Design Calculations of Lightning Protection Systems – Part Eighteen


In Article " Design Calculations of Lightning Protection Systems – Part Two ", I indicated the lightning protection design process involves a number of design steps as in below Fig.1.


Fig.1: The Lightning Protection Design Process



Step#1: Characteristics of the Structure to Be Protected




Step#2: Risk Assessment Study



Also, In above Article, I indicated that the risk assessment study can be done by (4) different methods as follows:

Methods Of Calculations For Risk Assessment Study
Articles
First: Manual Method (Equations And Tables Method) as per IEC 62305-2
    Design Calculations of Lightning Protection Systems – Part Two
    First: Manual Method (Equations And Tables Method) as per NFPA 780
    Second: Software Method For Performing The Risk Assessment Study
    Third: Excel Sheets Method For Performing The Risk Assessment Study

    Fourth: Online Calculators Method Used for Need for Lightning Protection calculations



    Step#3: Selection Of External LPS Type and Material





    Step#4: Sizing of Air Termination System Components



    In Article " Design Calculations of Lightning Protection Systems – Part Sixteen ", I explained the following points:

    • Types and forms of Strike Termination Subsystem,
    • Sizing of Air Terminals Based on IEC 62305-3 and Based on BS EN 62305-3,
    • Sizing of Natural Air Terminals,
    • Positioning / Placement of Air Termination System Components.
    • The Class of LPS/LPL influences on the (3) Positioning Methods.


    And I explained The Rolling Sphere Method (RSM) in Article " Design Calculations of Lightning Protection Systems – Part Seventeen ".


    Today, I will explain in detail the other two Positioning Methods for Air Termination system which are:

    1. The Protective Angle Method (PAM),
    2. The Mesh Method.


    For more information, please review the following Articles:





    Step#4: Sizing and Positioning of Air Termination System Components - Continued




    2- The Protective Angle Method (PAM)




    2.1 Application And Usage

    The protection angle method is most commonly used to supplement the mesh method, providing protection to items protruding from the plane surface (roof mounted structures like antennas, ventilation pipes) see Fig.2. Where the protection angle method alone is employed, multiple rods are generally required for most structures.



     Fig.2


    The protection angle method can be used on (see Fig.3):

    1. Simple shaped buildings with flat surfaces,
    2. Simple shaped buildings with inclined surfaces, where the height of the rod is the vertical height, but the protection angle is referenced from a perpendicular line from the surface to the tip of the rod.



    Fig.3: Type of Surfaces used with Protection Angle Method






    2.2 Differences between the Protection Angle Method and the Simple Cone of Protection Method

    The simple cone of protection method provided in BS 6651 which apply the simple 45° zone of protection differ from the protection angle method in the following points:

    1. The protection angle method uses the height of the air termination system above the reference plane, whether that be ground or roof level (note that the height of the air-termination is measured from the top of the air termination to the surface to be protected),
    2. In The protection angle method the protection angle is not fixed as 45º and can vary,
    3. The protection angle method depends on the class of lightning protection system and its corresponding rolling sphere.






    2.3 Relation between the Protection Angle Method PAM and the Rolling Sphere Method RSM

    The protection angle method is actually a derivative or a mathematical simplification of the rolling sphere method. This can be explained as follows (see Fig.4):

    • The protection angle is derived by initially rolling a sphere up to a vertical air termination e.g. an air rod (AB).
    • A line is then struck from the point where the sphere touches the air rod (A) down to the reference plane (D), finishing at point C.
    • The line must bisect the sphere (circle) such that the areas (shaded) of over and under estimation of protection (when compared to the rolling sphere method) are equal (have the same size).
    • The angle created between the tip of the vertical rod (A) and the projected line is termed the protective angle alpha (α).



    Fig.4: Derivation of the Protection Angle Method


    Note:

    The above procedure was applied to each Class of LPS using its corresponding rolling sphere.





    2.4 Determination Of Air Rod Protective Angle

    The protective angle afforded by an air rod located on a reference plane can be determined from Fig.5 or Table#1 in below.


