Ask the expert: Introducing the new PNB earthing system

How does PNB differ from other earthing arrangements, where is it used and what does its inclusion in BS 7671 Amendment 4 mean for electrical designers and installers?

Protective Neutral Bonding (PNB) is a specialised earthing arrangement used by Distribution Network Operators (DNOs) in the UK. Although PNB has existed for many years, it has gained increased attention as electricity networks expand to support electric vehicle charging, renewable energy and rural developments.

PNB is classified as a variation of the TN-C-S earthing system, but it differs from the more common Protective Multiple Earthing (PME) arrangement in several important ways.

PNB and PME arrangements

In a PNB system, the neutral conductor is connected to earth at a single point, usually close to the customer’s supply or at a dedicated transformer, rather than at multiple locations along the distribution network. This makes PNB suitable for individual properties or small groups of customers where installing a full PME network is impractical and costly. DNOs typically use PNB where only one or a few customers are supplied directly from a transformer, provided that strict design criteria, including earth electrode resistance and separation from high-voltage earthing systems, are met. 

The most common earthing system in the UK is TN-C-S (PME). In this arrangement, the Protective Earth and Neutral (PEN) conductor is connected to earth at multiple points throughout the distribution network. This provides a low earth fault loop impedance, allowing protective devices to disconnect quickly in the event of a fault. PME is economical, reliable and well suited to densely populated urban areas. However, it can present risks if the PEN conductor becomes damaged or broken, as exposed metalwork connected to the main earth terminal may rise to a dangerous voltage.

TN-S and TT systems

The TN-S system differs by providing separate neutral and earth conductors throughout the supply. The earth connection is normally provided by the metallic sheath of the supply cable. TN-S systems are generally found on older underground cable networks and are less vulnerable to broken PEN conductor faults than PME systems. However, they are becoming less common as older infrastructure is replaced. Main DNOs are choosing to identify their TN-S systems as PME due to maintenance work that may have been carried out on the network cable.

Another common arrangement is the TT system, where the electricity supplier provides only the live and neutral conductors, and the customer installs their own earth electrode. Because the earth fault loop impedance is usually higher than in TN systems, residual current devices (RCDs) are essential to ensure automatic disconnection during fault conditions. TT systems are frequently used in rural areas, farms and temporary installations.

Inclusion in BS 7671 Amendment 4

The PNB earthing system provides a practical alternative where a full PME network cannot be installed. While it shares many characteristics with PME/TN-C-S, it uses a single neutral-to-earth connection rather than multiple earth points. Compared with TN-S and TT systems, PNB offers a balance between safety, cost and performance, making it particularly suitable for isolated customers and small rural developments.

The introduction of this earthing system into BS 7671:2026 4th Amendment to the 18th Edition should not create too much worry for electricians and designers. On the Electrical Installation Certificate or Condition Report we will still select TN-C-S, regardless of it being PME or PNB.

cef.co.uk

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This article appeared in Electrotechnical News July/August 2026 edition – read it here