Table of Contents
Protection of special structures, such as those exceeding 30 m in height, structures with roofs of high inflammability, buildings with explosive or highly inflammable contents, fences, trees and structures near the trees, structures supporting overhead electricity supply, telephone and other lines, structures with radio and television aerials, tents, metal scaffoldings and similar structures, tall metal masts, tower cranes and revolving and travelling structures, farm buildings in area of high lightning incidence, sports stadium, raised motorways, bridges, dwelling houses etc is done in accordance with IS 2309 : 1989.
NEED FOR PROTECTION
In determining how far to go in providing lighting protection for specific cases or whether or not it is needed at all, we have to calculate the risk factor by taking into account the following factors.
- Usage of structure,
- Type of construction,
- Contents or consequential effects,
- Degree of isolation,
- Type of terrain,
- Height of structure, and
- Lightning prevalence.
The effect of these factors is given in detail in appendix I to the general specification of electrical works part I (Internal). However for certain types of structures, there will be little doubt as to the need for lightning protection. The examples of such structure are:
- Those in or near which large number of people congregate.
- Those concerned with the maintenance of essential public services,
- Those in areas where lightning strokes are prevalent,
- Very tall or isolated structures,
- Structures of historic or cultural importance, and
- Structures containing explosives and highly flammable materials.
CALCULATION OF NEED FOR PROTECTION
ESTIMATION OF EXPOSURE RISK:
The probability of a structure or a building being stuck by lightning in one year is the product of the ‘lightning flash density’, Ng , and the ‘effective collection area of the building’ Ac , which are defined below and is given by the following formulae:
The probability of strikes (risks) to the structure per year:
P = Ac × Ng × 10-6
Now to evaluate the overall assessment of risk, the probabilty of strikes should be multiplied by the the coefficient Ce , which is the multiple of various weightnig factors:
Ce = A × B × C × D × E
A Weighting factor for use of structure.
B Weighting factor for type of construction.
C Weighting factor for contents or consequential damages.
D Weighting factor for degree of isolation.
E Weighing facror for Type of country.
So we get overall risk as
Pd = Ce × Ac × Ng × 10-6
This overall risk should be compared with the accepted risk figure of 10-5 i.e. 1 in 100,000 per year. If the overall risk is considerably less than 10-5 then in the absence of other overriding considerations, protection does not appear necessary. If the result is considerably greater than 10-5 say 10-4, then protection becomes necessary.
However there are some anamolies:
- If the structure is so varied that any method of assessment may lead to anamolies, proper judgemet becomes necessary. For example, a steel framed building may be found to have a low risk factor, but an addition of an air termination and earthing system will greatly improve the protection and cost of providing this may be considered worthwhile.
- A low risk factor may result for chimneys made of brick or concrete. However where chimney are free standing, or where they project more than 4.5 m above the adjoining structure, they will require protection regardless of the factor. Such chimneys are therefore not covered by the method of assessment. Similiarly structures containing explosives or fllammable substances are also not covered.
Lightning Flash Density (Ng ):
It is the flashes to ground per square km per year and is estimated based on the thunderstorm days/year. The relation between the two is given in the table. Also the thunderstorm days for certain cities is given in another table.
|Thunderstorm days per year||Lightning flashes per sq. km. Per year|
|5||0.2||0.10 - 0.50|
|10||0.5||0.15 - 1.0|
|20||1.1||0.30 - 3.0|
|30||1.9||0.60 - 5.0|
|40||2.8||0.80 - 8.0|
|50||3.7||1.20 - 10.0|
|60||4.7||1.80 - 12.0|
|80||6.9||3.00 - 17.0|
|100||9.2||4.00 - 20.0|
Effective Collection Area (Ac):
It is the area on the plan of the structure extended in all the directions to take account of its height. The edge of the effective collection area is displaced from the edge of the structure by an amount equal to the height of the structure at that point. So far a simple rectangular building of length L, width W, and height H meters, the collection area has length (L + 2 H) meters and width (W + 2 H) meters, with four rounded corners formed by quarter circles of radius, H meters. So the collection area:
Ac = (L x W) + 2 (L × H) + 2 (W × H) + π × H² m².
