Monday, July 6, 2009

Lightning Protection System: An Overview

From the outset, a lightning seems far removed from an electrical system. Yet the protection from lightning strikes and from the damages caused by surges due to lightning strikes is an integral part of any electrical installation.

Men since the ancient times believed that a lightning strike was a demonstration of God's power, which He used as a weapon to punish those who commit great sins on earth.

Whatever the truth is, men today has developed a far more advanced knowledge and technology with regard to the lightning strikes and their effects than their ancient ancestors. Yet throughout the world the lightning strikes cause countless fatalities and material damages. Either through ignorance or an insufficient understanding of the principles underlying the theory and the practice of lightning protection.

Seen from the perspective of a building structure, lightning protection generally consist of 3 sections: aerial conductor network, the down conductors, and the earthing system. By design, the function of the aerial conductor network is to present at the highest point of the structures an area that will attract the lightning strike away from other parts of the building. The current of the strike will then be routed down through the down conductors to the earth grounding rod at the ground level or the lowest level in multiple basement structures.

The purpose of the earthing part is to help dissipate the discharge of the 200 kilo-ampere electrical energy into the earth mass as quickly as possible. This is very critical to the performance of the whole system, because a lower rate of dissipation will expose a higher risk of injuries or fatalities due to gradient voltage. Also damage to properties from flashover to ungrounded nearby metal parts.

Aerial Conductor Network

The aerial conductor network is designed to basically attract the lightning strikes. It is supposed to be where the lightning will strike when they attack a building structure. Therefore the aerial conductor network must be installed at the highest floor of the building structure.

Basically the intention is to create a network of conductors so that no point on the roof is more than five meters away from the nearest conductor of the network. Where a structure is raised above the roof level of a building (eg. lift motor room) then that part of the structure shall have their own aerial network conductor which shall be properly bonded to the overall buildings lightning protection scheme.

With the popularity of the new technology of lightweight all-metal roofing system, questions often arises whether the whole metal roofing system can be bonded to and form part of the lightning protection system without the need for a separate aerial conductor network.

Theoretically "Yes", these metal roofing system scan be used as the aerial conductor network and they have been practically installed to also serve that puspose for a while. Even though in many cases the aerial conductor network are still installed as a precaution.

However the development in the metal roofing system has resuled in the use of very thin roofing sheets that they require some form of metal parts to hold them together and to the roof structure. The effect is basically a complete isolation of a sheet fro the adjacent one, and to the roof structure. In these cases it is absolutely necesary to install a separate aerial conductor network.

Down conductors

The down conductor carries the 200kA electrical surge straight down to the earth mass. Due to the magnitude and the extremely steep front of the electrical surge, the current must be carried to the ground by the shortest possible route. Failure to do so will increase considerable the risk of side flashover.

The down conductor should be arranged to spread as uniformly as possible around the perimeter of the building at approximately 20 m spacing. They must follow the most direct path to the ground with a minimum number of bends. In difficult situations like at parapet walls, a loop-around turn must be avoided to prevent flashover. At 1.5 meter above ground, a test point must be provided at each down conductor. Here a bimetallic connector must also be installed if aluminum is used as the down conductor and the conductor below the test point to earth is copper.

Below the 1.5m test points, until terminations at the earth terminals inside the earth chambers, the down conductors must be installed inside a PVC pipe to protect it from the effects of corrosion and damage. Also to prevent a person or other metal parts from touching the conductors during lightning strike.

Copper used to be the conductor of choice for the whole lightning protection system including the down conductor in this country. But in the last ten years many engineers turn to aluminum for all parts above the test points. Parts of the reasons are the rising cost of copper materials. The other reason is vandalism. The vandalism factor is highly dangerous to the integrity of a lightning protection. This is due to the fact that it operates in silence and it is generally maintenance-free. A missing part at a few down conductors may not be noticed until the damages have been done.

This brings us to the very reason a lightning protection system must be regularly inspected. It will also ensure i is providing optimum protection. This will also check if there are other metal works within the flashover distance that is not properly bonded to the system.

Earth electrodes

The down conductors are routed along the most direct path to the ground and terminates to the lightning protection earthing system. These earthing systems usually steel reinforced copper rod half an inch in diameter, and driven a few feet deep into the ground.

Other methods are also used instead of the deep driven steel rods to provide the best possible contact with the earth mass. The choice depends on the prevailing ground conditions: the soils resistivity and the soil moisture content.In urban areas, a few earth rods are installed first and their combined resistance is measured. More rods are driven and connected until the 10-ohm requirement is met. However in locations away from recent developments, the soil conditions are not known. So the soil resistivity tests must be carried out to determine the earthing method most suitable and the extent of the work required.

At times when the soil is sitting on rocky subsoil below ground, a hole has to be drilled down 100 m or more and a long copper rod inserted deep into the hole to get the 10-ohm requirement. In more extreme cases, even this is not enough and soil conditioning agents like Bentonite or conductive cement need to be used. These compounds are prepared as a slurry or mix, and poured down the vertically drilled hole, creating a lower resistance between the embedded copper earth electrode and the surrounding soil.

The performance of a lightning protection is only as good as it weakest link. Any weakness along the flow path of the surge from the aerial conductor network down to the earth chamber can cause a side flashover to the building structure. A flashover means the surge current has taken an un-controllable path to the ground, causing damage to properties and possibly injuring persons along the way.Therefore it is vital that a lightning protection system is inspected properly and regularly. Recording of the tests must be done according to the latest international standards.

By the very nature of lightning strikes, no international standard can guarantee a total protection from their strikes. However, by obeying the latest international standards, we can ensure that the risk of damage and danger to human lives has been reduced to minimal.

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