Design Calculations of Lightning Protection Systems – Part Sixteen


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 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



    Today, I will I will continue explaining other steps of Lightning Protection Design procedure.


    For more information, please review the following Articles:







    Step#4: Sizing of Air Termination System Components




    1- Introduction




    As we stated before in Article " Conventional Lightning Protection System Components – Part One " that:




    Conventional Lightning Protection System LPS Components

    The Conventional Lightning Protection System consists of two main parts (as in below figure):


     External and Internal Lightning Protection System Parts


    1- The External Lightning Protection System, which includes:

    • Strike Termination Subsystem,
    • Conductor Subsystem,
    • Grounding Electrode Subsystem.


    2- The Internal Lightning Protection System, which includes:

    • Equipotential Bonding Subsystem,
    • Surge Protection Subsystem.



    Strike Termination Subsystem

    Usually called “Air Termination Subsystem “, the purpose of the strike termination subsystem is to intercept the lightning event and course it harmlessly into the conductor subsystem. Thus it is vitally important to use a correctly designed air termination system.

    When designing the external lightning protection system of a structure, we distinguish between two types of air-termination system:
      
    1. Non-isolated system,
    2. Isolated system.


    Isolated and Non-isolated Air Termination Subsystem


    Also, The strike termination subsystem can take many forms specified by the various engineering standards available, as follows:


    strike termination subsystem Forms


    strike termination subsystem Forms according to
     strike termination subsystem Type




    A- Vertical Air Terminals,
    B- Horizontal air termination network, which include two sub-forms:
    1. Meshed conductor network,
    2. Overhead wires (Catenary wires).

    C- Combination of Vertical Air Terminals and Horizontal air termination network,
    D- Natural Air Terminals.

    For more information about Air Termination Subsystem, please review Article " Conventional Lightning Protection System Components – Part One " mentioned in above.






    2- Sizing of Air Terminals




    First: Based on IEC 62305-3

    Configurations and minimum cross-sectional areas of air-termination conductors, air termination rods and down-conductors are given in below Table#1 and shall comply with the requirements and tests according to the future IEC 62561 series.


    Table#1


    Notes to Table#1:

    • a Mechanical and electrical characteristics as well as corrosion resistance properties shall meet the requirements of the future IEC 62561 series.
    • b 50 mm2 (8 mm diameter) may be reduced to 25 mm2 in certain applications where mechanical strength is not an essential requirement. Consideration should in this case, be given to reducing the spacing between the fasteners.
    • c Applicable for air-termination rods and earth lead-in rods. For air-termination rods where mechanical stress such as wind loading is not critical, a 9,5 mm diameter, 1 m long rod may be used.
    • d If thermal and mechanical considerations are important then these values should be increased to 75 mm².



    Second: Based on BS EN 62305-3

    The BS EN 62305-3 standards prescribe the minimum material requirements as summarized in Table#2. It should be noted that the standards do not prescribe any relative performance advantages between these choices. All are adequate to conduct the lightning current. When considering service life, aesthetics, galvanic compatibility with building materials and ease of installation, there is generally a preferred clear choice of material for a given structure.


    Table#2

      
    Notes to Table#2:

    • (1) 50 mm2 (8 mm diameter) may be reduced to 28 mm2 (6 mm diameter) where mechanical strength is not an essential requirement,
    • (2) For air-terminals of 1 m or less, 10 mm diameter may be used,
    • (3) 50 mm2 with a minimum 2.5 mm thickness may be used with aluminum alloy,
    • (4) Materials may be covered with PVC for aesthetic purposes.




      

    3- Sizing of Natural Air Terminals




    The parts of a structure should be considered and may be used as natural air termination components and part of an LPS, these parts are:

    a) Metal sheets covering the structure to be protected provided that:

    • The electrical continuity between the various parts is made durable (e.g. by means of brazing, welding, crimping, seaming, screwing or bolting),
    • The thickness of the metal sheet is not less than the value t’ given in Table# 3 (from IEC 62305-3) if it is not important to prevent puncture of the sheeting or to consider ignition of any readily combustible materials underneath,
    • The thickness of the metal sheet is not less than the value t given in Table# 3 if it is necessary to take precautions against puncture or to consider hot spot problems,
    • They are not clad with insulating material.


    Table# 3


    Notes:

    • Where hot spot or ignition problems may arise, it should be verified that the temperature rise of the inner surface at the point of strike does not constitute a danger. Hot spot or ignition problems can be disregarded when the metal sheets lies inside an LPZ0B or higher.
    • A similar Table to Table#3 but as per BS EN 62305 is provided in below.




    b) Metal components of roof construction (trusses, interconnected reinforcing steel, etc.), underneath non-metallic roofing, provided that damage to this non-metallic roofing is acceptable.

    c) Metal parts such as ornamentation, railings, pipes, coverings of parapets, etc., with cross sections not less than that specified for standard air-termination components.

    d) Metal pipes and tanks on the roof, provided that they are constructed of material with thicknesses and cross-sections in accordance with Table #1.

    e) Metal pipes and tanks carrying readily-combustible or explosive mixtures, provided that they are constructed of material with thickness not less than the appropriate value of t given in Table#3 and that the temperature rise of the inner surface at the point of strike does not constitute a danger.