Table: Weighting factor ‘A’ (Use of Structure)
|Use to which structure is put||Value of ‘A’|
|Houses and other buildings of comparable size.||0.3|
|Houses and other buildings of comparable size with outside aerials.||0.7|
|Factories, workshohops and laboratories.||1.0|
|Office blocks, hotels, blocks of flats and other residential buildings other than those included below||1.2|
|Places of assembly, for example churches, halls, theatres, museums, exhibitions, departmental stores, post offices, stations, airports and stadium structures||1.3|
|Schools, hospitals, children’s and other homes||1.7|
Table: Weighting factor ‘B’ (Type of construction)
|Type of construction||Value of ‘B’|
|Steel framed encased with any roof other than metal*||0.2|
|Reinforced concrete with any roof other than metal||0.4|
|Steel framed encased or reinforced concrete with metal roof||0.8|
|Brick, plain concrete or masonary with any roof other than metal or thatch||1.0|
|Timber framed or clad with any roof other than metal or thatch||1.4|
|Brick, plain concrete, masonary, timber framed but with metal roofing||1.7|
|Any building with a thatched roof||2.0|
* A structure of exposed metal which is continous down to ground level is excluded from these tables as it requires no lightning protection beyond adequate earthing arrangement.
Table: Weighting factor ‘C’ (Contents or Consequential effects)
|Contents or Consequential effects||Value of ‘C’|
|Ordinary domestic or office building, factories and workshops not containing valuable of especially susceptible contents||0.3|
|Industrial and agricultural buildings with especially susceptible* contents||0.8|
|Power stations, gas works, telephone exchanges, radio stations||1.0|
|Industrial key plants, ancient monuments and historic buildings, museums, art galaries or other buildings with specially valuable contents||1.3|
|Schools, hospitals, children’s and other places of assembly||1.7|
* This means specially valuable plant or material vunrelable to fire or the results of fire.
Consequential effects is not only intended to cover only material risks to goods and property, but also such aspects as the disruption of essential services of all kinds, particularly in hospitals, where if a lightning strikes, fire or panic can naturally result.
Table: Weighting factor ‘D’ (Degree of Isolation)
|Degree of Isolation||Value of ‘D’|
|Structures located in a large area of structures or trees of the same or greater height, for example in a large town or forest||0.4|
|Structures located in an area with few other structures or trees of similar height||1.0|
|Structures completely isolated or exceedind at least twice the height of surrounding structures or trees||2.0|
Table: Weighting factor ‘E’ (Type of Country)
|Type of Country||Value of ‘E’|
|Flat country at any level||0.3|
|Mountain country between 300 m and 900 m||1.3|
|Mountain country above 900 m||1.7|
Consider a telephone exchange at Shahpur (Gulbarga district), located in hill country and isolated from other structures of size 20 m × 16.5 m × 10 m (L × W × H).
In Gulbarga, thunderstorm days are 34, corresponding Ng may be taken as 2.50.
Now collection area Ac = (20 × 16.5) + 2 (20 × 10) + 2 (16.5 × 10) + π × 10 × 10 = 1375 m².
Probability of being stuck P = Ac × Ng × 10-6 = 1375 × 2.50 × 10-6 = 3.5 × 10-3
Applying the weighting factors: A = 1.3, B = 0.4, C = 1.0, D = 2.0, E = 1.0
So overall multiplying factor = A × B × C × D × E = 1.3 × 0.4 × 1.0 × 2.0 × 1.0 = 1.04
Therefore, the overall risk factor = 1.04 × 3.5 × 10-3 = 3.6 × 10-3 which far exceeds the accepted number of strikes of 10-5 , so protection is necessary.
PRINCIPLE OF PROTECTION
- The fundamental principle for the protection of buildings against lightning is to provide a conducting path between the general mass of earth and the atmosphere above the building by which a lightning discharge may enter the earth without producing dangerous potential differences in or near the building and also without passing through a non-conducting part of the building, for example, parts which are made of wood, brick, tile, stone or concrete. Damage to the building may be caused by the thermal and mechanical forces generated in such non-conducting parts by the lightning discharge, whereas in metal parts, discharge have negligible effect, provided the metal part has sufficient cross-sectional area.
