What is Striking Distance?
Striking Distance is termed as the shortest distance between a downward leader and a particular structure or object. It depends on the amplitude of the lightning current.
Where ‘I’ is the minimum peak current.
‘r’ is the striking distance i.e. radius of Rolling Sphere
The minimum values of peak current is given in IS/IEC 62305-1, which is 3kA, 5 kA, 10kA & 16kA for Level I, II, III, IV respectively.
The Striking Distance Approach:
- At a point the di-electric strength of the air breaks and ionization happens, due to which it starts conducting.
- For lightning flashes to earth, a downward leader grows step-by-step in a series of steps from the cloud towards the earth.
- A further “leader” discharge similar to the downward leader begins to grow towards the head of the downward leader.
- Upward leader will be launched at points of greatest electric field intensity and can move in any direction towards the approaching downward leader.
Relation between LPL and Striking Distance: According to the IS/IEC 62305-1, it is said that Level I, II, III & IV offers 99%, 97%, 91% & 84% probability that the Lightning Current may be greater than these levels.
Origin of Rolling Sphere Radius: So, lets take a look how the Rolling Sphere Radius is coming from?
So if we take a quick glance at the last page and look into the formula and put the values of Ip, lets see what we get.
Now, Lets put Ip as 3 kA as per LPL I.
- If we install a Lightning Protection System in accordance to Level I then 99% of all lightning strikes will be intercepted. There is only 1% probability that lightning strike of amplitude less than 3kA may not be intercepted.
Again it is to be noted that lightning strikes lesser than 3kA are very rare and they don’t potentially cause any damage to the structure.
- If we are designing a Lightning Protection System in accordance to Level IV, then the Air Terminal placement will be done using the Rolling Sphere Radius of 60 m. In this case the air terminal will capture the lightning strikes of 16 kA or more than that. Thus it is giving protection percentage to 84% at least.
To offer a greater lightning protection level a smaller striking distance (Radius of Rolling Sphere) is needed. The lesser the striking distance, the distance between the air terminal decreases thus increases the capability to intercept lightning strikes of lesser amplitude. In this way we can increase the total percentage of lightning strikes captured.
Why Copper is more preferable in Earthing ?
Reddish metal (Copper) is a noble metal that occurs naturally in its elemental form. And also copper is a versatile metal not only its unique properties of high electrical conductivity and high thermal conductivity but combination of these and other properties of copper.
Every metals has the properties of some amount of resistivity to flow of electrical current, which is why they require a power source to push the current through. Lower the resistivity, higher will be the electrical conductivity. Copper has low resistivity, so it is consider as an excellent electrical conductivity.
Copper is suitable for using in most of the earth electrode applications, except for acid, oxygenated ammoniac or sulphurous conditions. This is not only for high conductivity, but also for good corrosion resistance.
As per standard IS 3043, tests would be taken in wide variety of soils and it show that, the specimen of 150 mm x 25mm x 3mm of different material (copper, galvanized steel and mild steel without coating) has been buried in variety of soil for 12 years.
Copper whether it is tinned or not, is entirely satisfactory because the average loss of specimen in no case not exceed 0.2 percent per year.
Average losses of unprotected mild steel (cast iron, wrought Iron and mild steel) used in test shown as high as 2.2 percent per year.
Test showing the average loss of galvanized mild steel to be little superior to copper not greater than 0.5 percent per year.
Similarly, Copper earthing conductors, in general, need not be protected against corrosion when theyare buried in the ground if their cross-sectional area is equal to or greater than 25 mm2. So therefore, compare the average lossess among this material copper has high corrosion resistance.
Ease of joining
Copper can be readily joining by the following methods
- Brazing using zinc-free brazing material with a melting point of at least 600°C;
- Riveting and sweating; and
- Explosive welding.
Copper is both malleable and ductile metal, which means that it could be stretched to a good length without breaking or weakening it. They can be bended to fit around corners. The reddish metal stands well on this parameter.
