Lightning and its protection
Lightning is an unpredictable natural phenomenon that occurs within the atmosphere or between the atmosphere and the earth surface. This powerful discharge usually made during thunderstorm. Lightning current is a consequence of lightning.Direct lightning stroke is caused by the lightning strikes on buildings or specified zones. When the lightning strikes or induces power surges in power cables or transmission links, accidents are caused by indirect lightning stroke. Power surges are the short time voltage impulses that last less than a second and often reach 6000v and 3000 amps. Power surges cause damage of electronic equipment.
Lightning strikes are subdivided into four groups based on the point of strike.
S1 – Direct lightning strike to the structure
S2 – Lightning strike near a structure
S3 – Direct lightning strike to an incoming line
S4- Lightning strike near an incoming line
In order to protect buildings from direct lightning strikes, a lightning protection system is essential. It should meet the requirement of class of LPS III according to IEC 620305-3(EN 62305-3) is recommended as a minimum.
Components of a lightning protection system
- Air termination system
- Down conductor system
- Earth termination system
Air termination system design
The duty of air termination system is to intercept the lightning flashes from reaching the destination Down conductor system. Mainly there are three protection method for the design and installation of Air termination system.
- Mesh method, which is used for flat type of structures.
- Protection angle method, which is suitable for finials and single mast. Finials are hip-knob structures that makes the top or end of an object. Single mast is seen in boat to move forward and backward according to the direction of wind. This method has height limitations for its use.
- Rolling sphere method is a universal method for all type of structures. It is even suitable for the most complicated geographical arrangement. The radius of the rolling sphere is determined by the lightning protection level, which is derived from lightning protection risk assessment system. The rolling sphere is rolled in all the possible direction, upto and over the structure. The potential point of strike on the structure is considered as the parts or points where the sphere touches the structure. That point is called the air terminals.
Lightning protection
| Level(LPL) | Radius of sphere
|
| LPL IV | 60 m |
| LPL III | 45 m |
| LPL II | 30 m |
| LPL I | 20 m |
Down conductor system
The lightning flashes are guided towards the ground by the down conductor system. The lightning rods connected to the ground should have at least two down conductors located on opposite facades. The down conductors are connected outside the building. The proximity of electric cables and gas should be avoided. The path of the down conductor should be straight, any sharp bent or lift is to be avoided. The materials and dimensions for down conductor should follow IEC 62561-2 according to the nature of lightning strike.
Earth termination system
It is the continuation of air termination system and down conductor system installed on top of the building. A properly designed earth termination system can dissipate the lightning flashes to the ground. It provides equipotential bonding between down conductor and potential contact. Inorder to achieve an effective earth termination system, the resistance value should keep zero.