(B) ROLLING SPHERE METHOD: (Suitable for complex shape building)
- The rolling sphere method should be used to protect the areas of a structure when there is design limitation to use the protection angle method.
- The rolling sphere method is recommended as the main method to be used in the design of lightning protection system with location of air terminals for structures with complex shapes.
- This method is more accurate, and complex compared to other lightning protection schemes, because it specifies the exact number of spikes needed for each building and considers the worst-case scenarios, in which a lightning strike hits the side of the building.
- Position of Air Termination Rod:
- In this method, the positioning of the Air-Termination system is adequate so that no point of the structure to be protected comes in to contact with a sphere with radius ‘r’ depending on the class of LPS (see table) rolling around on top of the structure in all possible directions. In this way, the sphere only touches the air termination system (see figure).
- Radius of Sphere:
- The rolling sphere lightning protection method assumes the electrically charged field that produces a lightning strike has a radius “r” and the sphere with that radius rolling over the surface of the building. Any place the sphere touches the building is a location where lightning can strike the building. By installing air terminals, the sphere cannot touch the building because electrical charges flow through the lightning protection system into the ground.
- The radius of the rolling sphere is correlated with the peak value of the current in the lightning that strikes the structure: r = 10xIx0.65 where I define as kA.
- In the rolling sphere method, the radius of the sphere is selected in such a way that its radius is equal to the striking distance. Since the striking distance is a function of the prospective return stroke current, the radius of the sphere “r” is defined as a function of the probable return stroke current according to the relationship between the lightning striking distance and the peak return stroke current.
- The lightning stroke depends on the degree of risk considered. So, for a high-risk facility, the sphere radius is at its smallest, e.g. 20meter or a 40meter diameter ball. The smallest size ball means the amount of protection installed will be at its highest. Thus, lowering the risk profile and increasing the protection afforded.
- For a low-risk scenario method, the sphere radius is at its largest distance, 60 meters (120-meter diameter ball), which means a lot less hardware to install.
- The radius r of the rolling sphere depends on the class of LPS as per given Table.
| RADIUS OF THE ROLLING SPHERE | |
| Class of LPS | Rolling sphere radius, r (m) |
| CLASS I- (Very High Risk) | 20 Meter |
| CLASS II- (High Risk) | 30 Meter |
| CLASS III- (Moderate Risk) | 45 Meter |
| CLASS IV- (Low Risk) | 60 Meter |
- Figure shows the application of the rolling sphere method to different types of structures. The sphere of radius r is rolled around and over all the structure until it meets the ground plane or any permanent structure or object in contact with the ground plane which can act as a conductor of lightning.
- A striking point could occur where the rolling sphere touches the structure and at such points protection by an air-termination conductor is required.
- Any part of the structure that is in contact with the sphere is considered to be vulnerable to a direct lightning strike; the untouched volume defines a lightning protected zone.
- When the rolling sphere method is applied to the structure, the structure should be considered from all directions to ensure that no part protrudes into an unprotected zone a point which might be overlooked if only front, side and plan views on drawings are considered.
PENETRATION DISTANCE:
- The distance between the two air terminals should be chosen in such a way that protection is provided for all the objects placed on the surface to be protected.
- The protection of the objects placed on the surface can be ensured by calculating the penetration distance of the rolling sphere.
- The distance between the level of air terminals and the least point of sphere in the space between the air terminals is called penetration distance.
- Let us consider an object of height ‘h’ placed on the surface to be protected. Let ‘ht’ be the height of the air terminal, ‘p’ be the penetration distance and ‘d’ be the distance between the two terminals.
- In this case, the penetration distance ‘p’ should be less than the physical height of the air-termination rods above the reference plane minus the height of the objects to be protected.
- P<(ht-h)
DISTANCE BETWEEN TWO AIR TERMINALS:
- The penetration distance of the rolling sphere below the level of conductors in the space between the conductors can be calculated by using the below formula (IS 62305-3).
- p=r-√(r^2-(d/2)^2 )
- Were,
- p : penetration distance
- r : radius of rolling sphere
- d: distance between the air terminals
- For attaining a particular penetration distance, we can derive the required distance between the air terminals from the above equation.