    Fig.5: Determination of the Protection Angle


    Table#1: Simple Determination of the Protection Angle







    2.5 Restrictions for using Protection angle method PAM

    1- The height of the corresponding rolling sphere radius:

    • From above, we find that the angles for the protection angle method are obtained from a rolling sphere analysis and this is why the protection angle method is limited to the maximum height of the equivalent rolling sphere as in Figure which identifies the restrictions when using the protective angle method for the air termination system design.
    • When the structure/air rod/mast, relative to the reference plane, is greater in height than the appropriate rolling sphere radius, the zone of protection afforded by the protection angle is no longer valid (see Fig.6).i.e. the protective angle method is only valid up to the height of the appropriate rolling sphere radius.



    Fig.6


    For example:

    if the design was to a structural LPS Class II, and the structure’s height was 50m, then using the appropriate rolling sphere of 30m radius would leave the upper 20m needing lightning protection. If an air rod or a conductor on the edge of the roof was installed then a zone of protection angle could not be claimed because the rolling sphere had already identified that the upper 20m was not protected. Thus the protective angle method is only valid up to the height of the appropriate rolling sphere radius.


    2- the height of the reference plane

    If air-termination rods are installed on the surface of the roof to protect structures mounted thereon, the protective angle α can be different. In Fig.7 the roof surface is the reference plane for protective angle α1. The ground is the reference plane for the protective angle α2. Therefore the angle α2 according to Figure and Table is less than α1.


    Fig.7






    2.6 Shapes of Protection Zones Provided By Protection Angle Method PAM

    The protection zone can have on of the following shapes according to the used type of  Air-terminations (rods/masts and catenary wires) (see Fig.8) :


    Fig.8: Shapes of Protection Zones Provided By Protection Angle Method


    1- Cone-Shaped:

    The protective angle afforded by an air rod is clearly a three dimensional concept, Therefore a simple air rod is assigned a cone of protection by sweeping the line AC at the angle of protection a full 360º around the air rod.


    2- Tent-Shaped:

    At each end of the catenary conductor (A) a cone of protection is created relative to height h. A similar cone is created at every point along the suspended conductor. It should be noted that any sag in the suspended conductor would lead to a reduction in the zone of protection at the reference plane. This produces an overall ‘dog bone’ shape at the reference plane.





    2.7 Applications of protection using the Protection Angle Method

    Protection Angle Method PAM can be used with different types of Air-terminations (rods/masts and catenary wires) with one condition that these Air-terminations must be located so the volume defined by the protection angle covers the structure to be protected.


    Application#1: Protection Angle Method with Air rods or free standing masts

    Protection Angle Method with Air rods or free standing masts can be used in either isolated or non- isolated air-termination systems on roof-mounted structures as follows:

    A- For isolated air-termination systems on roof-mounted structures:

    • Air-termination rods as shown in Fig.9 are suitable for protecting smaller roof-mounted structures (with electrical equipment). They form a “cone-shaped” zone of protection and thus prevent a direct lightning strike to the structure mounted on the roof.

    Fig.9

    • The separation distance s must be taken into account when dimensioning the height of the air termination rod.


    B- For non- isolated air-termination systems on roof-mounted structures:

    • If the system does not need to be isolated from the structure then air rods fitted to the roof of the structure could be employed. See Fig.10A.


    Fig.10: non- isolated air-termination systems on roof-mounted structures

    • In a non-isolated system, an air rod (or multiple air rods) may be used to protect larger items of roof mounted equipment from a direct strike. See Fig.10B
    • The height of the air rods utilized is now a function of the protection angle (Class of LPS), the spacing between the air rods and the height above a particular reference plane.


    Application#2: Protection Angle Method with Catenary (or suspended) conductors

    One or more catenary conductors may be utilized to provide a zone of protection over an entire structure (See Fig.11).


    Fig.11: Protection Angle Method with Catenary (or suspended) conductors


    Application#3: Protection Angle Method with Meshed conductor network

    As with the rolling sphere method a meshed conductor network must be mounted at some distance above the roof. This is in order to provide an effective zone of protection using the protective angle method (See Fig.12).


    Fig.12: Protection Angle Method with Meshed conductor network






    2.8 Advantages and disadvantages of Protection Angle Method PAM

    A- Advantages:

    • Its simplicity in application.


    B- Disadvantages:

    • It is a further simplification of the rolling sphere method, hence may not be as reliable or efficient. Because the protection angle method is limited in application to heights that are equal to or less than the corresponding rolling sphere radius,
    • Where the protection angle method alone is employed, multiple rods are generally required for most structures,
    • Its main usage is to show the effectiveness of the designed protection system more than determination of which parts of a structure require protection.






