The choice over whether a mesh of roof conductors, vertical air terminals, or fortuitous elements of the building etc are used, is dependent on the Lightning Protection Designer and the nature of the structure itself. There is no such thing as a standard Lightning Protection Kit. Each building needs to be examined in its own right and a system designed to suit that particular structure and its use or purpose.
The type of conductor used – aluminium, copper, composite or PVC covered – has a bearing on which fittings to use, as does the shape and size of the conductor. Conductors can be supplied in a rectangular and solid circular section.
Kingsmill offer a comprehensive range of Air Terminals:
- Lightweight Lightning Mast (5m to 20m in height)
- Free-standing Air Terminals (1m to 10m in height)
- Conventional Small Air Terminals (0.5m to 3m in height)
- Air terminals for use with Insulated Lightning Conductor cable (3m to 7m in height)
This give the architect, designer, contractor and client flexibility over height, as well as installed aesthetics of the system.
Each system has its features and benefits which are summarised below:
CONSIDER ON ROOF
|Lightweight Mast||5m - 20m||Excellent - can be customised to blend into architectural or landscape setting||45+ years||Excellent||Simple, quick, lightweight||Civil contractor to cast in foundation bolts||Civil contractor to cast in foundation bolts||Excellent - conical design improves strength|
Resists wind deflection
|Lowers to ground on hinged base|
|3m - 10m||Average aesthetics||30 - 40 years||Good||Heavy, requires concrete blocks and guy wires for tall versions||Free-standing base||Weight of concrete blocks/tripod base and guys take up space||Subject to deflection in wind and storm conditions|
|Conventional small air terminals||0.5m - 3m||Average aesthetics||30 - 40 years||Good||Lightweight, easy fixing||Small base||Can be broken if hit by an object|
|Air Terminals used with Insulated Lightning Conductor cable||3m - 7m||Average aesthetics||30 - 40 years||Good||Heavy, requires concrete blocks and guys for tall versions||Free-standing base/wall bracket mounting||Weight of concrete blocks/tripod base takes up space||Subject to deflection in wind and storm conditions||Reduces separation distance/useful where roof space is limited|
From BS:EN 62305 . . .
The Lightning Protection Designer may need to consider the following when designing a system . . .
Roof mesh conductor system
BS:EN 62305 recommends the use of a protective mesh of conductors laid over the structure to be protected. This mesh can be supplemented with the use of Air Terminals. The risk assessment carried out from BS:EN 62305-2 determines both the Lightning Protection Level (LPL) and Lightning Protection Class (LPC) for a structure.
Roof mesh spacing
The roof mesh is spaced in accordance with guidelines contained within BS:EN 62305-3. This can be summarised below:
Example of a roof mesh system
Perimeter conductors should be placed close to the edge of the structure. If possible, place down conductors at each corner as well as spacing them equally around the structure.
A minimum of two down conductors is required, whether an Air Terminal, Mesh or Rolling Sphere concept is used.
Protective angle/zone of protection
The protective angle differs according to the class of Lightning Protection System (ie I, II, III or IV) and the height of the Air Terminal above the reference plane.
Examples of how the protective angle changes with height and LPS class are shown to the right.
Roof mounted terminal
In this circumstance there are two reference planes and angles:
A Roof to tip of Air Terminal
B Ground to tip of Air Terminal
The Rolling Sphere can also be used to determine protective area.
Some examples of Rolling Sphere applications . . .
The sphere is rolled all over the structure. Where the sphere touches the building, protection is required.
This protection is usually in the form of a mesh system of conductors.
Buildings over 60m in height should have both lateral (side) and roof conductors installed.
Spacing of conductors would still conform to the roof mesh conductor system tables (above).
When used in conjunction with a Lightning Mast . . .
The object being protected must be contained within the green protected area.
When used in conjunction with several Lightning Masts . . .
A separation distance needs to be maintained between the top of the object being protected and the bottom of the sphere.