TN-C Earthing Banned in New Installations — BS 7671:2018+A4:2026

A TN-C system is a low-voltage earthing arrangement in which the neutral and protective functions are combined into a single conductor — the protective-earthed-neutral or PEN conductor — for the whole of the distribution and the consumer's installation. The same metallic conductor carries return load current and is also the conductor to which exposed-conductive-parts of the installation are connected for automatic disconnection of supply.

Read against the system definitions in Section 312 of BS 7671, the "T" in TN-C means that one point of the source is connected directly to earth (the supply neutral is earthed at the source), the "N" means that exposed-conductive-parts of the installation are connected to that earthed point via protective conductors, and the "C" means that the neutral and protective functions are combined throughout — the PEN runs from the source, through the supply, through the consumer unit, and out to every accessory.

In a TN-C arrangement, every socket, every accessory, every fixed appliance has its earth terminal connected to the same conductor that carries the return load current. There is no separate circuit protective conductor (cpc) running alongside the neutral — they are physically the same wire. That is the configuration the 2026 amendment prohibits inside the installation. For a wider primer on what earthing means and why it matters, see our what is earthing guide.

The prohibition codifies a position that BS 7671 and CENELEC HD 60364-1 have been moving towards for decades. The combined PEN conductor presents three categories of hazard when run throughout a consumer's installation:

1. Open-PEN risk — if the combined PEN is broken anywhere between source and load, the neutral path and the protective path are lost simultaneously. Exposed-conductive-parts rise towards line voltage with respect to true earth, and there is no functional cpc to operate the overcurrent device.
2. Load-current on protective conductors — in TN-C the protective conductor carries load current at all times. It sits at a non-trivial voltage above true earth, proportional to load current and conductor impedance. This is incompatible with the modern expectation that protective conductors sit at earth potential.
3. Inability to use RCDs effectively — RCDs sense the imbalance between line and neutral. If the protective function shares the same conductor as the neutral function, earth fault current returns via the conductor the RCD treats as neutral; the RCD cannot distinguish a fault and will not operate. Section 411 increasingly requires RCDs as the primary means of automatic disconnection; TN-C is structurally incompatible.
4. Inconsistency with bonding strategy — Chapter 54 requires equipotential bonding to extraneous-conductive-parts. In TN-C the bonded metalwork connects to a conductor carrying load current, so touch-voltage becomes load-dependent — the opposite of what bonding is supposed to achieve.

The open-PEN risk does NOT disappear by switching to TN-C-S — the PEN still exists, but only up to the splitting point. Section 722 (EV charging) addresses open-PEN mitigations for PME supplies. A4:2026 narrows the problem to the supply side, but does not eliminate the need for open-PEN protection on PME EV installations.

A common point of confusion when the A4:2026 prohibition is reported is the assumption that TN-C-S is also banned. It is not. TN-C-S is the dominant earthing arrangement in UK distribution and remains fully compliant. The "C" in TN-C-S refers only to the portion of the system upstream of a defined splitting point.

For a new installation, the designer chooses the earthing arrangement based on the supply provided by the DNO (or the privately-owned source in the case of PNB) and the suitability of that arrangement for the loads to be served. The three compliant arrangements are TN-C-S, TN-S and TT.

For a deeper walk-through of when each arrangement is appropriate, including the design choice criteria and the recording of the arrangement on the EIC and EICR Section I, see the TN-S vs TN-C-S vs TT comparison.

A4:2026 prohibits TN-C in NEW installations. It does not retrospectively make existing TN-C unlawful. EICR coding follows the usual principle: code against the standard in force at installation, and record any departure from current standards as an improvement recommendation, with safety implications driving the code.

When an existing TN-C installation undergoes new work — consumer unit change, rewire, extension, EV charger — the new work falls under A4:2026 and must not extend the TN-C arrangement. The CU change becomes the natural opportunity to convert to TN-C-S (via a DNO PME terminal request) or to a TT system with a local earth electrode.

Alongside the TN-C prohibition, A4:2026 codified the distinction between PME and PNB on the model forms. Before A4, the Section I tick-box was a single "TN-C-S" option that did not distinguish distributor PME from consumer PNB. From A4 onwards, two separate tick-boxes appear and the inspector must record which arrangement is in use.

For the model form change and how to record PNB correctly, see our A4 TN-C-S (PNB) earthing guide.

The open-PEN failure mode is the engineering reason TN-C is prohibited inside an installation. It is also why TN-C-S (PME) installations require additional measures for higher-risk loads such as EV chargers. The regulatory boundary sits between the DNO (distribution network operator) and the installer.

A4:2026 does not change the Section 722 rules for EV chargers, but it rules out TN-C as an earthing arrangement for any new EV installation and makes the PME / PNB / TT distinction more important when assessing open-PEN risk.

There is no statutory requirement to retrofit an existing TN-C installation solely because A4:2026 has been published. The trigger is one of: the installation is being altered, extended or significantly added to; an EICR has identified a coded observation requiring remedial action; or the owner elects to upgrade voluntarily.

For any new installation, alteration or addition under A4:2026, the earthing arrangement is recorded on the relevant model form — the EIC, MEIWC, or EICR. Each form's Section I provides tick-boxes for the earthing arrangement.

For broader context on the A4:2026 certificate model form changes, see the A4:2026 summary.

The Elec-Mate certificate suite is aligned to BS 7671:2018+A4:2026 from publication. The EIC and EICR tools present the Section I earthing arrangement as a structured choice rather than free text, with TN-C disabled for new installations and flagged for observation on EICR.