Definition – A lightning arrester is a device that protects electrical system, buildings & towers from lightning (very high natural voltage) from damage as it provides a low resistance path for current that flows due to high voltage. It is also known as Surge Arrester.
The function of lightning arrestor (LA) is to divert a lightning discharge so that building structure could be protected from damage.
(Note:- IS2309-1989 considers conventional lightning protection system.)
Need for Lightning Protection – Following are the structures which need lightning protection –
- a) Structures which are having explosive material like factories, stores, dumps & fuel stores,
- b) Other structures – where large numbers of people congregate;
– Where essential public services are concerned;
– Where the area is one in which lightning strokes are prevalent,
– Where there are very tall or isolated structures;
– Where there are structures of historic or cultural importance,
– Structure with inherent explosive risks, for ex – explosive factories, stores and dumps & fuel tanks,
– Where there is use of electrical & electronic equipment/ components that are very sensitive to the effects of lightning like industry, transportation & communication.
TERMS WHICH ARE USED IN LIGHTNING ARRESTER SYSTEM-
Zone of protection – It is the volume within which a lightning conductor gives protection from direct lightning stroke. Zone of protections for – a) vertical conductor & b) horizontal conductor.
Zone for vertical conductor – The zone has been defined as a cone with its apex at the tip of the conductor its base on the ground.
Zone for horizontal conductor – The zone has been defined as volume generated by a cone with its apex on the horizontal conductor moving from end to end.
CONSTRUCTION OF LIGHTNING ARRESTER (L.A.) –
A conventional lightning Arrestor consists of following parts –
- Air termination,
- Down conductors,
- Bends & Joints,
- Testing points,
- Earth terminations network,
- Air termination– It consists of vertical or horizontal conductors or combination of both& air terminal as well,
Following points to remember for air termination –
- No part of roof should be more than 9m from nearest horizontal conductor,
- All metallic parts at terrace should be connected with air termination network including reinforcement,
- If height of structure varies considerably, the air termination of lower portion should be connected with down conductor of upper structure along with its own down conductor.
- If PVC covered horizontal air termination conductors are used, bare vertical rods of 0.3m high should be fixed at each intersections and at intermediate position along the horizontal conductors spaced no more than 10m apart.
- Air termination on a flat roof –a network of 10m x20m horizontal conductor should be used. Also ensure that edges of the roof are covered with horizontal conductor in any case.
- Air termination for flat roof at different levels – An air termination along the outer perimeter of the roof is required, & no part should be more than 5m from the nearest horizontal conductor, except that an additional 1m may be allowed for each meter by which the part to be protected is below the nearest conductor.
- Air termination for large areas of roof of various profile –If S ˃10 + 2H, additional longitudinal conductors are required so that the distance between conductors does not exceed 10m, where S= Space between two edges of roof design & H = height of edge. Secondly if the length of the roof exceeds 20m additional transverse conductors are required.
- Air termination for building less than 20m height with slopping roof –Air termination should be done as per design given below –
- Air termination for tall conducting/metallic structure – The horizontal conductors on the roof should be of 10m x 20m. There are bonds at the corner of the roof top, 20m interval around periphery & at the base of the tower, 0.5m above roof level. (Note- no down conductor is required in this case as steel structure itself work as a down conductor but bonding between structure &earthing should be proper).
- Down Conductor – The function of down conductor is to provide low impedance path between air termination & earth pit. Following things to be remembered while selecting number of down conductors –
- For up to 100m2base area – One down conductor,
- For base area more than 100m2 – Two situations are there so number of down conductor should be at least the smaller of the following –
- a) One plus an additional one for each 300m2
- b) One for each 30m of perimeter of the building to be protected,
- For tall structure like chimney (where regular inspection is difficult), at least two down conductors shall be required,
- For steal framed structure – no additional down conductor is required as metal structure will itself act as a down conductor but we have to ensure that metal structure is properly connected with earth pits.
The down conductor should follow the direct path between air termination & earth pits. Following points should be considered while selecting the routes of down conductors –
- The walls of light wells may be used for down conductors,
- Lift shafts should not be used for down conductors,
- Possibility should be considered of incorporating steel structure suitably disposed for purposes of acting as a down conductor.
(Note: In case external route is not available for down conductor, in such cases down conductor may be housed in air space provided by a non-metallic non-combustible internal duct & taken straight down to ground. The duct size should not be less than 76mm x 13mm or any suitable vertical service duct running the full height of the building may be used for this purpose provided it does not have any un-armoured or non-metal-sheath service cable. The duct should be sealed at each floor level for fire protection. Access to the interior of the duct should be available.)
- Bends & Joints –
Sharp bends & Re-entrant loops –
Sometimes straight routes are not practically available, hence down conductor bends & sometimes make re-entrant loops. These re-entrant loops can produce high inductive voltage drop so that the lightning discharge may jump across the open side of the loop.Following rule should be followed – the length of the conductor forming the loop should not exceed 8 times the width of the open side of the loop.
Welding joints are advisable for lightning protection system as other types of joints offer high conduction path. Also LA should have as few joints as possible.
Joints should be mechanically & electrically effective. For overlapping joints, the length of the overlap should not be less than 20mm for all types of conductor.
Joints should be protected against corrosion or erosion from the environment and should present an adequate area.
- Test Points – Each down conductor should be provided with test clamp in such a way that it should be convenient for use testing.