Copper can be easily shaped into pipes, flats and wires and the copper pipes are light weighted because they have thin walls.
As we know the earthing conductor takes from one place to other place. When high magnitude fault current is passes through a copper conductor, its surface temperature becomes high. Not every metal sustain this temperature fluctuation but copper conductor can.
As per UL 467. the melting point of copper conductor is 1083°. so it can be used for high temperature application. So, therefore copper has high temperature tolerance than other material using in earthing
Copper can be easily combined with other metals to make alloy and it is non-magnetic and non-sparking material.
Though, the material of earthing conductor is selected based on its resistance value of earthing conductor, other properties also considered for the better performance.
Copper is not only having lower resistance but also combination of unique properties at economical prices.
Every grounding system need lower resistance value as possible. Lower the resistance better will be the performance of system and also easy for channelizing the fault current in to general mass earth without cause any damage.
Testing Procedure of ESE Air Terminal
In the ESE system, the internal design arrangement senses lightning down streamers and triggers upward leaders prior to other passive objects in the structure. Since the upward streamer is triggered in advance when compared to other conventional air terminals, the area protected by the ESE terminal will be larger than that of conventional terminals.
NFC 17-102/2011 has given the testing sequence of the ESE lightning air terminal.
1) Mechanical Dimension Test
2) Environmental Test
3) Electrical Test
4) Early Streamer Emission Test.
Mechanical Dimension Test:
During testing, all the dimensions of the air terminals will be physically measured and the actual values will be verified with the dimensions given by the manufacturers.
2. Environmental Test:
NFC 17/102 explains about two different types of tests to be performed on the ESE air terminal sample to ensure its operation on adverse environmental conditions. The tests are as follows.
- Salt mist treatment test
- Humid sulphurous atmosphere treatment test
2.1 Salt Mist treatment
Salt mist treatment has to be done as per NF EN 60068 –52 standard with level 2 severity. The sample should be subjected to salt & fog (sodium chloride solution) atmosphere for 3 cycle -72 hours
2.2 Humid Sulphurous Atmosphere Treatment:
The sample should be tested in a humid sulphurous atmosphere with seven cycles and a sulphur dioxide concentration of 667 ppm (in volume).
Each cycle should be carried for 24 hours. There are two periods
- Heating Period
- Standing Period
The sample should be tested at a temperature of 40°C ± 3°C for 8 hours.
The sample should be tested for 16-hour standing period.
The ESE is subjected to the following tests without cleaning after done salt & Sulphur mist test.
The ESE air terminal should be tested with a minimum of 100kA lightning impulse current at 10/350 µs waveform
Early Streamer Emission Test:
ESE should be tested for the triggering time given by the manufacturers. The difference between reference wave & measuring wave is the triggering time ‘∆t’.
As per NFC 17-102/2011, the value of ‘∆t’ should be between 10µs and 60µs. An air terminal is considered as ESEAT only if the triggering advance time is greater than 10µs, also the value of ‘∆t’ being greater than 60µs, it is to be still considered as maximum 60µs for all design calculations.
Why do the sequences need to be followed?
The sequence of testing is kept in such a manner to inspect and confirm that the ESE sample after going through the rigorous environmental tests and current carrying tests, the sample is still be able to sense downward leaders and trigger upward leaders at an advanced time than the conventional lightning air terminals. That's exactly why the standard says that these tests are to be carried out in the same sequence with the same sample as mentioned in their requirements to qualify the sample as an ESE type Air Terminal.
ESE Lightning Air Terminal
ESE Air Terminal Rod is an active type lightning arrester which stimulates continuous upward leader before any other object does within its radius of protection. Due to its earlier triggering of upward leader, the area of protection is much larger when compared to a simple conventional air terminal rods. The area protected by an ESE terminal depends upon the time difference between the streamer raised from an ESE terminal and the streamer raised from other passive components located at same height. If this time difference is higher, the area protected by the air terminal will also be higher. That time difference is generally termed as triggering advance time and it is always expressed in microseconds (10−6s or µs).
