Understanding overvoltage protection: requirements of BS 7671:2018+A2:2022 | Hager

Understanding overvoltage protection: requirements of BS 7671:2018+A2:2022 | Hager

Hager’s Technical Training Manager, Paul Chaffers, discusses the importance of providing installations with surge protective devices (SPDs) and looks at requirements and considerations of BS 7671:2018+A2:2022.

In our everyday life, we’re surrounded by electrical equipment which may contain sensitive electronic components. Such products are vulnerable to transient overvoltages and can have their lifespan significantly reduced through degradation and damage.

Understanding overvoltage protection can be very complex, but from an electrician’s point of view, the Wiring Regulations are now much simpler than previous editions that required complex thunderstorm risk assessments.

Voltage disturbances in the form of overvoltage can originate from outside of an installation or from within the installation itself. In fact, about 35% of all transients come from outside the facility with the remaining 65% generated inside our homes and facilities – see Table 1 (pictured below) for examples of where such transients may originate from.

General requirements

Understanding how to use BS 7671 correctly is the key to success. Unfortunately, too often installers look at an isolated “Section” or “Part” of the Regs, without considering the general requirements that must always be applied. An example of this would be only looking at Section 443 (Protection against transient overvoltages of atmospheric origin or due to switching) and not considering the Fundamental Principles.

BS 7671 Chapter 13 deals with Fundamental Principles, with Regulation 131.1 providing a number of conditions in electrical installations that may present a risk of injury. Indent (iv) of Regulation 131.1 includes voltage disturbances (undervoltage and overvoltage) meaning that where there is a threat of this occurring, protection is required.

Furthermore, Regulation 131.6.2 requires persons and livestock shall be protected against injury, and property shall be protected against damage as a consequence of overvoltages.


Part 2 of BS 7671 does provide some clarity for understanding the consequences of direct lightning strikes or indirect lightning strokes, as follows:

Direct lightning strike

Lightning strike impacting a component of the network directly.

NOTE 1: Examples of components of the network are: conductor, tower, substation equipment, etc.

NOTE 2: Consequences of a direct lightning strike include the thermal, magnetic and mechanical effects of the discharge current as well as the surges resulting from its voltage drop along the discharge path.

Indirect lightning stroke

Lightning strike that does not impact directly any part of the network but that induces an overvoltage in that network.

NOTE: Consequences of an indirect lightning stroke for electrical installations are the associated overvoltages and the energy content of the incoming surge wave.

Should lightning strike near power lines (indirect stroke), then the magnetic effect of this lightning current rushing to earth will induce a voltage on anything conductive (i.e. power lines passing through this magnetic field). This is known as inductive coupling – see Fig 1.

Fig 1. Inductive coupling, where power lines pass through magnetic field.

The power cable could even be underground and if the lightning struck somewhere nearby a massive amount of current will be injected into the ground. As the current flows away, a voltage will be induced; this is known as resistive coupling which is the most common cause of indirect atmospheric transient overvoltages. Such transients occur when a lightning strike raises the electrical potential of one or more of a group of electrically interconnected buildings, as illustrated in Fig 2.

Fig 2. Resistive coupling, which can affect both underground and overhead cables.

Examples of vulnerable interconnections are:

● DNO feed from substation to building

● Submains from building to building

● Supplies to external equipment, lampposts, CCTV, EV charge points etc

● Telephone lines

● Data and communication lines

Overvoltage control

Chapter 443 deals with protection against transient overvoltages of atmospheric origin, or overvoltages which are associated with switching activity within an installation, with Regulation 443.4.1 prescribing:

‘Protection against transient overvoltages shall be provided where the consequence caused by overvoltage could:

Result in serious injury to, or loss of human life, or

● Result in significant financial or data loss.

For all other cases, the default position is to provide protection against transient overvoltages unless the owner of the installation states that it is not required due to any loss or damage being tolerable. If the client makes this decision, obviously they will be accepting the risk of equipment damage and any consequential loss.

The modern home will have an abundance of electronic equipment which could easily be damaged by overvoltage and for this reason SPDs are often included.

It should be noted that where the word “shall” is used in BS 7671, the implication of that Regulation is a requirement and normative.

Transient overvoltages caused by equipment

Overvoltages associated with lightning are severe, but overvoltages associated with switching are more common and in fact can be longer in duration and contain more energy than transient overvoltages of atmospheric origin.

This means that where inductive or capacitive equipment is installed, it is likely that switching overvoltages or disturbances will occur. Regulation 443.4.2 deals with transient overvoltages caused by equipment and advises that protection against overvoltages shall be considered.

Examples of inductive or capacitive equipment likely to cause such disturbances within an installation are:

● Motors

● Transformers

● Capacitor banks

● Storage units

● High current loads

The examples provided in Regulation 443.4.2 are associated with commercial installations, however consideration for protection against switching overvoltages is equally as important for residential applications. High inrush currents associated with the switching of LED light fittings is one example of equipment likely to cause voltage disturbance and a good example of the type of sensitive equipment that requires protection.

Regulation 443.1.1 explains that switching overvoltages typically have a lower amplitude than transient overvoltages of atmospheric origin. This means that where protection is provided for transient overvoltages of atmospheric origin, it will usually protect against switching overvoltages.

Where the threat of lightning is low and no protection is provided, protection will still be required for switching activity. For this reason, Hager recommends consumer units are fitted with Type 2 SPDs and can supply consumer units with SPD kits factory fitted, saving contractors time otherwise spent installing the individual parts.

Download Hager’s 18th Edition Amendment 2 Bitesize Guide here

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