- Lightning discharges to buildings and structures tend to travel in those metal parts which extend in the general direction of the discharge. Hence, if adequately earthed metal parts of proper proportions are provided and spread properly on and around the building, damage can be largely prevented. The protective system should, therefore, be simple, mechanically strong and properly maintained. However, because lightning has such a wide range of characteristics, it is difficult to provide protection under all conditions although the degree of protection can be increased if the installation is properly made and maintained.
- Metallic water pipes, metal sheaths and armouring of electrical cables and other conductors, long sections of which are buried in the ground and which are not connected or inductively coupled to the lightning conductor system, remain at true earth potential throughout the lightning process. As the point of strike on the lightning protective system may be raised to a high potential with respect to true earth, there is the risk of a flashover from the lightning protective system to metal on or in a structure. If such a flashover occurs, part of the lightning current would be discharged through internal installations with consequent risk to the occupants and the fabric of the building. Such a flashover, a so-called side flash, must therefore be guarded against.
- The required conditions of protection are generally met by placing all the air terminals, whether in the form of vertical finials or horizontal conductors, on the uppermost parts of the buildings or its projections with lightning conductors connecting the air terminals with each other and to the earth. By properly distributing the lightning conductors all over the building or other structures a satisfactory degree of protection can be achieved. These lightning conductors, if desired, may be so placed as to give minimum interference with the contour and appearance of the building.
- A lightning conductor is incapable of discharging a thunder cloud without a lightning stroke. Its function is to divert to itself a lightning discharge which might otherwise strike a vulnerable part of the structure to be protected. The range over which a lightning conductor can be attract a lightning discharge is not constant but is believed to be a function of the severity of the discharge. The range of attraction is thus a statistical quantity. On the other hand, the range of attraction is little affected by the configuration of the conductor so that the vertical and horizontal arrangements of air terminations or lightning conductors are equivalent. The use of pointed air terminations or vertical finials is, therefore, not regarded as essential except where directed by practical considerations.
- The comprehensiveness of the lightning protective system depends on the prevalence of lightning in the locality, the frequency and extent of occupancy of the building and the nature and value of its contents and the nature of the soil. Other things being equal the more elaborate the protective system the more complete the protection will be.
ZONE OF PROTECTION
- The zone of protection of a lightning conductor denotes the space within which a lightning conductor provides protection against a direct lightning stroke by diverting the stroke itself. For a single vertical conductor, this zone is described as a cone with its apex at the highest point of the conductor and with an angle the so-called protective angle, between the side of the cone and the conductor to which different authorities have given different values depending on the degree of protection required and base at the ground. For a horizontal conductor, this zone is defined as the volume generated by a cone with its apex on the horizontal conductor fron end to end. Experience has, however shown that a conductor cannot be relied upon to provide complete protection within any particular zone.
- The protective angle conot be precisely stated, since it depends on the severity of the stroke and the presence within the protective zone of conducting objects providing independent paths to earth. All that can be stated is that protection afforded by lightning conductor increases as the assumed protective angle decreases.
- In general for the purpose of providing an acceptable degree of protection the protective angle of any single component part of an air termination network, namely other one vertical or one horizontal conductor is considered to be 45° (see figure 1 and 2). Between two or more vertical conductor of equal height spaced at a distance not exceeding twice their height the equivalent protective angle within the space bounded by the conductors may be taken as 60° to the vertical, while the protective angle away from the conductor is still taken as 45° to the vertical (see figure 3)
- For structures of exceptional vulnerability by reasons of explosibve or highly infllamable content, all possible precaution should be provided, even against the rare occurance of lightning dicharge within the protected zone. For this purpose a reduced zone protection and other precaution as per IS 2309 should be taken.
COMPONENT PARTS AND THEIR INSTALLATION
The principal components of a lightning protective system are:
- Air terminations,
- Down conductors,
- Joints and bonds,
- Testing points,
- Earth terminations.
- Earth electrodes, and
The material for air termination, down conductors, earth termination etc of the protective system shall be reliably resistant to corrosion or be adequately protected against corroison. The material shall be any one of the following:
Copper: Solid or flat copper strip of at least 98% conductivity conforming to relevant IS shall be used.
Copper Clad Steel: Copper clad steel with copper covering permanently and effectively welded to the steel core shall be used. The proportion of copper and steel shall be such that the conductance of the material is not less than 30 % the conductance of the solid copper of the same total cross-sectional area.