Triggering Advance Time (t):
The triggering advance time is defined as the difference in triggering time of an early streamer lightning rod and a simple conventional air terminal rod obtained when both rods are exposed to the same atmospheric & electrical conditions.
∆t = TSRAT - TESEAT.
TSRAT = The mean triggering time of the upward leader of a simple conventional air terminal rod.
TESEAT = The mean triggering time of the upward leader of a ESE air terminal rod.
As per NFC 17-102/2011, the value of ‘∆t’ should be between 10µs and 60µs. An air terminal is considered as ESEAT only if the triggering advance time is greater than 10 µs., also the value of ‘t’ being greater than 60µs, it is to be still considered as maximum 60µs for all design calculations.
Radius of Protection - NFC 17- 102/2011:
The radius of protection is the distance between the point where you want to place the (ESE) air terminal rod and the farthest point from the structure or building to be protected. For calculating the radius of protection, it is very important to get the triggering advance time of the device and the height of the mast on which the ESE air terminal rod is mounted upon. The ESE Air terminal should be installed at least 2 meters over the surface of the structure to be protected.
Calculation for Radius of Protection:
For the Early Streamer Emission air terminal, the radius of protection for different levels can be calculated using the following equation:
For h ≥ 5m
h - Height (m) of the mast above the considered surface
∆ - ∆ = ∆t x 10−6 s
r – Depends on the selected level of protection ( in meter)
r = 20 for Level 1 protection
r = 30 for Level 2 protection
r = 45 for Level 3 protection
r = 60 for Level 4 protection
For buildings taller than 60 meters, minimum of 4 down conductors should be used
Selection of Materials for Lightning Protection System
The components of Lightning Protection System (LPS) are exposed to direct lightning strikes and corrosive atmosphere. To achieve efficiency, the materials should have high current withstanding capacity and the materials should be less corrosive. The current carrying capacity of a material depends on the cross sectional area and the materials should be selected based on the local environment.
A material which is most preferable for some site conditions might be the least preferred material for other site conditions due to its chemical properties.
Apart from the corrosion of materials due to the local environment, the contact of two dissimilar materials also lead to galvanic corrosion.
Hence, IEC 62305 suggests the materials that can be used for different corrosive environments and it also given minimum cross sectional area for different materials of LPS Components.
Minimum Cross sectional area of different materials:
- Copper, tin plated copper strip, cable, Copper coated steel Should have minimum 50mm² cross-sectional area.
When mechanical strength not considered the cross sectional area of copper material can reduce up to 25mm²
- Minimum 176mm² cross sectional area should be considered for all materials of air terminals and 70mm² crosssectional area considered for sites where mechanical stress such as wind loading is not critical
- Normal cross sectional area of stainless strip, and conductor is 50 mm² and it can be increased upto 75 mm² when thermal & mechanical factors are considered.
Protection against corrosion:
- The LPS should be constructed of corrosion-resistant materials such as copper, aluminium , stainless steel and galvanized steel.
- The material of the airtermination rods and should be electrochemically compatible with the material of the connection elements and the mounting elements.
- Connections between different materials should be avoided.; otherwise they should be protected against galvanic corrosion.
- Copper parts should never be installed above galvanized or aluminium parts unless those parts are provided with protection against corrosion.
- Aluminium conductors should not be directly attached to calcareous building surfaces such as concrete, limestone and plaster, and should never be used in soil.
- Lead-sheathed steel conductors are not suitable for use as earth conductors
- Lead-sheathed copper conductors should not be used in concrete nor in soil with a high calcium content.
Aluminium has very good electrical conductivity but it more prone to corrosion on soil and concrete medium. Hence, they can be used for air terminal and down conductor systems above the ground level and connected to earthing system of GI/Copper/SS using proper bimetallic connectors. The fasteners or sleeves for aluminium conductors should be of similar metal and of adequate cross-section to avoid failure by adverse weather conditions.
