Protection Angle Method - Lightning calculation

Protection Angle Method (PAM)

Introduction:

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

1)   Intercept a lightning flash to a structure. (Air terminal system)

2)   Conduct the lightning current safely towards the earth. (Down conductor system)

3)   Disperse the lightning current into the earth. (Earthing system)

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.

1)   Protection Angle Method

2)   Rolling Sphere Method and

3)   Mesh Method

Protection Angle Method:

The position of the air termination system is considered to be adequate if the structure to be protected is fully situated within the protected volume provided by the air-termination system. PAM can be used for the placement of following air termination systems,

-      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.

Example:

Let us consider an air-terminal with overall height of 10m. We can find the protection angle for all the four levels from the graph.

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.