The main and most effective measure for protection of structures against physical damage is considered to be the lightning protection system (LPS).
An external Lightning Protection System is intended to
The air termination system can be composed of rods, catenary wires or meshed conductors.
The following methods are used to determine the position of air termination system.
Protection Angle Method is the conventional method of providing protection to the structures. In this method the air terminals are placed at the top of the structure to be protected. Based on the height of air terminal from the reference plane and the level of protection considered, the volume protected by the air terminal varies.
The air terminals in the following configurations shall be used for PAM.
- Vertical rod air-termination system
- Wire air-termination system
- Wires combined in a mesh
The volume protected by a vertical rod is assumed to have the shape of a right circular cone with the vertex placed on the air-termination axis, semi-apex angle α, depending on the class of LPS and on the height of air-termination system from the reference plane.
From the above figure,
H – height of the building
h1 – physical height of air terminal rod
h2 – overall height of air terminal from ground reference plane.
α1 – protection angle corresponds to height h1
α2 – protection angle corresponds to overall height (H + h1)
The protection angle for various heights of air-terminals can be found by using the following graph. Based on the height and protection angle, we can calculate the protected area.
The angle will not change for values of h below 2m. In PAM, each level of LPS has its limitation on the height of air-terminal (LPL I -20m, LPL II -30m, LPL III – 45m & LPL IV- 60m), beyond which this method is not applicable.
Similarly, we can find the protection distance for any height and any levels of protection. If a structure to be protected cannot be covered using a single air terminal we have to go for multiple terminals placed as per the coverage distance and all the terminals should be inter connected to form a network.
Using the same method, we can calculate the protection area for wire air-termination system also.
From the above analysis, we can conclude that the protection provided by the air terminal depends on the two factors.
Among these two factors, the level of protection is determined by performing risk assessments. Each level has its limitation on the height of air terminal from the reference plane. Hence for particular class of LPS, the distance protected by the air terminals purely depends on the height of air terminal from the reference plane considered.