motorway-services electrical infrastructure. Any site where the customer takes HV supply and steps it down to LV themselves typically operates under PNB rather than PME.
The choice between PME and PNB affects several specific BS 7671 decisions:
Regulation 543.4.2 (NOTE) of BS 7671 and the ESQCR both require that the PEN conductor is separated into individual neutral (N) and protective (PE) conductors at the incoming supply termination. The combined PEN must not continue as a single combined conductor within the consumer installation. Inspectors should verify this separation is present and correctly bonded at the boundary — record any deficiency as a C2 or C3 observation on the EICR as appropriate.
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Section I of the A4:2026 EICR model form has the earthing-arrangement tick-boxes. The inspector ticks one of:
Trace the supply backwards from the consumer unit to where the conductor splits from PEN into separate N and PE. If that split is at the cut-out / supply terminals provided by the distributor, it's TN-C-S (PME). If the split is downstream of a privately-owned transformer (substation on the customer's land), it's TN-C-S (PNB). When in doubt, consult the original installation documentation or the distributor / private network owner.
IET Guidance Note 3 (9th Edition, Chapter 8, Reg 8.2) requires inspectors to verify and record the following on PME (TN-C-S) installations: (1) PEN conductor integrity — confirm the combined PEN is terminated correctly at the supply boundary and has not continued within the consumer installation contrary to ESQCR. (2) Continuity of main protective bonding conductors — verify that all extraneous-conductive-parts are correctly bonded and continuity is satisfactory. (3) Adequacy of precautions — confirm that any special precautions required for PME supplies (such as supplementary bonding in high-risk locations) are in place. Record any deficiencies in Section K of the EICR.
EICRs issued before 15 April 2026 used the previous model form which had a single "TN-C-S" option without the PME/PNB distinction. Those reports remain valid for their stated periodic inspection interval — A4 is not retrospective.
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