The sixth part of the article "Lightning rods on the roof (lightning protection of the roof)"
Lightning protection specialists have had a long-term offense with the designers for their neglecting of the catenary wire lightning rods. There are reasons for this offense. The merit of catenary wire lightning rod is not only in possibility to make a closed protective contour around the object, thanks to which, the possibility of using multi-electrode rod lightning rods of minor elevation was proved. Advantages of catenary wire lightning protection are much more remarkable. Almost anything we know about protective action of lightning rods was achieved with the help of wires. For many years, the International Committee of CIGRE has been summarizing the experience of using catenary wire lightning rods on power lines. The controlled length of ground wires is counted by many hundreds of thousands of kilometers. Their use experience is credible and reliable. It is enough to say, that the method of calculating protection of rod lightning rods, which is used in almost all national regulatory documents on lightning protection, was tested and approved on the results of ovserving the wires.
But that is not the main point. As a rule, rod lightning rod, is placed directly near the protected object, or on its roof, as we have just seen. At a lightning strike into the rod lightning rod, its current is distributed directly over the metal lightning rod, and then along its down conductors. It doesn't remote from the object, and that is why, the electromagnetic field of the lightning current fully influences the object. Today, in the epoch of micro-processor technology, electromagnetic pickups may be more dangerous that the consequences of direct contact of constructions with plasma channel of lightning. Of course, rod lightning rod can be removed from the object, but it will be expensive, since to englarge the zones of protecting lightning rod, it will be necessary to increase its height, at least in direct proportion to the zone sizes. It is quite expensive and little effective.
The situation is principally different with the wires. A ground wire can be directly located over the protected object and wire supports can be removed to the side even to dozens of meters, almost not increasing their height and cost. Then the lightning current will be introduced into the ground on a significant distance from the track of underground utility systems of the obect, thanks to which their pulse pickups will reduce sharply. It is a great advantage for the protection of microprocessor technology.
Fig. 7. Comparison of efficiency of rod and catenary wire lightning protection
Вероятность прорыва – breakthrough probability
Объект 100 на 100 на 20 м – object 100 x100 x 20 m
Стержни - rods
Высота молниериемника м – lightning rod height, m
The use of a closed catenary wire lightnign rod is promising not only on the building's roofs. When installed directly on the ground surface, they possess the same efficiency. In fig. 7 the results of calculation of probability of a lightning breakthrough on the territory of 100 x 100 m, thickly filled with structures up to 20 m high. 4 Rod lightning rods are used for protection, which were removed 10 m from the protected territory due to electrical safety rules. To provide protection from direct lightning strikes on I level with the probability of 0,98 (probability of a breakthrough to the protected object 0,02) the height of rods should be not less than 38 m. If to fix a closed ground wire with the same location of supports, its height can be reduced to 27,5. Estimating this result, we should remember that at the reduction of height of supports, not only the cost of lightning protectio is reduced, but also the number of lightning strikes into lightning rods, which determine the frequency of close electromagnetic effects. Today it is even more important than the cost indexes.
Wire system must not necessarily be closed. It is enough to place the wires to 10-20 m further outside the protected territory. And it is important to fulfil two conditions at that. First of all, supports on each end of ground wires should present themselves a double rod lightning rod, the protection zone of which has no downfall in the middle, for what, as it was mentioned before, the distance between these supports should be less than LC (see formulas for calculating protection zones of a double rod lightning rod in IS-153-34.21.122-2003). Secondly, it is necessary to provide the so-called negative protgection angle between the ground wires and the protected object.
It is not a rare thing, when the third wire is required to fulfil these conditions, as it is shown in fig. 8, borrowed from the real project of lightning protection of a tank farm with tanks of 20000 m3. Protection reliability is very high in this case.
Fig. 8. Exaple of tank farm catenary wire protection
No one has doubts about reliabiliy of catenary wire protection. When you read this article, please do not forget, that the reliability of electric power supply of your computer and Internet on the whole, is guaranteed by catenary wire lightning rods. They protected the overhead power lines all over the plant in the thunderstorm season.
E. M. Bazelyan, DEA, professor
Energy Institute named after G.M. Krzyzanowski, Moscow
- How to choose lightning rods after all? (article by Professor E.M. Bazelyan)
- Lightning protection of roof in operaton (video recording and transcript of the webinar with professor E.M.Bazelyan)
- Free webinars for designers of grounding and lightning protection
- Grounding and lightning protection projects in dwg, pdf formats
- Free consultations and assistance in calculation of grounding and lightning protection