MEE looks at new scientific data on lightning prediction and how to protect your buildings from the possibility of strikes.
Ateam of international scientists studying the branching effects of downward lightning strikes has developed an improved formula for accurately estimating the expected number of lightning strikes to ordinary ground structures (such as low or high-rise industrial facilities), towers, and antennas.
Besides predicting the frequency of strikes, this formula provides the first available method for directly comparing lightning protection devices such as lightning rods with the more high-tech Dissipation Array Systems (DAS).
The devastating effects of a direct or secondary strike are responsible for millions of dollars in damage annually to industrial parks, manufacturing plant, hospitals, military installations, public buildings, emergency centres, corporate centres, chemical plant, oil refineries, and nuclear plant.
With only one direct strike, loss of life, equipment destruction, fire damage, production and inventory loss can occur. In addition, secondary strike damage to robotics, communication lines, computer equipment and security systems can result in extended facility downtime. Still, many facilities managers consider maintenance costs for repairs due to storm activity as 'normal' not knowing that most, up to 85 per cent, can be eliminated.
Research
There is still much about thunderstorms and lightning that is not clearly understood. However, a joint research team of international scientists continue to study the affects of lightning in an effort to ultimately provide better protection for residential and commercial facilities.
Dissipation Array Systems (DAS) - such as those offered by Lightning Eliminators and Consultants (LEC) - are based on the 'point discharge' principle or charge transfer and are designed to prevent lightning strikes from even terminating within protected areas. A sharp point in a strong electrostatic field will leak off electrons by ionising the adjacent air molecules, providing the point's potential is raised 10,000 volts above its surroundings.
Thousands of Dissipation Array Systems have been installed to date worldwide, in applications ranging from communications towers to tank farms, electric power lines to public buildings.
DAS has been used to protect facilities as large as three square kilometres and structures as high as 1,700ft. In the study, scientists were trying to determine what happens when a branch of lightning progresses toward a ground object protected with DAS, prior to other branches reaching the ground surface or some other grounded object.
Although branched downward lightning discharges are a common occurrence, they have rarely been studied in this level of detail because speeds are measured in single digit microseconds. To study this phenomenon, advanced computer simulations were required.
At the time of the research, no reliable method existed to calculate the number of lightning strikes expected to hit an object or facility that took into account the effect of coronae.
In the report, E M Bazelyan, of the Moscow Power Engineering Institute, writes, "Available formal methods (for instance, the electro- geometric method) are unsuitable for these calculations because they do not take into account the injection of corona space charge into the atmosphere and the interaction between the injected space charge, and the production of a cloud and a counter leader developed upward from a ground object," - required to make the connection to earth.
The concept of corona space charge is specifically important to the operation of the DAS. The DAS employs the point discharge principle by providing thousands of points with specific point separation, which simultaneously produce ions over a large area, creating a 'corona space charge' above the system. This prevents the formation of a rising conductive path, or counter leader, from the object, which is a necessary precursor of a lightning strike.
Ionisation
This ionisation process also creates a flow of current from the point(s) into the surrounding air. Under storm conditions, this ionisation current increases exponentially with the storm's electrostatic field, and the approach of a lightning leader, which can reach levels as high as 30,000 volts per metre of elevation above earth during a mature storm, prior to discharge propagation. The charge induced on the site by the storm is removed from the protected area and transferred to the air molecules, which form a shield above the site.
During a storm, the electrostatic field within the protected site has been measured and found to be suppressed by up to 7,000 per cent when compared with that outside the protecting influence of the DAS.
Due to the corona space charge created by DAS, the formation of counter leaders are delayed, prevented long enough to allow other branches of a downward leader to terminate on some other unprotected ground object. As the study demonstrates, after the contact between the downward leader and ground, the lightning channel is neutralised and the object protected with DAS remains undamaged.
"It is absolutely necessary to take into account the effect of corona space charge," Bazelyan writes. "Without this factor, all previously developed computer models are unsuitable for the analysis of lightning strikes to DAS."
Formula
With this in mind, the international team of scientists was able to develop the improved formula for estimating the number of lightning strikes to a facility. The formula, which takes into account not only the height of the object, but the shape (an important factor when considering DAS), could then be used to effectively estimate the risk of strikes to the installation with and without the DAS - the first time such a calculation has been made available.
Based on the formula, calculations showed that the installation of DAS on a ground object leads to a considerable reduction in the risk of lightning strikes to a protected site. This conclusion, combined with decades of documented field test data, demonstrates the effectiveness of the DAS system in reducing the risk of a strike to a rate of 99.7 per cent when properly engineered, designed and installed.
© Middle East Electricity 2004