Lightning Protection for High Rise Buildings
Taller buildings in general attract lightning strikes, hence an efficiently designed LPS is highly critical to safeguarding these structures from the destruction caused due to lightning. Also, such structures should make sure quality earthing systems and surge protection devices are installed to protect the lives and expensive electronic equipment housed in these structures. National Building Code guidelines to be followed while designing LPS for such tall buildings.
Design of Lightning Protection System for high rise building:
1. A lightning protection system for structures could be done by any of the following methods.
- Protection angle method
- Mesh method
- Rolling sphere method
2. The protection angle method is suitable for simple-shaped buildings. It also has limitations on the height of the air terminal.
3. When a building height is more than 60m at level 4, Then the protection angle method is not applicable.
4. The rolling sphere method can be used in all types of buildings but by using this method we can place air terminals on the upper part of the structure. If the height of the building is greater than the radius of a sphere the possibility of a side flash may hit the structure.
5. Since it is a taller structure, major lightning flashes will hit on the top, horizontal leading edges, and corners but IEC 62305 -3 suggests providing lateral protection, reduces side flashes (i.e) if the building height exceeds 60m the 20% of the top should be protected by the lateral protection system.
6. The mesh method is suitable for the protection of plane surfaces and the lateral surfaces to protect against side flashes
Design of Protection for Oil Storage Tank against Lightning Strike
Lightning is one of the most destructive phenomena of nature. It causes maximum damage to living beings and to the equipment across the globe. Due to the storage of explosive and hazardous materials in the Oil & Gas Industry, a direct lightning strike or secondary surges could lead to major disasters leading to loss of lives, resources, and equipment since the average temperature of it can be around 20,000 degrees Celsius. In this article we have explained the design of protection for oil storage tanks against lightning strikes given in OSID_GDN 180 and IS/IEC 62305-2 . The calculation of the design of lightning protection can be done by the Rolling sphere method.
IS/IEC 62305-2 has mentioned at least level 2 protection is necessary for the structure with risk of explosion.
Lightning protection in oil tanks:
A properly bonded and earthed metallic tank is self-protected When the steel tank shell or Roof thickness is of a minimum of 4.8mm, a separate air terminal and down conductor are not needed. Separate lightning protection is needed when the steel tank shell or roof are of less than 4.8mm
Lightning protection for storage tanks can be done by three methods
• By using an air terminal
• By using Lightning mast
• By using overhead line
Use of Air terminals:
In this method, air terminals will be placed on the surface wall of the storage tank. The protection zone provided by the air terminals depends on the diameter of the tank, height of air terminals, and Distance between the air terminal around the perimeter of the wall. The use of air terminals can be applied to floating roof tanks for reducing the probability of rimfires due to lightning strikes since the space around the rim has a relatively higher possibility of flammable atmosphere being present due to leakage from improper sealing.
OSID_GDN 180 has given the calculation of the number of air terminal
When the air terminals of height 6m and the distance between the air terminals are 20m will protect an area up to 18m from the periphery of the tank with a diameter of 30.
Use of lightning mast around the tanks:
The following basic needs to be considered for the protection of storage tank by lightning mast:
1. The Lightning mast should not be placed 30m away from the tank
2. Mast shall be placed 5 to 6m from the tank to reduce the effect of side flashing.
The protection zone given by the lightning mast depends on the height of the mast, the diameter of the tank.
The height of the mast should be greater than the height of the storage tank.
The number of lightning mast calculations is the same as the calculation of the air terminal placed on the wall. Earthing system of the lightning mast will be bonded with the earthing system of the tank.
Lightning masts are much more expensive as compared to the air terminals on the shell.
Use of overhead line:
To provide complete protection the system of overhead lines can be calculated by using the Rolling sphere method. To protect a storage tank with a diameter of 6 to 8m can be protected by a single overhead earth wire with a minimum clearance of about 8 m above the highest point of the tank. For the protection of 8 to 30 diameter tank shall need two parallel overhead earth wire, while 30 to 80 m diameter tank shall need a minimum of three overhead earth wire. To avoid side flash the minimum distance between the supporting mast holding the overhead line and tank shall be 6m. For earthing the supporting mast will be bonded with the tank’s earthing system.
