Design Calculations of Lightning Protection Systems – Part Nineteen

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 Methods for Positioning of Air Terminals in the following Articles:


    Method for Positioning of
    Air Terminals
    Article
    The Rolling Sphere Method (RSM)
    The Protective Angle Method (PAM)
    The Mesh Method



    Today, I will explain Recommendations for the Best Positioning of Air Terminals.

    For more information, please review the following Articles:




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




    4- Recommendations for the Best Positioning of Air Terminals




    4.1 Recommendation on positioning of Air-terminals

    The positioning of air terminations on corners, exposed points and edges should be compliant with:

    1. The used type of Air Termination (i.e. Rod, conductor, natural components),
    2. The design method used (i.e. rolling sphere, protection angle or mesh method).


    And this can be explained as follows.


    1- Air-Terminals (Rods)

    • The IEC standards allow any height of air-terminal (rod) to be used for lightning protection without assigning how much this air-terminal (rod) is close to corners, exposed points and edges of the building. For example, air-terminals (rods) may be required on a parapet edge and a good design will allow sufficient height so that protection is provided regardless of whether the rods are installed on top of the parapet, or on the inside or outside edge.
    • While modern research shows that the effective protection provided by air-terminal (rod) depends on:

    1. The minimum height of air-terminal (rod),
    2. The relationship of the height of the air termination (rod) to distance from the structure edge.

    •  Based on this modern research, the following recommendations are given (see Fig.2):

    1. For air-terminals of less than 0.5 m height, these need to be placed less than half their height from the edge to be most effective.
    2. For air-terminals with a minimum height of 0.25 m be selected (0.5, m preferred), these need to be placed as close as practical to any edge being protected (at least within 0.5 x rod height).



    Fig.2: Recommended distances for placement of Air-terminals (rods)



    2- Air-Terminals (Horizontal Conductors)

    Case#1: horizontal conductor is a part of the air-termination network


    A- If Design made by The Mesh Method

    • This modern research does not bode well for horizontal conductors placed on the building via the mesh or other design methods, as the height of the conductor is the conductor thickness, typically 2 to 8 mm.
    • The above research concludes that the conductor would need to be installed 1 to 4 mm from the structure edge. Generally, this is impossible due to the need to install clips to fasten the conductor. As these are often screwed in place, they cannot be installed close to the edge.
    • Due to performance concerns, it is not recommended to use the mesh method where horizontal conductors (for use as air-terminations) are installed directly onto the surface to be protected. The addition of vertical air-terminations (rods) improves the performance of the system considerably.



    B- If Design made by The Rolling Sphere Method

    • In accordance with the design requirements of the rolling sphere method, any horizontal conductor on a building edge (and intended to be part of the air-termination network) would need to be virtually on the exact edge to stop the rolling sphere from touching the edge of the structure. The IEC standards do not give any assistance or recommendations on this issue. BS 6651-1991 infers that 0.1 m maximum distance is acceptable, yet this appears to be unsubstantiated.
    • Raising any horizontal air terminations at least 0.25 m above the surface improves performance.

      

    Case#2: horizontal conductor is not part of the air-termination network

    • If the horizontal conductor is not part of the air-termination network (i.e. is part of the bonding network joining air terminals), then placement is not as critical provided the other air-terminations provide protection to the desired level.



    3- Air Termination (Natural Components)

    • If the building edge uses a coping (metallic covering), then provided that the requirements for natural air-terminations are met, the coping eliminates the conductor placement concern. In many cases, at the design stage of the building, the use of coping, installation of metallic hand rails or careful selection of building materials and other structural items can significantly improve performance and reduce the visual impact of the lightning protection system.






    4.2 Recommendation for Masts and Antennas


    1- Masts and Flag Poles do not contain electrical circuits:

    • Masts and flag poles, etc, on the structure can be used as part of the air-termination system where they meet the requirements of natural components. The masts should be bonded into the LPS system.


    2- Antennas and Masts with electrical equipment (see Fig.3):

    • Antennas (communication and TV aerials, etc) and masts with electrical equipment (e.g. obstruction lighting) should be protected by an air-termination system, preferably an isolated system where the antenna and its mast does not conduct lightning currents.
    • For simple applications such as domestic roof top TV antennas it is permissible to simply bond the mast to the air-termination system, but damage to antenna/mast and cabling can be expected. To avoid this damage:

    1. If the antenna is exposed to lightning flashes, then surge protection (Class I) must be installed.
    2. If the antenna is protected by an air-termination, then surge protection (Class II) should be installed.



    Fig.3: Protection Of Antenna

      
    Note:

    The preferred location for the SPD is as close as possible to the entry point of the cable into the structure, and where possible the cable should enter into the building near to, and be connected to an equipotential bonding bar.


