WHY PROTECT AGAINST LIGHTNING
reasons are immediately apparent:
Reduce the danger of structural damage to buildings
Reduce the damage to electrical equipment
Reduce the danger to people and animals
DANGER OR DAMAGE TO STRUCTURES.
Lightning damage to structures can vary from a burn mark on a wall to the complete demolition of a structure. For structures with thatched (grass) roofs the danger from lightning is of major concern, when a thatch is old, it is often covered with loose fibers, which can be easily ignited. It is also possible that when thatch is damp, flammable gases can be evolved. In addition to the risk of a direct strike, there is the risk of sparking in the roof due to induced voltages. To explain, often a thatch is reinforced with wire mesh, bundles of thatch are secured with metal binding wire, and metal-coated insulating sheets may be included in the thatch to reduce the fire risk. Usually these metals are not bonded together or earthed. Metal water pipes and electric wiring is often mounted just under the roof. In the event of a lightning strike all these metals can be at different potentials giving rise to sparking in the thatch with the resulting fire risk.
There are different risk factors for other types of structures, the risk involved for an explosives magazine is reasonable apparent, but what about a flourmill? The danger from lightning may not be apparent one site had bottles of acetylene stored in the open, the danger being that the physical shock from a lightning strike could be sufficient to start the acetylene decomposing, and start a major explosion (A bottle of acetylene fell from a truck in Durban at the harbor it was suspended from a rope and thrown into the sea. 48 hours later it was still to hot to handle). A normal dwelling may suffer from broken tiles in the event of a lightning strike (in the UK Chimney pots were regularly blown off the roof).
Damage to a structure is caused by a combination of effects these include:
When the lightning current flows the heat generated is dependant on the energy content of the lightning current wave form. From
SABS IEC 61024-1-1:1993 it can be seen that 95% of the first negative stroke have an energy of at least 6 x 103 J/O (Equivalent to A2. Second). This amount of energy flowing in a conductor will produce a significant temperature rise. The risk of fire has to be taken into account when determining the possible route to earth of a lightning strike.
At the attachment point of the arc the lightning current is focused on a small area, this produces a high temperature, which may burn through a thin skin.
If you have been close to a lightning strike you will know that you can feel the shock wave rather than hear the discharge. At the beginning of the first high current stroke, there is a rapid pinching of the arc channel due to the increase in the magnetic field, and this produces a radial acoustic shock wave, which can cause indentations on metal skins. At the attachment point of the arc there are more local (axial) acoustic forces and these combined with the magnetic forces also present may enhance the thermal damage. The severity of the acoustic shock depends on both current magnitude and the rate of rise of the current.
DAMAGE TO EQUIPMENT.
From the previous discussion it is obvious that any equipment situated on a roof is in danger of being damage from a direct lightning strike. However more equipment is damaged from the indirect effects of a lightning strike. These can range from a surge induced on the main electricity supply, or incoming services (e.g. telephone lines, pipelines). Damage can also result from the uncontrolled lightning currents inducing voltages in building wiring, and causing earth potentials to rise to different values in the same part of the building. The social and economic costs from lightning damage as can be seen are substantial, but in many cases lightning protection systems that could minimize these costs are not implemented.
DANGER TO PEOPLE AND ANIMALS.
Statistics that relate to death or injury due to lightning are difficult to obtain and are often inaccurate, especially when you consider the rural areas of South Africa. Figures that are released from the USA show that there are more deaths due to lightning that any other weather related causes with the exception of flash floods and river floods. Deaths and injuries are either caused directly by being struck by lightning or indirectly form fires caused by lightning. In an incident a few years ago a soccer match was being played in Johannesburg during a thunder storm, the lightning hit the stand, due to the potential difference (Step voltage) in the ground several of the players were rendered unconscious. In another case three giraffe were found dead in a clear area. The ground was sandy and there had been thunderstorm activity, none of the giraffe showed signs of having been hit by lightning. The conclusion reached was that lightning struck the ground near by, sandy soil has a high resistivity therefore a high step voltage would be present. The potential difference between the forelegs and hind legs was sufficient to kill the animals. There are many documented instances of sheep and cattle being killed in this manner. Usually if a person is unlucky enough to suffer a direct strike from lightning they are killed outright, however there are cases of people surviving. The fact that direct injuries from lightning usually take place out doors may explain why men are struck by lightning more often than women. Lightning injuries are varied and take many different forms, the most dangerous immediate complications are cardiovascular and neurologic. Deep burns are often a feature of a lightning strike.