Surge protection: household appliances

Counter strike

3 July 2018

Preventing electrical surges is crucial given the number of appliances used in most buildings today. Gary Parker summarises the relevant guidance

Expectations of electrical installations have grown over the years as the use of electrical and electronic devices has increased.

A typical dwelling 20 years ago would have had a single television, maybe a PC and some kitchen appliances. Now, the range of products that can be purchased at relatively low cost is astonishing, ranging from ultra-HD TVs to smart thermostats and even voice-controlled home assistants.

These devices make our lives more pleasurable and convenient, but they come at a cost: they are susceptible to damage caused by electrical surges, and in extreme cases can catch fire easily as they use the combustible insulation also common in fridges and freezers.

Electrical surges, or transient overvoltages, are spikes in the power supply that, for the briefest of time, increase the voltage that the equipment experiences. They are a natural phenomenon occurring during thunderstorms, when lightning can cause transient overvoltages to enter buildings and sensitive electronic and data equipment to become damaged and potentially combust.

Another cause of transient overvoltage is electrical switching. Whenever a conductor carries a current a magnetic field is formed around it, and when the circuit is de-energised then this field collapses and induces a voltage in the conductor.

Such voltages can, briefly, cause a transient overvoltage to occur, and their effect is increased when there are large inductive loads, such as motors. The electricity supply to a premises can be one source of such overvoltages.

Surge protection

There is only one guaranteed way to prevent transient overvoltages, and that is to use no electrical or electronic equipment. But as this isn’t practical, electrical installations can be protected against transient overvoltages by the use of surge protection devices (SPDs).

SPDs are different to circuit-breakers and residual current devices in that they are specifically designed to protect the installation and components against the harmful effects of transient overvoltages.

Their role is to take the transient overvoltage and dissipate it to earth, safely away from the installation and the products they are protecting. They are therefore to be used to supplement circuit-breakers and similar devices to offer total protection for an installation.


The primary electrical safety guidance in the UK is BS 7671: 2008 Amendment 3: 2015, on the requirements for electrical installations. However, the impact of transient overvoltages and the risk of lightning is so specialist a subject that it is detailed in another series of standards, BS EN 62305 on protection against lightning.

This series is split into four parts and totals more than 450 pages. It would not be possible to condense this information into a short article such as this, so the following summarises the requirements of BS 7671: 2008 Amendment 3: 2015. Please note that, at the time of writing, BS 7671: 2018 is at draft stage and will be published in July.

There are two distinct sections in BS 7671 relating to protection against transient overvoltages: section 443, protection against overvoltages of atmospheric origin or due to switching; and section 534, devices for protection against overvoltage. In essence, the first helps you assess whether you need overvoltage protection, and the second helps specify how you do so and what devices you might need.

These devices make our lives more pleasurable and convenient, but they come at a cost: they are susceptible to damage caused by electrical surges

Section 443 has two options for determining necessary protection against transient overvoltages. The first is by use of an AQ rating, a measure of the number of thunderstorm days per year in an area. However, this is for many an outdated concept as there is nowhere in the UK that the number exceeds the arbitrary figure of 25 quoted in the standard.

The second, more sensible approach is to undertake a risk assessment of the installation’s consequential risk level based on the requirements of BS 7671 regulation 443.2.4.

Risk assessments

Risk assessments of this nature split installations into five categories:

  • those that have consequences that concern human life, such as safety services and hospital medical equipment;
  • those that would affect public services, IT centres or museums;
  • those with consequences for commercial or industrial activity, such as hotels, banks, industries, commercial markets or farms;
  • those where there would be consequences for groups of individuals, such as large residential buildings, churches, offices or schools; and
  • those whose failure would affect individuals, such as small or medium-sized residential buildings, or small offices.

For the first three, protection against overvoltage shall be provided as per regulation 443.2.4. For the fourth and fifth categories, protection requirements depend on the result of a calculation detailed in regulation 443.2.4 of BS 7671.

The information needed to complete this calculation successfully is so detailed that it will not be considered here; however, it is clear that many installations fall under the first three levels of consequence and will therefore require SPDs to be installed. On top of this, BS 7671 regulation 534. requires that SPDs be installed where there is a structural lightning protection system fitted to a building.

Once it has been determined that protection against transient overvoltages is necessary, section 534 comes in to play. This provides guidance on what device should be installed, and again makes reference to BS EN 62305 as well as to other relevant standards. This article will therefore simply summarise the requirements.

To help selection, SPDs are classified as one of three main types according to their location and use:

  • Type 1 SPD: this will be located at the point at which the supply transfers from the distributor to the client, typically where electrical services enter a property, and offers protection against direct lightning strikes.
  • Type 2 SPD: this will be located at sub-distribution boards throughout the installation, and offer protection against indirect lightning strikes or strokes and electrical switching surges.
  • Type 3 SPD: this will typically be a separate device used with an outlet or appliance to protect circuits in sensitive equipment.

Many manufacturers make devices that incorporate multiple protection types, so a combined type 1/2 device can be used at multiple points in the installation.

However, it is vital that whatever devices are used they are suitably coordinated with one another to ensure that those in series provide the necessary protection and function properly. The easiest way to do this is to use devices from a single manufacturer throughout the installation to achieve selectivity; that is, to be certain they work in conjunction with all other SPDs in the system.

Another vital consideration is the maximum length of the conductors used to connect the SPD to the distribution board being protected. BS 7671 regulation 534.2.9 requires that the cables be as short as possible, preferably less than 0.5m in length but in no case exceeding 1m. This can be a restriction in installations that are to be refurbished, and should be considered before specifying where SPDs should be fitted.

According to BS 7671, there are many installations that require protection against transient overvoltages, but in reality even more could benefit from SPDs due to the increased use of sensitive electronic equipment.

While it would be wrong to say that when in doubt, fit a SPD – this would be extremely costly and would not always benefit the building owner or occupier – it would be sensible to increase the industry’s awareness of such devices and the need to consider their use, where appropriate, to offer another level of protection for equipment and buildings.

RICS itself is aware of an increase in electrical fires caused by electrical surges, and a risk assessment approach is recommended.

Gary Parker is a senior technical support engineer at Electrical Contractors’ Association

Further information