    3- Antennas Cables:

    • Screened cables should have the screen bonded to the antenna/air termination and the equipotential bonding bar.
    • Isolated down-conductor system provides an air-terminal and isolated down-conductor that can be mounted directly on the mast/antenna structure to reduce the risk of direct or partial lightning currents (see Fig.4). The isolated down-conductor has a special construction that allows it to be mounted directly on the mast, but provides the equivalent separation distance of 1000 mm of air but its special insulation properties allow it to be mounted directly on the object to be protected.



    Fig.4: Isolated mast and Isolated Down-conductor protecting a roof-top antenna

      
    • The system is installed on the structure (like a non-isolated LPS), but has many of the advantages of an isolated LPS. The use of this system dramatically simplifies the bonding and separation issues normally associated with a LPS. Additionally, the system eliminates the conduction of lightning currents in the antenna system.






    4.3 Recommendation for Protection of other items protruding above the roof

    The design of the lightning protection system should be such that air-terminations are positioned to provide protection against lightning flashes to the roof and all items located upon it (vents, skylights, air-handling units, pipes, etc). However, in some cases, protection is not required for smaller or non-conductive items. Table#1 summarizes the requirements for determining if air termination protection is required.



    Roof fixtures do not require protection if these dimensions are not exceeded:

    Metal Roof Fixtures

    Height above the roof level:
    0.3 m

    The total area of the superstructure:
    1.0 m2

    The length of the superstructure:
    2.0 m

    Nonconductive Roof Fixtures

    Protrusion above the surface formed
    by the air termination system:

    0.5 m

    Table#1: Roof fixtures not requiring protection


    Note that the bonding requirement for these items requires separate consideration; refer to Part#4 in this Article.

    Conductive connections into the structure may be one of the following:

    1. Electrical circuits or,
    2. Non-electrical pipes such as metal pipes for water or gas, or air services.


    Items that have conductive connections into the structure interior require air-termination protection. Otherwise, considerable currents can be conducted into the structure interior.


    Methods to protect Conductive connections above the roof:


    1- Using a LPZ 1 or higher:

    IEC 62305-1 Section 8.3.2 requires internal systems to be located inside a LPZ 1 or higher. LPZ 1 are zones protected against direct flashes and only exposed to limited lightning or induced currents. To meet this requirement:

    • Conductive connections (e.g. water or gas pipes, electrical power or electronic control cables) are required to be bonded to the LPS. For the bonding of electrical/electronic circuits, surge protective devices (SPDs) are required.


    2- Replacing conductive connections with insulated (non conductive) connections

    • In some cases, it may be possible to replace conductive connections with insulated (non conductive) connections (twice the length of the separation distance requirement), thus overcoming these requirements.
    • For example a small pressurized air reservoir (< 0.3 m high, < 1 m2, less than 2 m width or length and meeting natural air-termination thickness requirements), but with metallic pipes entering into the facility, requires air-terminals and bonding of pipes. However, if the connecting pipes were replaced with non conductive pipes, then air-terminals and bonding would not be required.


    Special applications

    A- Vents (see Fig.5):

    • A plastic vent such as those used for plumbing would not require protection if protruding less than 0.5 m above the protection zone.
    • If greater than 0.5 m above the protection zone, then air terminal protection would be required. One way of doing this would be to use a wire with a diameter of 8 mm up to a maximum free length of 0.5 m, as shown in Fig.6.



    Fig.5: vents Lightning protection requirements


    Fig.6: Additional air-termination system for ventilation pipes


      
    B- Chimneys:

    While most chimneys are tall enough to require protection, in the case of non-conductive chimneys, or similar vents, protection is still recommended, as soot deposits can effectively make these items conductive.


    C- Electronic Or Electrical Equipment:

    All items on the roof that contain electronic or electrical equipment require protection via air-terminations (see Fig.7). Additionally for electrical and electronic items, protection is also required if wall mounted in a location where a lightning flash may occur (i.e. in a location touched by rolling sphere method). Surge protective devices must be installed on the circuits to limit current entering into the internal environment.


    Fig.7: Air-termination protection required to wall mounted electronic / electrical equipment.







    4.4 Recommendation for Bonding of Roof Top Fixtures


    Lightning Equipotential Bonding Of Roof Top Fixtures

    Separation distance requirements between the air-terminations and all conductive roof fixtures (including conductive flush mount fixtures) should be evaluated to determine if bonding of the item. You can review the Separation distance requirements in Article " Conventional Lightning Protection System Components – Part Seven ".


    Equipotential bonding of roof top fixtures ,generally, governed by (3) scenarios as follows :

    Scenario#1: If the roof mounted equipment is not protected by the air termination system but can withstand a direct lightning strike without being punctured.

    In this case, