What BNBC stated about Lightning Protection System!!!
Lightning Protection System
1.3.33 Lightning Protection of Buildings
Whether a building needs protection against lightning depends on the probability of a stroke and acceptable risk levels. Assessment of the risk and of the magnitude of the consequences needs to be made.
Whether a building needs protection against lightning depends on the probability of a stroke and acceptable risk levels. Assessment of the risk and of the magnitude of the consequences needs to be made.
1.3.33.6 Integral parts of a lightning protection system
A smallest complete lightning protection system shall consist of (i) An air spike or air terminal, (ii) A down conductor, (iii) A roof conductor and (iv) An earth electrode.
An air spike or air terminal is that part which is intended to intercept lightning discharges. It consists of a vertical thick conductor of round cross section mounted on the highest part of the building to protect the required area. However, in general there may be more than one air spike or air terminal. In such a case roof conductors (made with copper strips or PVC insulated Annealed Stranded copper cables) need to be used to interconnect the Air Spikes or Air Terminals. Usually, for each Air Spike or Air Terminal there shall be one down conductor (made with copper strips or PVC insulated Annealed Stranded copper cables) going down up to the Earth Electrode pit and connected to the Earth Electrode. In all junctions, appropriate type of copper or brass junction plates or brass clamps must be used to ensure low resistance, firm and long lasting connection.
A smallest complete lightning protection system shall consist of (i) An air spike or air terminal, (ii) A down conductor, (iii) A roof conductor and (iv) An earth electrode.
An air spike or air terminal is that part which is intended to intercept lightning discharges. It consists of a vertical thick conductor of round cross section mounted on the highest part of the building to protect the required area. However, in general there may be more than one air spike or air terminal. In such a case roof conductors (made with copper strips or PVC insulated Annealed Stranded copper cables) need to be used to interconnect the Air Spikes or Air Terminals. Usually, for each Air Spike or Air Terminal there shall be one down conductor (made with copper strips or PVC insulated Annealed Stranded copper cables) going down up to the Earth Electrode pit and connected to the Earth Electrode. In all junctions, appropriate type of copper or brass junction plates or brass clamps must be used to ensure low resistance, firm and long lasting connection.
(a) Air spike/air terminal
An air spike or air terminal shall be made with copper rod of minimum 12 mm diameter with tin coating on top. The terminal shall have a copper/brass base plate for mounting on top of roof, column, parapet wall using rowel bolts. The minimum dimension of such a base plate shall be 152 mm x 152 mm x 13 mm. The length and width may need to be increased depending on the number of connection of the down conductors and the roof conductors. Such connections are to be made using hexagonal head brass bolts and nuts of 10mm diameter with brass washers.
An air spike or air terminal shall be made with copper rod of minimum 12 mm diameter with tin coating on top. The terminal shall have a copper/brass base plate for mounting on top of roof, column, parapet wall using rowel bolts. The minimum dimension of such a base plate shall be 152 mm x 152 mm x 13 mm. The length and width may need to be increased depending on the number of connection of the down conductors and the roof conductors. Such connections are to be made using hexagonal head brass bolts and nuts of 10mm diameter with brass washers.
A Down Conductor shall be made with copper strip or Stranded PVC insulated annealed copper cable.
(c) Roof conductor
A Roof Conductor shall be made with copper strip or Stranded PVC insulated annealed copper cable. This shall run along the periphery of the roof to link all air spikes and all down conductors installed on top of a building. The joints shall be made using clamps made of copper strips (of 1/8 inch minimum thickness) and appropriate brass bolts and washers of 3/8 inch minimum diameter.
(d) Earth electrode
The Earth Electrode is exactly of the same type as the Earth Electrode of the Electrical Distribution (Electrical Installation for Buildings) system described earlier in this document. Considering the practical situation in this country and Pipe Earth Electrodes are suggested. For each Air spike one Earth Electrode is an ideal solution.
(e) Earth inspection boxes
A 18 SWG GI sheet made Earth Inspection Box must be provided for each down conductor 1000 mm above the plinth level of the building (concealed inside the wall) which will contain a copper strip made clamp on the insulation peeled down conductor to check the continuity of the Earth Lead Down Conductor and the Earth Electrode and also to measure the Earth Resistance of the system. The box shall have a GI sheet made cover plate.