- d=2x√(2 x p x r-p^2 )
- If there are no objects protruding from the structure to be protected, then the penetration distance can be increased up to the height of the air terminal to provide maximum protection. At this condition, the distance can be calculated by substituting the value of height of air terminal (ht) in place of penetration distance (p).
- d=2x√(2 x ht x r-ht^2 )
- The distance between the air terminals(d) in rolling sphere method depends on two factors.
- Height of the air terminal and
- Radius of the rolling sphere
- Among these two factors, the radius of rolling sphere is a constant value which depends on the class of LPS as specified by IS/IEC 62305-3. Hence for particular class of LPS, the distance between the air terminals purely depends on the height of air terminal.
| Distance between Air Terminals (Meter) | ||||
| Height of Air Terminal (ht) | Radius of Rolling Sphere(r) | |||
| LPS-I | LPS-II | LPS-III | LPS-IV | |
| r=20 Meter | r=30 Meter | r=45 Meter | r=60 Meter | |
| 0.5 Meter | 8.8 | 10.9 | 13.37 | 15.45 |
| 1 Meter | 12.48 | 15.36 | 18.86 | 21.81 |
| 1.5 Meter | 15.2 | 18.7 | 23 | 26.66 |
| 2 Meter | 17.43 | 21.5 | 26.53 | 30.72 |
| 3 Meter | 21.07 | 26.15 | 32.31 | 34.36 |
| 4 Meter | 24 | 29.9 | 37 | 43 |
| 6 Meter | 28.56 | 36 | 44 | 52.3 |
- Example:
- Conclude the equipment installed on Terrace is whether protected by LPS System or not by LPS system installed on Building (Calculate penetration height) having following details.
- LPS Level is -IV (Low Risk).
- The maximum height of equipment is 1 meter from Terrace Floor.
- The distance between the two Air terminals is 10 meters.
- Height of Air Terminal is 2 Meter.
- Calculation:
- First, we calculate the maximum distance between two Air terminal according to LPS level.
- Here Height of Air terminal (ht)= 2Meter.
- Height of equipment (h)=1 Meter
- According to LPS-IV Radius of rolling sphere (r) = 60 meter
- Distance between two Air terminals (d) =2√(2*ht*r-ht^2 )
- Distance between two Air terminals (d) =2√(2x2x60-2^2 )
- Distance between two Air terminals (d) =30.72 Meter
- Distance between actual installed Air terminals is 10 meter which is less than maximum calculated distance between two Air terminals.
- Now to calculate penetration height.
- penetration height (p)=r-√(r^2-(d/2)^2 )
- penetration height (p)=60-√(60^2-(10/2)^2 )
- penetration height (p)=0.20 Meter.
- Now Height of Air terminal (ht)-Height of equipment(h) = 2-1 =1Meter.
- Check condition of P< (ht-h)
- Here 0.2 <1 meter
- Hence equipment installed on terrace which height is 1 meter is protected from installed LPS System.
SIDE FLASHES IN TALL STRUCTURE
- On all structures higher than the rolling sphere radius “r”, flashes to the side of structure may occur. Each lateral point of the structure touched by the rolling sphere is a possible point of strike. However, the probability for flashes to the sides is generally negligible for structures lower than 60 meters.
- For taller structures, a major part of all flashes will hit the top, horizontal leading edges and corners of the structure. Only a few percentages of all flashes will be to the side of the structure.
- The probability of flashes to the sides decreases rapidly as the height of the point of strike on tall structures when measured from the ground.
- Therefore, consideration should be given to install a lateral air-termination system on the upper part of tall structures (typically the top 20 % of the height of the structure). In this case the rolling sphere method will be applied only to the positioning of the air-termination system of the upper part of the structure.
(1) Buildings Taller Than 120-meter High
- For structures taller than 120 meters, the standard recommends that all parts above 120 meters be protected. It is expected that due to the height and nature of such a structure, it would require a design to LPL I or II (99% or 97% protection level).
- For tall buildings, the actual risk of flashes to the side are estimated by the industry to be less than 2%, and typically these would be the smaller lightning flashes, e.g., from branches of the downward leader. Therefore, this recommendation would only be appropriate for high-risk locations or structures.
