Surge protection: what you need to know

Dominick Sandford, Director of Merchandising & Marketing at ElectricalDirect looks at the requirements for surge protection, installer responsibilities and how to select the correct surge protection devices.

Electrical and electronic equipment of all types, from computers to industrial machinery to medical devices, are now a central part of our lives and crucial to business operations and public services. However, they are vulnerable to power surges, also known as transient over-voltages. While these high voltage spikes can last just a fraction of a second, they can degrade, damage or even destroy equipment attached to the power supply.

Around 35% of transient over-voltages come from external events such as lightning strikes, utility grid switching or electrical accidents. The other 65% come from sources within the building, most commonly electrical switching within appliances and equipment. This is where the energy stored in the magnetic field generated by electrical equipment is suddenly released when the current is interrupted and dissipates as a high voltage transient.

Surge protection requirements

Successive versions of the Institution of Engineering and Technology (IET) Wiring Regulations (BS 7671) have updated the guidance on surge protection devices (SPDs). The current 18th Edition, released in July 2018 and applicable from January last year, outlined new criteria for where SPDs should be installed and provided revised advice for contractors.

The new regulations introduced a simplified assessment for when SPDs are required. It states that:

“Protection against transient over-voltages shall be provided where the consequence caused by over-voltage effects:

  1. Results in serious injury to, or loss of, human life or;
  2. Results in interruption of public services and/or damage to cultural heritage or;
  3. Results in interruption of commercial or industrial activity, or;
  4. Affects a large number of co-located individuals.”

Furthermore, any commercial, industrial or public building that is supplied by overhead lines also requires surge protection. This means that the majority of buildings will require surge protection because one or more of the criteria apply.

While single residential properties are not included if the value of equipment does not justify the protection, apartment buildings would fall under the category of affecting a large number of co-located individuals. Single dwelling properties are not required to have surge protection, therefore it is up to the homeowner to decide if they want to take preventative measures. Often the deciding factor will be the level of risk and the value of the equipment that the devices would be protecting. In these situations, installers have a role in advising the homeowner on the best course of action.

For non-residential properties that do not fit into these categories, a risk assessment must be carried out. This process has been simplified in the 18th Edition and details of how to calculate the level of risk can be found in section 443.4 of the IET Wiring Regulations. Where this risk assessment is not conducted the regulations require that surge protection is implemented.

It is important to note that Overcurrent Protective Devices (OCPDs), such as circuit breakers and fuses are not intended to provide over-voltage protection. They are designed to protect homes, businesses and human life from the dangers of a short circuit or overload and serve a different function to SPDs in the electrical installation.

Selecting and installing SPDs

All SPDs work by diverting surge currents to earth to reduce the over-voltage to a level that will not damage the components of the system or equipment connected to it. There are three types of surge protection device. A Type 1 SPD is designed to provide protection against surges caused by direct lightning strikes. These often feature spark gap technology which can handle very high voltages by creating a short to ground when a level of current is reached.

A Type 2 device offers protection against over-voltages from switching and indirect lightning strikes. This type more commonly uses a metal oxide varistor (MOV) to divert the current away.

Type 3 SPDs provide local protection for sensitive equipment. As these have a relatively low discharge capacity, they should always be installed in addition to a Type 1 or 2 device. These are available as hard-wired components but are also commonly found in surge protected sockets, adapters and extension leads.

A Type 1 or 2 must be installed at the origin of the installation, where the power supply enters the building. However, determining which type is required depends on whether there is a structural lightning protection system (LPS) installed. A Type 1 should always be installed where there is an LPS, as it forms part of the system, and a Type 2 can be fitted where there is not.

To determine the location of other SPDs, the lightning protection zones (LPZ) have to be considered. External zones (LPZ 0a and 0b) are areas where there is a threat of full lightning voltage or electromagnetic field. The internal zone (LPZ 1) are sections protected from full lighting surge voltages by an SPD at the boundary. LPZ 2 is an area deeper inside the installation that is protected by additional layers of surge protection. A surge protection device should be installed whenever a cable crosses to an internal zone from the external LPZ. SPDs are also required where a cable crosses an internal zone boundary.

Combining different stages of surge protection in a ‘cascade’ will maximise protection as it improves the system’s diversion capacity, whilst maintaining a low voltage. For example, a Type 1 or 2 will provide broad protection against the highest voltage spikes, while further Type 2 or Type 3s installed close to the equipment will provide an additional buffer. These secondary or tertiary levels also provide protection against electrical switching from within that zone.

Protecting electrical installations from the threat of over-voltages is essential to ensure the safety of building occupants and the longevity of the equipment connected to the system. The requirements of the updated wiring regulations will undoubtedly mean that a large proportion of buildings will now need surge protection measures to be installed.

ElectricalDirect stocks a range of surge protection devices. To find out more visit

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