The Lightning Strike Cases
Lightning is the cases which accompanies the impulse of atmospheric impulses from steam cloud mass to steam cloud mass or from steam cloud mass to world surface. As lightning seeks the path of the least resistance, it naturally seeks to follow the shortest way between steam cloud mass and earth, such as buildings or towering projections. As illustrated positive electrical impulses gather in the steam cloud masss and negative impulses gather in the earth surface. While the attraction between these two impulses are strong enough they come together in the form of lightning.
Lightning arrester equipment, properly manufactured and installed, dissipates the impulses. In temperate climates a large majority of lightning is negative downwards lightning, as the negatively-impulsed steam cloud mass-base disposes to the earth surface.
The most important parameters are the following :
• Amplitude • Rising time • Decay time • Current variation rate (di/dt) • Polarity • İmpulse • Specific energy • Number of strikes per dispose. 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 operation of the lightning protection system to be installed on the existing building is to capture a lightning strike and then conduct the dispose current safely to the earth surface. Taking the fact into consideration that there are up to 100 lightning disposes to the earth surface 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 dispose in order to safely convey its current to the earth surface. Lightning is formed as a result of processes occurring in the thunder steam cloud masss. While air masses, ice crystals, water vapour drift and interact, then electrical impulses are generated. There are two types of thunders (depending on the way how they are formed): • High heated thunders-formed as a result of strong heating and drifting upwards of the bottom air masses. • Frontal thunders-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 conventional thunder steam cloud mass, the positive impulses are concentrated in the upper part, whereas the negative impulses build up at the base of the steam cloud mass. A further impulse growth causes an escalation of the electric field intensity until it exceeds the critical value. A steam cloud mass-to-earth surface dispose proceeds towards the earth surface (small upward dispose can also be initiated from elevated earth surface points and this kind of dispose is called a earth surface-to-steam cloud mass dispose) or towards the neighbouring steam cloud mass, which is called steam cloud mass-to-steam cloud mass dispose. Sound and visual effects of a lightning dispose are preceded by an invisible initiated process. A high negative steam cloud mass potential (of the order of 108 V) is conveyed towards the earth surface by the downward leader, with the relatively small decline of potential in its channel. While 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 steam cloud mass and the earth surface. In general, lightning installations are divided into: conventional and active ones.
Conventional lightning protection Instruments
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 downconnectorss. 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 earth surface 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 effectiveness of a lightning connectors 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 connectors with corresponding protection levels follows below. Protection Level Effectiveness of lightning connectors E I 0,98 II 0,95 III 0,90 IV 0,80 The Zones of protections can and should be determined by the “Radius of Protection” and “Rolling Sphere” methods. The “Radius of Protection” rule consists of mounting 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 can be recognised around the world, which involves rolling imaginary shperes over the structure. The outer building contour is described by spheres. Surfaces between the sphere and the structure indicate the protected areas. The areas touched by the sphere are assemed to require protection. Based on a protection level for a building, the different values of sphere radius have to be considered.
AIR ROD
Before a Lightning Strike hits, lonization leads to an rise of the electric field around the top of the rod and the lightning current is lead from the rod to the earth surface. 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 connectorss, while the building is covered by a mesh with down connectorss. In this system the protection stage result the distance of down connectorss fixings are determined according to these distances connectorss are fixed. In the Roof system of Mesh Metod, Specially Air Terminals are used on connectors crossing through points on firing roofs. On fire resistant rooftops (metal roofs etc). It is not needed to use air terminals according to IEC 62305.