Galvanised steel: Steel throughly protected against corroison by a zinc coating shall be used.
Galvanisation Test For Stel Conductors: A test piece should be dipped seven times, each time for a duration of one minute, in a solution consisting of two parts of copper sulphate and five parts of water. The test piece shall, after each dipping, be carefully rinsed in water and checked that continuous copper coating is not formed. The zinc shall be so strong and plastic that it does not crack or break when the test piece is wounded on a cylinder of 50mm diameter.
Aluminum: Aluminum 99 % pure and with sufficient mechanical strength and protected against corroison shall be used.
Aluminium should not be used underground or in direct contact with walls.
All air terminations shall be of GI and all down conductors shall be of GI or Aluminum, except where atmospheric conditions necessitate the use of copper or copper clad steel for air termination and down conductors. The recommended shape and size of conductors for use above and below ground is given below:
|Sl. No.||Material and Shape||Minimum Size|
|1.||Round copper/copper clad steel wire||6 mm diameter|
|2.||Standard copper wire||50 sqmm or 7/3.00 mm dia|
|3.||Copper strip||20 mm × 3 mm|
|4.||Round galvanised iron wire||8 mm diameter|
|5.||Galvanised iron strip||20 mm × 3 mm|
|6.||Round aluminum wire||8 mm diameter|
|7.||Aluminum strip||25 mm × 3 mm|
|Sl. No.||Material and Shape||Minimum Size|
|1.||Round Copper/Copper clad steel wire||8 mm diameter|
|2.||Copper strip||32 mm × 6 mm|
|3.||Round galvanised iron wire||10 mm × 6 mm|
|4.||Galvanised iron strip||32 mm × 6 mm|
Air Termination network may consists of vertical or horizontal conductors or a combination of both. The pointed air terminations may be divided as below:
The ESE’s triggers an upward steamer/leader at the time earlier than the time of a simple lightning rod and manufactures claim that they cover larger areas, but it has not been proved scientifically. Morever for the purpose of lightning protection, the vertical and horizontal conductors are considered equivalent and the use of pointed air termination, or vertical finial, itself is not considered essential.
A vertical air termination, where provided, need not have more than one point, and shall project at least 300mm, above the object, salient point or network on which it is fixed.
For a flat roof, horizontal air termination along the outer perimeter of the roof shall be used. For a roof of larger area a network of parallel horizontal conductors shall be installed. No part of the roof shall be more than 9 m from the nearest horizontal protective conductor.
For strips in parallel or grid formation, the distance between two strops shall be minimum 2.4 meters.
Horizontal air terminations shall be carried along the contours such as redges, parapets and edges of flat roofs, and where necessary, over flat surfaces, in such a way as to join each air termination to the rest and should therefore form a closed network.
All mettalic projections including reinforcements, on or above the main surface of the roof which are connected to the general mass of the earth, should be bonded to form a part of the air termination network.
If portions of a structure vary considerably in height, any necessary air termination or air termination network for the lower portion should be binded to the down conductor of the taller portion, in addition to their own down conductors.
The function of down conductor is to provide a low impedance path from the air termination to the earth electrode so that lightning current can be safely conducted to the earth. The number of down conductors depend on the form of the building, and some may be the part of building structure itself.
The position and spacing of down conductors on large structures are often governed by the aesthetics. The number of down conductors is decided as below:
- For a structure having a base area not exceeding 100 m², only one down conductor should be provided, except ehen it is built on a bare rock or where testing is difficult.
For a structure having a base area more than 100 m², the number of down conductors is: One for first100 sqm and then one for each 300 m² OR one for every 30 m of perimeter, whichever is smaller, subject to a minimum of two nos of down conductors.
- For tall structures, where testing and inspection could be difficult, at least two down conductors should be required.
A down conductor should follow the most direct path possible between the air terminal network and the earth termination network. When more than one down conductor is used, the conductors should be arranged as evenly as practicable around the outside walls of the structure and due consideration should be given to side flashing.
If most direct route is not possible and there are unavoidable sharp bends, re-entrant loops in a conductor can produce high inductive voltage rops and lightning discharge may jump across the open side of the loop. This risk arises when the length of conductor forming the loop exceeds 8 times the width of the open side of the loop. This should be avoided and holes may be made in the cornices/parapets through which conductor can pass freely.