(f) Earthing pit
Earthing pits shall be provided as described in the earthing topic above
1.3.33.7 Number of lightning arrestors required and their installation
Number of Lightning Protection Air Spikes in a building will depend on the nature of the roof top, on the total area of the roof top, on the height of the building, height of the adjacent buildings, height of the nearby towers or other similar structures. However, as a thumb rule, for every 80 m2 area at least one air spike should be chosen at the beginning. During placement of the air spikes the total number may have to be increased or adjusted.
Number of Lightning Protection Air Spikes in a building will depend on the nature of the roof top, on the total area of the roof top, on the height of the building, height of the adjacent buildings, height of the nearby towers or other similar structures. However, as a thumb rule, for every 80 m2 area at least one air spike should be chosen at the beginning. During placement of the air spikes the total number may have to be increased or adjusted.
1.3.33.8 Protection zone
The zone of protection is the space within which an air spike provides protection by attracting the stroke to itself. It has been found that a single vertical conductor attracts to itself strokes of average or above average intensity which in the absence of the conductor would have struck the ground within a circle having its centre at the conductor and a radius equal to twice the height of the conductor. For weaker than average discharges the protected area becomes smaller. For practical design it is therefore assumed that statistically satisfactory protection can be given to a zone consisting of a cone with its apex at the top of the vertical conductor and a base radius equal to the height of the conductor. This is illustrated in Figure 8.1.5. A horizontal conductor can be regarded as a series of apexes coalesced into a line, and the zone of protection thus becomes a tent-like space (Figure 8.1.6).
(a) When there are several parallel horizontal conductors the area between them has been found by experience to be better protected than one would expect from the above considerations only. The recommended design criterion is that no part of the roof should be more than 9 m from the nearest horizontal conductor except that an additional 0.3 m may be added for each 0.3 m or part thereof by which the part to be protected is below the nearest conductor.
(b) The earth termination is that part which discharges the current into the general mass of the earth. In other words, it is one or more earth electrodes. Earth electrodes for lightning protection are no different from earth electrodes for short circuit protection systems. The total resistance of an electrode for a lightning protection system must not exceed 2 ohms.
(c) The down conductor is the conductor which runs from the air termination to the earth termination. A
building with a base area not exceeding 100 m2 shall be provided with one down conductor. For a larger building, there shall be one down conductor for the first 80 m2 plus a further one for every 100 m2 or part thereof in excess of the first 80 m2. Alternatively, for a larger building one down conductor may be provided for every 30 m of perimeter. Ideally, every air spike should have a down conductor going down up to the earth electrode.
building with a base area not exceeding 100 m2 shall be provided with one down conductor. For a larger building, there shall be one down conductor for the first 80 m2 plus a further one for every 100 m2 or part thereof in excess of the first 80 m2. Alternatively, for a larger building one down conductor may be provided for every 30 m of perimeter. Ideally, every air spike should have a down conductor going down up to the earth electrode.
(d) The material used for lightning conductors must be copper. The criterion for design is to keep the
resistance from air termination to earth electrode to a negligible value.
(e) Recommended dimensions for various components of lightning arrester are given in Table 8.1.29.
Larger conductors should however be used if the system is unlikely to receive regular inspection and
maintenance.
(g) When a lightning conductor carries a stroke to earth, it is temporarily raised to a potential considerably above that of earth. There is, therefore, a risk that the discharge will flash over to nearby metal and cause damage to the intervening structure. This can be prevented by either, (i) providing sufficient clearance between conductor and other metal or (ii) by bonding these together to ensure that there can be no potential difference between them. The necessary clearance is obtained from:
(h) Surge arrester selection
A surge arrester is a protective device for limiting surge voltages by discharging, or bypassing, surge current through it. It also prevents continued flow of follow-through current while remaining capable of repeating these functions. It is used to protect overhead lines, transformers and other electrical apparatus mostly in an outdoor substation from lightning voltages traveling through the overhead lines.
(i) Horn-gap lightning arresters
Horn-gap lightning arresters are commonly used for low and medium voltage overhead lines. The rating of the surge arrester shall be equal to or greater than the maximum continuous phase to ground power frequency voltage available at the point of application.
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