- Earth termination network –
An earth electrode should be connected to each down conductor. Each of these earths should have a resistance not exceeding the product given by 10Ω multiplied by the number of earth electrodes to be provided. The whole of the lightning protection system should have a combined resistance to earth not 10Ω without taking into account of any bonding.
If the value of resistance exceeds 10Ω, a reduction can be achieved by extending or adding to the electrodes or by interconnecting the individual earth terminations of the down conductors by a conductor installed below ground.
The advantage of resistance less than 10Ω is that the potential gradient around the earth electrode when discharging lightning current. It also reduces the risk of side flashing to metal in or on a structure.
FIXING OF LIGHTING CONDUCTOR –
Fixing arrangement of lightning conductor should be purpose made for each size of strip – the dimension should be equal to the thickness of the strip and should be equal to the width plus 1.3mm for expansion. Fixing arrangement for circular conductor should be treated similarly.
All fixings should be securely attached to the structure; mortar joints should not be used.
INSPECTION OF LIGHTNING PROTECTION SYSTEM–
Inspection of all lightning protection should be done by a competent person to ensure that installation has been done as per the code. Visual inspection should be done at fixed intervals but not exceeding 12months. During visual inspection ensure mechanical condition of all conductors, bonds, joints, & earth electrodes are checked & observations are noted. Due to any reason, if it is not possible to inspect any part of the installation, this should also be noted.
Following measurements should be done after completion of installation or modification done & results should be recorded in a log book –
- Resistance of earth of whole installation & each earth termination,
- Electrical continuity of all conductors, bonds & joints.
Tests should be done at fixed intervals but not exceeding 12 months.
(Note –If the value of earth resistance exceeds 10Ω, the value of earth resistance should be reduced to below 10Ωby using various methods. If earth resistance is less than 10Ω but significant higher than the previous value, investigation should initiated to know the reason for higher resistance & appropriate action should be taken to reduce the value.)
MAINTENANCE & UPKEEP–
Periodic inspections/tests will show the need of maintenance if any. Following points should be taken care for –
- Earth resistance value,
- Evidence of corrosion,
- Alteration or addition to the structure which may affect the lightning protection system.
Diagram given below will help to understand more about the LA System
MINIMUM DIMENSIONS COMPONENTS PARTS (as Per IS:2309-1989)
||Al, Cu, GI strip
||20 x 3
||Al, Al alloy or, phosphor bronze and galvanized steel rod
||Al, Cu, GI strip
||20 x 3
||Al, Al alloy or, phosphor bronze and galvanized steel rod
||Hard drawn copper rods for direct driving into soft ground
||Hard drawn or annealed copper rods for indirect driving or laying under ground
||Phosphor bronze for hard ground
||Copper-clad or galvanized steel rods
- ESE(EARLY STEAMER EMISSION) TYPE –
ESE stands for Early Streamer Emission. It works on ESE technology. ESE rod produces ionization directed at the clouds so that any possible electrical discharge can be channeled from the very beginning.
It consists of two parts; one is of spherical shape which surrounds the second one which is pointed. Spherical part has no contact with the pointed part (there is a gap). The pointed part is connected with earthing pit through a cable. So, we can say that spherical & pointed parts form a potential difference. The spherical will have the same potential as in the air & pointed one has zero potential as it is connected with earth.
During rainy season when there is a thunder storm, the clouds get charged with very high potential (lower side is negative charged & upper parts of cloud is positive charged) &this high voltage led clouds produces a high (negative) potential field. When this high potential field hit high rise building, the building may get damaged with high voltage if building is not protected with lightning.
So, in such situation when high negative voltage field passes through the spherical part of ese LA, it gets negative charged & pointed part gets positive charged (due to capacitive effect). This positive charge is creates an electric filed which is called upward steamer. This streamer (positive field) attracts downward leaders (negative charge) of lightning. When these two streamer & leader meat each other, the clouds high voltage gets grounded through LA.
The basic difference between conventional Rod LA & ESE is that ESE produces a positive charge field in the form of upward streamer which attracts the negative charged lightning while conventional LA works when negative charged lightning reaches LA.ESE produces upward streamers up-to a certain range.
How ESE Lightning Arrester works –
It consists of two parts (or armatures), one part (pointed) is connected with earth & other (spherical) remains separate with first part and on atmospheric charge (which is normally negative due to clouds). Now there is a potential difference between two parts. During normal weather p. diff is less but during storm p. diff increases & when p.d. exceeds a certain limit, LA generates upward steamer which tries to contact the down streamer from clouds. Once they meet, the negative charged clouds get discharged to earth through ese LA.
Ese LAs are available with counter so that number of strikes could be counted. Counter is connected with internal circuit of LA. No external source of power is required for the operation of ESE LA. The potential difference between two parts works as a source of power for LA.
The ESE LA protects the building radially & the radius which covers the building depends on the height of LA. Height of LA is more; the coverage area of building will be more. The relation between height of LA & coverage radius depends on the design of LA by different manufactures.
Difference between conventional Lightning Arrester Rod &ESE –
||ESE Lightning Arrester
|It collects the charge& directs it to ground.
||It attracts the charge, hold it till sufficient potential difference is not achieved & then it directs to ground.
|Multiple rods are required for the protection of the building.
||It covers radially a large area & less ese are required
|Number of lightning strikes can’t be counted
||Number of lightning strikes can be counted through a counter.