Lightning Protection System Design - Rolling Sphere Method (IEC62305)

Introduction:

There are no methods or devices capable of modifying the natural weather phenomena to the extent that they can prevent lightning discharges. Lightning flashes to, or nearby structures or lines connected to structures are hazardous to people, to the structure themselves, their contents and installations as well as to lines. This is why the application of lightning protection measures is essential.

IS/IEC 62305-3 specifies the following three methods to determine the position of air termination system.

  1. Protection Angle Method
  2. Rolling Sphere Method and
  3. Mesh Method

Protection angle method is suitable for simple shaped buildings. It also has limitations on the height of the air terminal. The mesh method is suitable where plane surfaces are to be protected. Whereas rolling sphere method can be used in all the cases.

Rolling Sphere Method:

In this method, the air terminals are placed by rolling an imaginary sphere around and on top of the structure to be protected in all the possible directions. The sphere should not make any contact with the structure to be protected and touches only the air termination system and the ground.

The radius of the rolling sphere for different classes of LPS is as follows

The variation of protection zones for different levels of Lightning Protection Levels are shown below.

Variation of protected area with respect to different levels of protection

The radius of the rolling sphere is corelated with the peak value of the current in the lightning that strikes the structure,

Where,
I - peak lightning current in kA.
The point on which the sphere touches the structure will be exposed to lightning strikes and at such points protection by an air-termination conductor is required.

Let us consider a rod air terminal placed at the top of a building. The protection provided by the air terminal as per rolling sphere method for different classes of LPS are shown below.

From the above image, we can find that the protected region varies for different classes of LPS and different heights of air terminal. The air terminals should be placed in such a way that the sphere doesn’t make any contact with the structure to be protected.

Penetration distance:

The distance between the two air terminals should be chosen in such a way that, the protection is provided for all the objects placed on the surface to be protected. The protection of the objects placed on the surface can be ensured by calculating the penetration distance of the rolling sphere. The distance between the level of air terminals and the least point of sphere in the space between the air terminals is called penetration distance. By comparing the penetration distance with the height of the object placed between the air terminals, we can find whether the object placed is protected or not.

Let us consider an object of height ‘h’ placed on the surface to be protected. Let ‘ht’ be the height of the air terminal, ‘p’ be the penetration distance and ‘d’ be the distance between the two terminals. In this case, the penetration distance should be less than the difference between the height of air terminal and height of the object to be protected

Distance between air terminals:

The penetration distance of the rolling sphere below the level of conductors in the space between the conductors can be calculated by using the below formula (IS 62305-3).

Where,
p – penetration distance
r – radius of rolling sphere
d – distance between the air terminals
For attaining a particular penetration distance, we can derive the required distance between the air terminals from the above equation.

The penetration distance should always lesser than the height of the air terminal. If the penetration distance and the height of air terminal are same, then the sphere just touches the reference place. The distance at this condition can be calculated by substituting the value of height of air terminal (ht) in place of penetration distance (p).

Conclusion:

From the above analysis, we can conclude that the distance between the air terminals(d) in rolling sphere method depends on two factors.

1) Height of the air terminal and
2) Radius of the rolling sphere

Among these two factors, the radius of rolling sphere is a constant value which depends on the class of LPS as specified by IS/IEC 62305-3. Hence for particular class of LPS, the distance between the air terminals purely depends on the height of air terminal.

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