    Notes:

    • If the conditions for thickness are not fulfilled, the pipes and tanks shall be included into the structure to be protected.
    • Piping carrying readily-combustible or explosive mixtures shall not be considered as an air termination natural component if the gasket in the flange couplings is not metallic or if the flange-sides are not otherwise properly bonded.
    • A thin coating of protective paint or about 1 mm asphalt or 0,5 mm PVC is not regarded as an insulator.
    • It is not desirable to use vessels and pipe work which contains gas or liquids under high pressure or flammable gas or liquids.
    • Where combustible materials are not present, and water ingress can be tolerated from a puncture due to lightning, then thinner material is permitted for air-terminations.


    Don't forget that The requirements for natural air-terminations are:
    • To withstand the ohmic heating and electromechanical/magnetic forces,
    • To withstand the heat of the lightning plasma arc.






    4- Positioning / Placement of Air Termination System Components




    The three basic methods recommended for determining the position of the air termination system components are:

    1. The Rolling Sphere Method (RSM),
    2. The Protective Angle Method (PAM),
    3. The Mesh Method.


    Any of these methods can be used to determine:

    1. The optimum location of the air-termination System Components,
    2. The resulting down-conductor,
    3. earthing requirements.


    Notes:

    • Different design methods can be applied to different regions of a single lightning protection system, provided the zones afforded by each method overlap to protect the entire structure. I.e. we can use the (3) methods together in designing one complete LPS for given structure.
    • Generally most of the standards consider the three methods as equivalent, although there are limits on the application of the protection angle and mesh methods as in below table:


    The Rolling Sphere Method (RSM)
    The rolling sphere method is recommended as the most universal and most effective method.
    The rolling sphere method generally provides the most optimized design and the vertical air-terminal is far more effective at capturing lightning flashes than mesh conductors installed upon, or just above structure surface. For example; Rod air-terminations of height in the region of 0.5 m are preferable to shorter rods or conductors on the building surface.

    The Protection Angle Method (PAM)
    The protection angle method can only be used with limited vertical distances (limited height).
    The Mesh Method
    The mesh method is more suitable for the protection of flat/plane surfaces.


    Suitability Of Air-Termination Forms and Design Methods

    Meshed conductors used as air-terminations should not be confused with the mesh method. While the mesh method requires the use of surface mounted meshed conductors (a grid) to protect flat surfaces, the rolling sphere and protection angle method can also be used to determine protection provided by elevated meshed conductors to protect a variety of compound surfaces. The below Table shows the Suitability Of Air-Termination Forms and Design Methods.


    Air-Termination

    Protection Method
    Rolling Sphere
    Mesh Method
    Protection Angle
    Rod
    √√√

    √√
    Meshed conductors – (on structure surface)

    (1)

    Meshed conductors – (elevated from structure)
    √√

    √√
    Catenary wires
    √√

    Note (1): Mesh method is appropriate for the evaluation of the protection of the bound flat surface. Rolling sphere and protection angle methods can be used to determine protection of adjacent areas.







    The Class of LPS/LPL influences on the (3) Positioning Methods


    According to IEC 62305-3, Based on the class of the lightning protection system, the below table indicate Maximum values with each class for:

    1. Rolling sphere radius,
    2. Mesh size,
    3. Protection angle permitted,


    Class Of LPS (Lightning
    Protection Level)
    Rolling Sphere Radius (M)
    Mesh Size (M)
    Protection Angle
    I
    20
    5 x 5
    See below figure

    II
    30
    10 x 10
    III
    45
    15 x 15
    IV
    60
    20 x 20



    Protection Angle with corresponding height and Class of LPS


    Notes:

    • h is the height of air–termination above the surface to be protected,
    • The angle will not change for values of h below 2 m,
    • Lightning protection class I, II, III & IV relate to protection level I, II, III, & IV respectively as in below table:


    Class of LPS
    Lightning protection
    level (LPL)
    I
    I (highest protection)
    II
    II
    III
    III
    IV
    IV (lowest protection)

    For example if the risk assessment determines that a lightning protection system with lightning protection class II is required to reduce the risk to below the tolerable level, then the design of the lightning protection system will need to be in accordance with the requirements of lightning protection level II (or higher). The greater the level of lightning protection (LPL I being the greatest), the larger the resulting material requirement for the lightning protection system.


    From the above Table and Figure, the following table can be used for Positioning of air termination according to the level of protection


    Protection level
    h (m)
    20
    30
    45
    60
    Mesh Width d (m)
    R (m)
    α0
    α0
    α0
    α0
    I
    20
    25
    *
    *
    *
    5
    II
    30
    35
    25
    *
    *
    10
    III
    45
    45
    35
    25
    *
    15
    IV
    60
    55
    45
    35
    25
    20
    * Rolling sphere and mesh methods only apply in these cases

    And the parameters h, R and α are indicated in the below figure:


    Positioning of air termination according to the level of protection


    Where:

    h is the height of air–termination above the surface to be protected,
    R is the radius of the rolling sphere,
    α is the protective angle of the conic space in degrees.





    In the next Article, I will explain in detail the (3) Positioning Methods for Air Termination System. Please, keep following.




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