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Understanding Surge Protection Devices (SPDs)


The introduction of the 18th Edition Wiring Regulations saw a change in the requirements surrounding overvoltage protection a.k.a. surge protection that will result in a much higher prevalence across a range of different properties. Here, we aim to have a look at the consequences of electricity surges, related regulatory requirements, the different types of surge protection device (SPD) available, and where and when to install them.

18th Edition Surge Protection Requirements

Whereas the 17th Edition worked on the basis of the likelihood of a lightning strike - the principal cause of electricity surges - following the AQ criteria, BS7671:2018 instead considers the consequences of a strike and associated surge. Namely, regulation 443.4 states that an SPD must be installed where consequences caused by overvoltage include:

  • ‘Serious injury to, or loss of, human life’
  • ‘Interruption of public services and/or damage to cultural heritage’
  • ‘Interruption of commercial or industrial activity’
  • ‘Affecting a large number of collocated individuals’

This effectively means that everything but small domestic dwellings must now have adequate surge protection.

Causes and Consequences of Electrical Surges

Most electricity surges are caused either by lightning strikes or electricity switching. A bolt of lightning can carry a current of 200,000A and is particularly dangerous as the flashover can cause fire or electric shock. Switching surges caused by shutting down large inductive loads tend to be less extreme but more repetitive, potentially limiting the operating life of system components.

In terms of SPDs, they serve to prevent damage to all parts of an electrical installation and attached appliances. This is caused by transient overvoltage exceeding the rated withstand limit of electrical equipment, thus causing damage such as burned components, degraded insulation and even melted wires.

In view of the essential equipment that might be affected by surges and the fact that parts of Eastern England average more than 14 days with thunderstorms per year, SPDs are essential in maintaining vital services.

Types of Surge Protection Device

There are three principal types of surge protection device in common use, depending on where they are installed within an installation.

Firstly, Type 1 SPDs are capable of discharging partial lightning current with a waveform of 10/350µs. This makes them suitable for installation on the supply side of the main service entrance, protecting the whole installation including the service panel from overvoltage. Typically, they use a spark gap to direct the surge to earth and prevent it reaching the building.

Secondly, Type 2 SPDs are intended to protect equipment attached to an installation. They are positioned on the load side of the main service entrance across phase-neutral or phase-phase and prevent the spread of overvoltage through a metal oxide varistor (MOV) with an 8/20µs current wave. MOVs maintain very high resistance until surge voltage is encountered, at which point resistance drops dramatically and excess current can be channelled to earth.

Finally, Type 3 SPDs are much smaller scale and should only be used in conjunction with a Type 2 device. They are specifically designed for protecting sensitive equipment such as televisions and computers from overvoltage. Their voltage waves tend to be around 1.5/50µs and current waves tend to be around 8/20µs.

Coordination of Surge Protection

Once the correct type of surge protection has been selected, the 18th Edition offers guidance on how to position and coordinate SPDs. Table 443.2 indicates the impulse withstand category different sorts of equipment and denotes the minimum impulse withstand voltage. SPDs should have a protection level significantly lower than the rated impulse withstand voltage. SPDs should also always be from the same manufacturer.

An important factor to consider when installing surge protection is the total length of the leads connecting the SPD to the installation. They should be as short as possible, preferably totalling less than 0.5m and in no case totalling more than 1m. SPDs should always be installed according to the manufacturer’s instructions and preferably on the supply side of RCDs.

For the full 18th Edition Wiring Regulations and accompanying On-Site Guide, please visit our Books and Publications section.