The forth part of the article "Lightning rods on the roof (lightning protection of the roof)"
On the photo fig. 5 three-meter rod lightning rods on the roof of a European industrial building. They protect climat-control machines and, as the use experience shows, succesfully manage with this task. Is it possible to make something similar in domestic pracice, not violating the requirements of normative documents? Fortunately, there is a solution and it is quite simple. The IS 153-34.21.122-2003 (p. 220.127.116.11) recommends to use closed catenary wire lightning rods. The efficiency of their protective action is high, especially, when the catenary wires are located outside the protected territory, covering it along the external perimeter. For example on fig. 6 you see the probability dependence of lightning breakthrough to the roof of the building 50x15 m, 20 m high from the height of the closed lightning protection wire hanging.
Fig. 5. Multi-rod system of lightning rods on the roof of the building for protection of ventillation machines of a production shop
The calculation is performed for different deviations D to the outer side. So, for deviation D=1m, the probability of a lightning breakthrough did not exceed 0,01 (protection reliability 0,99) at the height of the wire suspension relative to the ground level in 22,5 m and at D=2 m, the required height for this suspension reduced to 21,7. This way, the wire turned out to be effective at the exceeding of less than 2 m over the roof!
Fig. 6. The dependence of probability of a lightning breakthrough on the roof of the building on the height of the closed catenary wire lightning rod suspension regarding the ground level at the external deviations of the wire from the roof edge of 1 and 2 m
Вероятность прорыва – breakthrough probability
Здание 50 на 15 м высотой 20 м – building 50x 15 m, 20 m high
Высота грозотроса, м – catenary wire height, m
It means, that the protective acton of a couple of rods, installed with the distance of not more than LC between each other, on the length of this interval is equivalent to the catenary wire. Consequently, a closed catenary wire, hanged above the roof with the exceeding of Δh on the roof of any size, can be susbstituted by the system of rods Δh high on a roof of any size, if they are installed anove the whole length of the protected perimeter with the pitch LC=2,25 Δh. Rod lightning rods 2,5 m high will be required for the example viewed above, they should be installed with the distance of about 5,5 m along the perimeter of the part of the roof, which requires protection from direct lightning strikes. At that, one should not forget about provision of a very desirable deviation of rods outside the protected space, like it is prescribed in p.18.104.22.168 of the IS - 153-34.122-2003 for a closed catenary wire.
E. M. Bazelyan, DEA, professor
Energy Institute named after G.M. Krzyzanowski, Moscow
Read more "5. Features of lightning protection installation"
- 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