    3- Mesh Method




    3.1 Usage

    Mesh Method used for protection of plane (flat) roof structures and should not be used on curved surfaces. so, it can be used on the following surfaces regardless of the height of the structure (see Fig.13):

    1. A horizontal flat-roof structure,
    2. A sloped-roof structure,
    3. A compound flat roof structure,
    4. A compound shed roof structure such as industrial roofs,
    5. Vertical sides of tall buildings for protection against flashes to the side.



    Fig.13: Types of Surfaces used with Mesh Method






    3.2 Conditions for Application of Mesh Method Protection

    The mesh method is considered to protect the whole bound surface if the following (5) conditions are verified:


    Condition#1: Air-termination conductors are positioned on:

    1. Roof edge lines,
    2. Roof overhangs,
    3. Roof ridge lines, if the slope of the roof exceeds 1/10 (5.7°).


    Notes:

    • 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.
    • As modern research on lightning inflicted damage has shown, the edges and corners of the roofs are most susceptible to damage. So on all structures particularly with flat roofs, the perimeter conductors should be installed as close as possible to the outer edges of the roof as is practicable.


    Condition#2: The mesh size of the air-termination network is in accordance with Table#2.

    LPL
    Mesh Size
    I
    5 m x 5 m
    II
    10 m x 10 m
    III
    15 m x 15 m
    IV
    20 m x 20 m

    Table#2: Mesh size for mesh method.


    Condition#3: No metallic structures protrude outside the volume protected by air-termination systems

    Notes:

    • The protection provided by meshed conductors not placed in full accordance with the mesh method, e.g., those raised above the building surface, should be determined with an alternative design method, i.e., PAM or RSM, applied to the individual conductors.
    • If the RSM is used, Table#3 provides a simple rule of thumb for determining what minimum distance above the building surface the mesh conductors would be required to be raised in order to conform to the rolling sphere method. It can be seen that this distance is 0.31, 0.83, 1.24 and 1.66 m for mesh method grids spaced to requirements of LPL I, II, III and IV respectively.



    Table#3


    Condition#4: From each point, at least two separate paths exist to ground/earth termination system (i.e. no dead ends), and these paths follow the most direct routes

    Note:
    • Larger number of down-conductors results in reduction of the separation distance and reduces the electromagnetic field within the building.


    Condition#5: The air-termination conductors follow, as far as possible, the shortest and most direct route.





    3.3 Using Natural Components In Mesh Method

    • Natural components may be used for part of the mesh grid, or even the entire grid system if the required minimum dimensions for natural components of the air-termination system are complied with the conditions stated before in Article Design Calculations of Lightning Protection Systems – Part Sixteen " part: Sizing of Natural Air Terminals.
    • Also we can use the ridge and the outer edges of the structure as a part of the mesh grid, so the individual meshes can be sited as desired (see Fig.14).



    Fig.14: Using Gutter as a part of the Mesh Grid



      


    3.4 Special Cases for Protection by Mesh Method


    1- Mesh method on Vertical sides of tall buildings for protection against side flashes

    The mesh method is recommended for the protection of the sides of tall buildings against flashes to the side as follows:

    Case#1: For Buildings above 60 m High

    • The topmost 20 % of lateral surfaces should be equipped with air terminals (like a mesh with the same size). For the part of this surface to be protected which is below 60 m the protection can be omitted.
    • 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.


    Notes:

    • For structures between 60 m and 75 m in height, the area protected need not extend below 60 m.
    • If sensitive parts (e.g. electronic equipment) are present on the outside of the wall in the upper part of the building, they should be protected by special air-termination measures, such as horizontal finials, mesh conductors or equivalent.


    Case#2: For Buildings Less Than 60 m High

    • Note that for structures less than 60 m 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.



    Note for Buildings Taller Than 30 m:

    • 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.



    2- Adjacent areas to the mesh

    • The protective area provided by the mesh method is the area bounded by the mesh. The protection to areas adjacent to the mesh (e.g. building sides and lower structural points) is determined by the protection angle method or rolling sphere method (see Fig.15).

    Fig.15: The protection to areas adjacent to the mesh

    • If there are external areas of the structure situated in heights which are higher than the radius of corresponding rolling sphere, an air termination system has to be installed applying the mesh method (see Fig.16).



    Fig.16: external areas of the structure with heights more than the radius of corresponding rolling sphere




    In the next Article, I will list The Best Recommendations for Positioning of Air Terminals. Please, keep following.





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