Surges are short-term high voltage impulses often called ‘spikes’, ‘transients’ or ‘glitches’. The duration of these surges are typically measured in micro seconds.

Surges are typically produced from internal tripping events such as a lift motor in a hotel or office, lighting being turned on and off.

When there is a high energy inrush, this generates a surge and continues on around the circuits and can damage sensitive electronic equipment.

Lightning strikes also produce huge surges, up to 2km away from a direct strike. Surges can be induced in cabling systems, even when there is no direct connection to the point of a lightning strike.

Without adequate protection the greatest consequence is a risk to human life, particularly in circumstances where medical equipment is involved and could be damaged.

Secondly, there is the potential downtime in production e.g. in a workshop, retail outlet or office.

Finally, there is the inconvenience of replacing valuable and sensitive equipment, insurance claims, and increased insurance premiums.

Note:- Currently, cabling for data, telephones and satellites are outside the scope of the 17th edition, but installers and specifiers should still consider providing surge protection for these systems.

Section 443 refers to why and when SPD’s should be fitted.

Is simply a risk assessment,and asks the question if not installing an SPD, represents an issue to human life, public service or industrial activity. If the answer is ‘Yes’ then fitting of the SPD is mandatory.

Where some confusion arises is the reference to risk of lightning in relation to the number of thunderstorm days per year. The UK on a whole has less than 25 thunderstorm days per year.
Therefore, it is not generally required to fit SPDs to all domestic properties.
However, where higher risk to personal injury or prevention of commercial activity, further risk assessment is required.

The main driver for installation of SPDs will be insurance companies, with the potential for reduced premiums if SPDs are fitted, or the possibility of non-payment of insurance in the event of any damage or injury if not fitted.

Section 534 refers to device selection.

Refers to how to divert the surge current to earth before this surge has a way to propagate about the wiring of the installation and cause damage, by considering which device to install.

A house or building is split into Lightning Protection Zones (LPZ), as a potential surge passes between one zone to another, this is where the appropriate SPD should be fitted, e.g. between the outside (LPZ0A) to the inside of a building (LPZ1) -SPD fitted in a consumer unit.