(2) Buildings Above 60-meter High
- In the IEC standards, for buildings above 60-meter, protection is required to the sides of the upper 20% of height. The same placement rules used for roofs should apply to the sides of the building.
- While the mesh method is preferable, particularly if using natural components, protection is permitted using horizontal rods and rolling sphere method. However, horizontal rods on most structures are impractical due to window washing access equipment, etc.
(3) Buildings Less Than 60-meter High
- Note that for structures less than 60 meters high the risk of flashes to the sides of the building is low, and therefore protection is not required for the vertical sides directly below protected areas.
(4) Buildings Taller Than 30 meters:
- For buildings taller than 30 m, additional equipotential bonding of internal conductive parts should occur at a height of 20 m and every further 20 m of height. Live circuits should be bonded via SPDs.
(C) THE MESH METHOD (Suitable for all flat surface building)
- The mesh method is the simplest and most flexible method for LPS because it does not depend on the height of the structure. However, it requires flat but non curved surfaces. The Flat surface may be horizontal or vertical surfaces.
- It is mostly used for simple Building like domestic households, mainly for perfectly square or rectangular buildings.
- In the mesh method, a mesh is created by a flat conductor and placed on the structure. The separation distance of the conductors is based upon the class of protection determined during the risk assessment.
- Mesh Size:
- According to IEC 62305, mesh conductor size is based on the selected class of LPS and that is totally dependable on user requirements.
- In the Mesh method, a conducting mesh with a cell size determined by the minimum return stroke current that is allowed to strike the protected structure.
- In order to avoid a direct strike, the mesh has to be located at a critical distance above the flat surface to be protected. This procedure is called “protective mesh method”.
- The maximum mesh size should be in accordance with the table below.
| Mesh Size | ||
| Class of LPS | Mesh Size (M) | |
| CLASS I-(VERY HIGH RISK) | 5 X 5 METER | |
| CLASS II-(HIGH RISK) | 10X 10 METER | |
| CLASS III-(MODERATE RISK) | 15 X 15 METER | |
| CLASS IV-(LOW RISK) | 20 X 20 METER | |
- The following conditions shall be considered while selecting the Mesh Method.
- (a) Air-termination conductors are positioned, on roof edge lines, on roof overhangs, on roof ridge lines, if the slope of the roof exceeds 1/10.
- (b) The mesh dimensions of the air-termination network are not greater than the values given in Table.
- (c) The network of the air-termination system is constructed in such a way that the lightning current will always encounter at least two distinct metal routes to earth-termination.
- (d) No metal installation protrudes outside the volume protected by air-termination systems.
- (e) The air-termination conductors follow, as far as possible, the shortest and most direct
- Location of Mesh
- The corners and edges of roofs are most susceptible to damage due to lightning. Therefore, designers and installers should place the conductors as close to the edge of the roof as possible.
- IEC 62305 allows for the use of conductors under the roof of a structure. Thus, the natural components of a structure can be used as part of the mesh grid, or even the whole grid. These components may be the rebar structure underneath the roof or dedicated lightning protection conductors, but they must be connected to the air termination rods that are mounted above the roof.
- For structures, with a protruding metallic structure, the Protective Angle Method is generally used as a supplement to the Mesh Method
- Mesh Method with combination of other Methods:
- For Medium to large scale buildings mesh can be implemented, but due to its limitations, it does not come alone. It must be merged with other types of LPS, either protection angle or rolling sphere, subject to the suitable class number of each type.
- Air termination conductors and down conductors should be inter-connected by means of conductors at the roof level to provide sufficient current distribution over the down conductors.
- Conductors on roof and the connections of air termination rods may be fixed to the roof using both conductive or non-conductive spacers and fixtures. The conductors may also be positioned on the surface of a wall if the wall is made of non-combustible material. The fixing centers shall be minimum 1 meter apart.
- For each non-isolated LPS, the number of down conductors shall be not less than two. A down conductor should be installed at each exposed corner of the structure, where this is possible.
- Limitations:
- The mesh method is suitable for horizontal and inclined roofs with no curvature.
- The mesh method is suitable for flat lateral surfaces to protect against side flashes.
- If the slope of the roof exceeds 1/10, parallel air-termination conductors, instead of a mesh, may be used provided the distance between the conductors is not greater than the required mesh width.
