Lightning Protection Overview
Lightning protection is built into the majority of electrical specifications for new build projects, and for good reason. The primary risks identified by the latest British Standard, BSEN 62305 2011 are, “loss of human life” and “loss of service to the public”.
In simple terms a Lightning Protection System can be broken down into five main categories.
- An Air termination network to receive the strike.
- Bonded connections to items of exposed metalwork to equalise earth potentials and prevent dangerous flashover.
- Down conducting paths to carry the discharge from the air termination network to earth.
- An Earth termination network to offer a low resistance connection into the general mass of earth.
- Surge protection devices (SPDs) to reduce the effect of transient overvoltages entering a structure through its services.
A grid of traditional flat tape or solid circular conductors of either aluminium or copper are usually installed to provide an air termination network linked to a series of down conducting paths. PVC sheathed conductors and fixings are available in a range of colours to mask their presence.
Items of exposed metalwork are connected directly to the lightning protection system (including the main earthing terminal) to prevent dangerous flashover between two items of differing electrical potential. Alternatively under certain criteria, items such as roof plant can be isolated by the use of air terminals.
At low level, copper conductor is installed from a test facility in the down conductor to an earth electrode, typically a 2.4m deep vertical driven rod surmounted by an inspection chamber or a copper lattice mat installed within a bed of conductive aggregate. Under certain circumstances it is necessary to link all down conductors with a buried ring conductor.
Where the requirements of BSEN 62305 permit and provided they are correctly designed into a lightning protection system by an ATLAS accredited designer, substantial metallic building components such as sheet roofing, architectural features, guttering, plant screens, etc, together with the steel framework, or steel reinforcing bars within a concrete structure are all acceptable for use as part of the lightning protection system negating the need for extensive visual traditional conductors.
Where external conductors are required they can be designed in at an early stage of the buildings conception and are regularly built into cavities, behind render and decretive curtain walling. In addition, conductors can be hidden by rain water pipes, installed below roofing tiles and beneath the surface of the increasingly popular ‘green’ and ‘brown’ eco style roofs.
In addition to structural protection, all services entering or leaving a structure and from any roof mounted equipment, must be provided with suitable surge protection devices (SPDs) to prevent transient overvoltages entering the structure as a result of a direct or indirect lightning strike