Protection of Building Against Lightning

Protection of Building Against Lightning Points : protection of building against lightning, zone of protection, component parts and their installations, air terminations, down conductors, earth termination, earth electrodes , completion report General The protection of building against lightning shall generally and those of special structures like livestock in fields, structures for highly combustible materials etc. shall strictly be done in accordance with I S 2309-1969. For the purpose of protection of buildings against lightning both the vertical and horizontal conductors may be used as air terminations depending upon the type of building to be protected. Horizontal air terminations may be used in buildings with flat roof extending over a large area such as R.C.C. framed structure, multistoried buildings etc. Vertical air terminations may be used in buildings having towers and domes etc. A system of both vertical and horizontal conductors might be necessary for protection of bigger buildings. The materials of lighting protective system shall be copper or galvanized steel as specified. Brief specifications are described here below: Zone of protection The zone of protection of a lightning conductor denotes the space within which a lightning couductor provides protection against a direct lightning stroke by diverting the stroke to itself. For a single vertical conductor, this zone is described as a cone with apex at the highest point of the conductor and with an angle, called as protective angle, between the side of the cone and the conductor. Normally, this angle may be taken to be 45 degrees. Component parts and their installations The principal components of a lightning protective system are: Air terminations Air terminations may consists of a vertical conductor or a horizontal conductor, or a system of both, horizontal and vertical conductors, depending upon the type of building as mentioned above. In the case of a vertical termination, it need not have more than one points and shall protect at least 30 cm above the object on which it is fixed. Horizontal air terminations should be so inter-connected that on part of the room is more than 9 metres away from the nearest horizontal conductor. For a flat roof, horizontal air-terminations along the outer perimeter of the ridge, parapet etc. shall be used. The air terminations shall be joined with each other so as to form a closed network and should cover all salient points of the structure. The air termination should be fixed permanently so as to avoid over turning due to any reason. Down conductors The number and spacing of down conductors shall largely depend upon the size and shape of the building and upon aesthetic considerations. The minimum number of down conductors may, however, be decided on the following considerations:

A structure having a base area not exceeding 100 m2 may be have one down conductor only if the height of air termination provides sufficient protection. However, it is advisable to have at least two down conductors except for very small buildings. For structures having a base area, exceeding 100 m the number of down conductors required should be worked out as follows:

(1) One of the first 100 m2 plus one more for every additional 300 m2 or part thereof.
(2) One for every 30 metres of perimeter. The smaller of the two shall apply.

Down conductors should be distributed round the outside walls of the structure, preferably, along the corners and other projections. Lifts shafts shall not be used for fixing down conductors. The down conductors shall follow the most direct path between the air terminations and the earth terminations. Sharp bends, upturns and kinks should not be provided. The down conductors should be protected against mechanical damage. Metal pipes should not be used as protection for the conductors, in buildings of cantilever construction the down conductor shall be taken straight down to ground. All metallic items extending beyond the top of the structure and running vertically through the structure should be bonded to the lightning conductor at the top and the bottom.

The lightning protection system shall have as few joints in its as possible. Where joints in the down conductor above ground level are necessary, they shall be mechanical and electrically effective. In the down conductor below ground level, there shall be no joints. The joints may be ‘clamped, screwed, or welded as may be necessary. External metal on or forming part of a structure may have to discharge the full lightning current, therefore, the bond to the lightning protection system shall have a cross-sectional area not less than that employed for the main conductors. Bonds shall be as short as possible.

Conductors shall be securely attached to the building or other objects to be protected by fasteners which shall be substantial in construction, not subject to breakage and shall be of galvanized steel or other suitable materials to avoid corrosion. The lightning conductors shall be secured at not more than 1.20 metres apart for horizontal run and 1.00 metres for vertical run. Earth termination Each down conductor shall have an independent earth termination. The interconnection of all the earth terminations is preferable. It should be capable of isolation for testing purpose by “Testing Joints”. Water pipes should be installed away from the earth terminations and should not be bonded to it. The gas pipe should in no case be bonded to the earth termination system. The whole of the lightning protective system should have a combine resistance to earth not exceeding 10. ohms before any bonding has been effected to metal in or on a structure or to surface below ground. Earth Electrodes Earth electrodes shall be constructed and installed in accordance with IS 3034-1966 (Code of Practice for Earthing). G.I plate shall be used as earth electrode except where it is unavoi4able to use copper plate earth electrode due to corrosive soil conditions. When soil contains sulphur, copper electrode shall be adequately tinned. The earth eletrode should be installed at places where best contact with earth are obtainable. These should be spread out and not squeezed together.

The minimum dimensions of the electrode shall be as under:
(a) G I plate electrode-60 cm 60 cm X 60 mm
(b) Copper plate electrode-60 cm x 60 cm x 3 mm

The electrodc shall be buried in ground with its face vertical and top not less than 5 metres from ground level or not les than 60 cm below summer water level whichever is more. Medium class, G.I. pipe of 20 mm diameter, with a funnel with mesh, at the top of the pipe, shall be provided from ground to electrode of watering the earth. The watering funnel attachment shall be housed in a masonry enclosure of not less than 30 cm x 30 cm X 30 cm size.

A cast iron frame with hinged cover having locking arrangements shall be suitably embedded in the masonry enclosure. For artificial chemical treatment of soil, the electrode shall be surrounded by charcoal/coke and salt in alternative layers to a thickness of not less than 15 cm on all the sides.

The down conductor shall be securely bolted to the plate (earth electrode) with two G.I. or copper bolts, nuts, check nuts and washers. The down conductor from the electrode onwards upto the building shall be suitably protected from mechanical injury by 40 mm diameter medium class G.I. pipe. The portion of the protective pipe within ground shall be buried at least ,30 cm deep and shall be increased to 60 cm ‘in case of road crossing and pavements. No earth electrode shall have greater ohmic resistance than 5 ohms as measured by an approved earth testing apparatus. In rocky soils, the resistance may be upto 8 ohms. Completion report After completion of the installation, test results on the prescribed Proforma (Appendix E) and -layout of the different components duly- marked on the building plan shall be submitted. The portion of the building etc. damaged during erection of installation shall be repaired properly to original finish and colour of the building etc.