Protection of Protruding Roof Fixtures From Lightning Strikes
When structures are containing protruding or flush-mounted structures, the Lightning protection is designed in such a way that those parts lie within the protection given by air terminal rods mounted over the structure.
The above picture depicts that the protruding structure lies within the region of protection given by the lightning rods. However, additional protection is required if
- The total area of the protruding structure is more than 1 m²
- The length of the protruding structure is more than 2 m.
- The height of the protrusion is above 0.3 m from the roof level.
If those protruding structures are non-conductive, then additional protection from the lightning rods is not required if they are within 0.5 m height.
When a Chimney is made of non-conductive material, the ash or soot deposits at the inner wall can cause conductivity and can even result in forming upward leaders of greater length resulting in a lightning strike on the Chimney. This can happen even in the absence of rain, and so the lightning rod over the Chimney should be at such a height that it lies under the area of protection given by the lightning rod.
Protection of Solar Panel From Lightning Strikes
Lightning is one of the most destructive phenomena of nature. It can cause damage to humans, structures, electrical and electronics equipment. Currently, due to global warming, the entire world is moving towards Renewable Energy, and Solar Panels are at high risk due to the possibility of destruction by lightning strikes due to its elevation and the widespread vacant land areas chosen for the installation of such structures. Redesigning or modification of Lightning Protection System post the installation of these structures isn’t advisable as it would attract heavy expenses and hence a properly designed LPS as per relevant standards is mandated.
Lightning could strike and cause damage to the solar panels either directly or indirectly. External protection of solar panels against direct lightning strikes needs an air terminal to intercept the lightning strike, a down conductor to provide a dedicated path, and an efficient earthing system to dissipate the lightning current into the earth.
Internal protection of solar panels needs an appropriate surge protection device to protect these solar panels from getting damaged from a surge current caused due to lightning strikes.
Non-Insolated lightning protection:
In a non-isolated Lightning protection System, the air terminal is mounted on the panel itself. The number of air terminals to be installed to protect a panel depends on the dimensions of such solar panels.
The air terminal height (h) should not exceed 0.5m to prevent the shadow of the Air terminal from falling on the panels, which could hamper the performance of these solar panels. Positioning of these air terminals shall be derived by either adapting to the protection angle method or the rolling sphere method as per IEC 62305. The down conductor should be a minimum of two runs from the air terminal and shall be connected to a minimum of two earth electrodes per down conductor.
Isolated lightning protection:
In isolated lightning protection, the air terminal is mounted on a mast of a certain height to provide appropriate elevation as per the design and is placed away from the solar panel/panels to be protected. This separation distance between the solar panel and the Lightning Protection System shall be calculated as per relevant standards.
As the influence of the shadow of the Lightning arrestor arrangement on the solar panel could hamper the performance of the entire solar system, a safe separation distance between the LA arrangement and the solar panel should be maintained. The separation distance can be calculated based on the following parameters.
- Height of the supporting mast.
- Latitude and Longitude of the site.
- Time of operation and
- Seasonal variation
Again as in the Non Isolated LPS, the down conductor should be a minimum of two runs from the air terminal and shall be connected to a minimum of two earth electrodes per down conductor.
Nowadays more such solar farms are being protected by the Early Streamer Emission technology-based Lightning Protection System as per the French NFC 17-102 2011 or Spain’s UNE21186 standard for Protection against lightning strikes. Due to its larger protection area as per its working principle, the number of ESE type Lightning Protection Systems is considerably very less when compared to conventional type Lightning Protection Systems, thus being the most preferred technology for Lightning Protection for most of the companies in India and in many other countries involved in the Solar Power Generation. ESE type Lightning Protection Systems are isolated and installed away from these panels to be protected which avoids shadow issues on solar panels.