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Lightning Protection Australia

LONGMONT

Lightning Protection Protection

A general discussion paper prepared by Terry Mulligan MIEEE, MACA MIICA February 1996 and modified August 2003


Introduction


Science tells us that the eventual death of the planet will be entropic. That is to say, all forces will eventually tend to neutralize such that life cannot be sustained. The chill of outer space will reduce the surface of the planet to a -273degrees centigrade; there will be no gravitational effect; there will be no wet or dry states; all gasses will have solidified or otherwise dissipated and therefore no charged state will arise. Planet Earth will be another cold rock in space. As a consequence at this time there will be no lightning. Between now and this time of entropic death, we will have a continuing change of temperature, gravity will continue to apply and electrostatic charges will continue to arise. Being aware of this, we know we will still have the effects of lightning to mitigate. Knowing how a charge can arise may assist our understanding of how best to mitigate any potential problem caused by lightning strike or that charge release.

I use the word mitigate for a good reason. Because of the many factors, atmospheric and ground based, involved in development and discharge of that electrical condition called lightning, all we can reasonably hope to cost effectively achieve is a reduced level of effect

LIGHTNING PROTECTION

from a strike. Although possible to completely negate any effect, galvanic, capacitive or inductive, the cost becomes increasing prohibitive as the level of protection rises. As such, we select the level of protection that is most cost effective for the specific application. This protection moves from lightning protection through coarse and close surge protection. The end result being the cost effective level of security necessary to allow confidence that our assets will be protected under most lightning strike conditions.


This discussion considers the ground strike and effects only.


Lightning


Ground Strike

That phenomena we call lightning is the visible element of ionisation and electron movement with the consequential current flow that occurs when two charged atmospheric states equalize their charges. The level of charge is measured in volts. When the charge is high enough for the voltage to overcome the resistance of the air, ground and cloud interface, there will be a break over and the charges will equalize. That is when the negatively charged clouds equalize with the positively charged ground below; the instantaneous movement of electrons between the two charged areas is seen as a brilliant flash of light. The heat generated by this exchange of electrons causes the air to rapidly expand and then contract. This physical movement of air is heard as a loud noise, we call thunder. The total effect on any object subject to or in the vicinity of such a discharge can be any or a combination of thermal, electrical and mechanical.

The greater the collection area of charge, the greater the number of charged particles to be dissipated at the time of charge equalisation. Additionally, the dryer the air between the negative clouds and the positive earth, the higher the break over voltage necessary to allow equalisation and consequently the higher the current; the higher the resistance of the earth at the point of equalisation with the ground, the higher the break over voltage necessary to effect equalisation.

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