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Earthing
- 1. LIGHTNING An abrupt electro-staticdischarge from cloud to cloud or, from cloud to earth accompanied by a FLASH Separation and Accumulation of Electrical Charges in the clouds via micro- and macro- physical During rain storms Clouds get electrically charged
- 9. LIGHTNING… • Negative chargesare near and +ve charges away from, the ground • When the charges build up enough Electrons tend to move towards the +ve charges • air gets ionized and the discharge path becomes a plasma with high conductivity • This discharge is in jumps called “stepped-up leaders”
- 10. How lightning occurs? • The movement is along the path of “least resistance” and hence Zig-Zag Form • When electrons approach the +ve charges, leaders (called streamers) develop towards the center of positive charge • Downward leaders start from the clouds and upward leaders from the earth • The two leaders join causing a “stroke”
- 11. How lightning occurs? • Up leaders causing strokes are called return strokes • Point of intersection of two leaders (about 10~ 30M from the center of the +ve charges” is the “center of the stroke • Large amount of heat is dissipated, producing FLASH & THUNDER
- 12. Lightning parameters • Cloudpotential - very small to 100 MV • Charge in a cloud - very small to > 300 C • Lightning flash - Has many strokes – upto 40 -Typically 4 to 5 strokes/Flash - Peak current is highest for the first stroke • Current in a lightning discharge to ground - negative stroke – 200 kA positive stroke – 300 kA • Rate of rise – 1011 A/s (max) = 100 kA/μs • More than 70% of the strokes are negative • Number of strokes/second – 2000 to 5000 (all over the world)
- 14. Lightning characteristics • 1~ 100 kA Current • Few hundred micro second duration • Very rapid current rise (up to 1010A/sec) • Average Dissipated Energy ~ 55 KW Hr • Velocity : 1/3 speed of light • Rise is fast 1~ 50 µS ec • Decay is slow: Time for half the max. value 50~250 µ sec
- 15. Characteristics Lightning Fast Riseof current means: Inductance of wires and skin effects of the conductors
- 16. Characteristics Lightning.. • Frequencyof thunderstorm occurrence 0.1T ~ 0.2 T, Where T: Number of thunderstorm days/year/sq.Km) • Only some thunderstorms are dangerous • Lightning is purely a random phenomenon • Researches being conducted for diverting lightning
- 17. Protection scheme • Capturethe lightning strike. • Safely convey this energy to ground • Dissipate energy into a low impedance grounding system • Bond all ground points together. • Protect incoming AC power feeders. • Protect low voltage data/telecommunications circuits.
- 18. Function of lightningarrestor The purpose of an arrestor is to • launch a successful up-leader before any other point on the structure to be protected • There by “Guide the charges to discharge through a safe passage to the ground, and dissipate the energy”
- 19. Lightning Arrestors Models.., •Lighting Rod (Franklin Rod) • Rolling Sphere • Parabolic-like lightning charge “Collection Volume” method • A conducting Rod (Franklin Rod) with sharp tip protrudes the installation. • It offers a “Cone of Protection” as shown in another slide
- 21. MOV • metal oxidevaristor (MOV) is an electronic component with a significant non-ohmic current- voltage characteristic • The name is for variable resistor • When triggered by high voltage, they will shunt the current • Often used to protect circuits against excessive transient voltages
- 22. MOV… • stacks ofMOV as the nonlinear element • virtually no current to flow during steady state operation and very large amounts to flow during over voltages • 90% of all MOVs: ZnO • The disks-shaped blocks of ZnO are made by a process involving grinding, mixing, pressing, and sintering • number of disks depend on dielectic properties of the material
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