The walls of light wells may be used for fixing of the down conductors, but lift shafts should not be used for this purpose.
Metal pipes leading rainwater from the roof to the ground may be connected to the down conductor, but cannot replace them, such connections should have disconnecting joints.
In deciding the route of down conductor, the accesibility for inspection, testing and maintenance should be considered.
Each down conductor should be provided with a test joint in such a position, that while not inviting unauthorized interference, it is convenient for use when testing.
Where the provision for external route for down conductor is impracticable, it may be housed in an air space provided by a non-mettalic or non-combustible internal duct or it may be taken in any covered recess not smaller than 76mm x 13mm or any vertical service duct, running the full height, provided it does not contain unarmoured or non-metal sheathed cable. In cases where unrestricted duct is used, it shall be sealed at each floor level for fire protection. Access to the interior should be normally available.
An earth station comprising one or more earth electrodes, as required and specified, should be connected to each down conductor. Each earth station should have resistance not exceeding 10 multiplied by the number of earth electrodes provided therein. It shall be capable of isolation for testing by means of testing joints. Whole lightning protection system, including any ring earth, should have a combined resistance R <10 before bonding. If the value exceeds 10 , additional electrodes or interconnection by ring earth should be done.
A reduction in earth electrode has the advantage of further reducing the potential gradient around the earth electrode and risk of side flashing is reduces. Plate earthing should invariably be used for earthing in preference to the pipe earth elctrode system.
A typical arrangement of protection system is shown below:
BONDING AND ISOLATION
All metals in or forming a part of the structure, or any building services having mettalic parts which are in contact with the general mass of the earth, should be either isolated from or bonded to the down conductor. This also applies to all the exposed large metal items having any dimension greater than 2 m whether connected to the earth or not.
All metallic finials, ducts, vent pipes, railings, gutters, metallic flag staff etc on or above the main surface of the roof of the structure shall be bonded to and form part of the air termination network.
Structure supporting overhead electric supply, telephone and other lines must not be bonded to the lightning protection system without the permission of the appropriate authority. Gas pipe in no case shall be bonded to the lightning earth termination system.
Isolation requires large clearances between the lightning protective system and other metal parts in the structure. To find out the approximate clearances, the following two factors should be taken into account:
- The resistive voltage drop in the earth termination
- The inductive voltage drop in the down conductors.
The resistive voltage drop requires a clearance of 0.3 m/ohm of earthing resistance while the inductive voltage drop requires a clearance of 1 m for each 15 m of structure height. For two or more down conductors with a common air termination this distance should be divided by the number of down conductors. The total clearance required is the sum of the two distances and may be expressed by the following simple equation:
D = 0.3 R + H / 15 n,
where D = required clearance in meters, R = the combined earthing resistance of the earth termination in ohms, H= structure height in metres, and n= number of down conductors connected to a common air termination.
The above clearance may be halved if a slight risk of a side flash occurring can be accepted. The drawbacks of isolation lie in obtaining and maintaining the necessary sale clearance and in ensuring the isolated metal has no connection via the water pipes or other services with the earth. In general, isolation can be practiced only in small buildings.
- The lightning protection system should be so installed that it does not spoil the aesthetic beauty.
- The entire lightning protection system should be mechanically strong enough to withstand the mechanical forces produced in the event of lightning.
- Conductors shall be securly attached to the building, or any other object by means of fasteners, which shall be substantive in construction, not subject to breakage and shall be of galvanised steel or other suitable material, with suitable precaution to avoid corroison.
- The lightning conductor shall be secured not more than 1.2 m apart for horizontal run and 1.0 m for vertical run.
- All air terminals shall be effectively secured against overturning either by attachment to the objects protected or by substancial bracing and fixing which shall be permanently and rigidly attached to the building.
- The lightning protection system should have as few joints as possible. Where joints are there they should be mechanically and electrically effective and minimum overlap should be 20mm.
- Joints of dissimiliar materials should be protected against corrosion or erosion from the elements or environment and should present adequate contact area.
- External metal on or forming part of a structure may have to discharge full lightning current and its bond to the protective system should have a cross sectional area not less than that employed for main conductor.
- All Installation should confirm to IS 2309.
|Name of Place||Thunderstorm days per year|
|Car Nicobar I||10|