THE LIGHTNING PHENOMENON

Lightning is the phenomenon which accompanies the discharge of atmospheric charges from cloud to cloud or from cloud to earth. As lightning seeks the path of the least resistance, it naturally tends to follow the shortest way between cloud and earth, such as buildings or towering projections. As illustrated positive electrical charges gather in the clouds and negative charges gather in the ground. When the attraction between these two charges are strong enough they come together in the form of lightning.

Lightning arrester equipment, properly manufactured and installed, dissipates the charges. In temperate climates a large majority of lightning is negative downwards lightning, as the negatively-charged cloud-base discharges to the ground.

The most important parameters are the following :

• Amplitude
• Rise time
• Decay time
• Current variation rate (di/dt)
• Polarity
• Charge
• Specific energy
• Number of strikes per discharge.

The first three parameters are independent in terms of statistics .
The expected effects of the characteristic lightning parameters are as follows:

• Optical effects
• Acoustical effects
• Electro-chemical effects
• Thermal effects
• Electro-dynamic effects
• Electro-magnetic radiation

LIGHTNING PROTECTION

Lightning Protection system are seperated two parts ;
External Lightning Protection System (LPS) and Internal Lightning Protection System.

External Lightning Protection System - LPS
External LPS is designed for protection of structures opposite to the direct lightning strokes.
For external LPS the following three different methods are used,

• Air Rod
• Mesh Method
• Early Streamer Emission Lightning Rods

The protection level must be calculated before one of the above mentioned methods can be chosen. (IEC 62305- 1 - 1 and standarts of Europe)

LIGHTNING PROTECTION THEORY

Requirements for the the safety of people staying at work, home etc., made it necessary that special tasks were assigned to design engineers to take care about the quality of lightning protection system on every higher building. The main function of the lightning protection system installed on the existing building is to capture a lightning strike and then conduct the discharge current safely to the ground. Taking the fact into consideration that there are up to 100 lightning discharges to the ground every second throughout the world, it is always possible that a lightning strike hits the place which is particulary close and important to you. The lightning protection system exists to intercept an atmospheric discharge in order to safely convey its current to the ground. Lightning is formed as a result of processes occurring in the storm clouds. When air masses, ice crystals, water vapour drift and interact, then electrical charges are generated. There are two types of storms (depending on the way how they are formed):

• heat storms-formed as a result of strong heating and drifting upwards of the bottom air masses.
• frontal storms-caused by the impact of a front of cold air on a warm moist air mass which is lifted above the advancing cold front.

In a typical storm cloud, the positive charges are concentrated in the upper part, whereas the negative charges build up at the base of the cloud. A further charge growth causes an escalation of the electric field intensity until it exceeds the critical value. A cloud-to-ground discharge proceeds towards the ground (small upward discharge can also be initiated from elevated ground points and this kind of discharge is called a ground-to-cloud discharge) or towards the neighbouring cloud, which is called cloud-to-cloud discharge. Sound and visual effects of a lightning discharge are preceded by an invisible initiated process. A high negative cloud potential (of the order of 108 V) is conveyed towards the ground by the downward leader, with the relatively small decline of potential in its channel. When one of the upward leaders comes into contact with the downward leader, a conductive path of ionized air is created allowing a powerful current to flow equalizing the potential difference between the cloud and the ground. In general, lightning installations are divided into: conventional and active ones.

Conventional lightning protection system

Conventional lightning systems are based on the protection of a structure by making an installation of horizontal or vertical air terminals which are connected to the earth with the help of downconductors. By means of the procedure described below, it is possible to decide according to the chosen protection level whether the lightning protection is required or not. On the ground of our observations and experiences gained in this field, we recommend to install the lightning protection on the structure regardless of the existance of a strike hazard level.

The selection of the protection level allows to minimize the risk of damage to people as well as complex and sophisticated equipment and structures. The higher the efficiency of a lightning conductor is, the lower the risk of damages caused by lightning strikes will become. A protection level selection depends on the kind of building, its structure and value.

A List of the effectiveness of lightning conductor with corresponding protection levels follows below.

The Zones of protection can be determined by the "Cone of Protection" and "Rolling Sphere" methods.

The "Cone of Protection" rule consists of placing the protected structure in the zone of protection of high vertical air terminal.

Angle α for the protection level 0.95 is a 45º "Rolling Sphere" rule, which involves rolling imaginary shperes over the structure. The outer structure contour is described by spheres. Points between the sphere and the structure indicate the protected areas. The areas touched by the sphere are deemed to require protection. Depending on a protection level for a structure, the different values of sphere radius have to be considered.

AIR ROD

Before a Lightning Strike hits, lonization leads to an increase of the electric field around the top of the rod and the lightning current is lead from the rod to the ground. According to table 1, the protected area connects the level of protection according to the protective angle, the length of the rod, the height of the rod above the surface to be protected. Air Rods are used for the mesh method and stretched technique.

This is done by connecting the Air Rods to the down conductors, while the building is covered by a mesh with down conductors. In this system the protection stage result the distance of down conductors fixings are determined according to these distances conductors are fixed.

In the Roof system of Mesh Metod, Specially Air Terminals are used on conductors crossing points on firing roofs. On f i re resistant roofs (metal roofs etc ...) It is not needed to use air terminals (IEC 62305)

Mesh Method Details

Protection Levels According to Height and α Angles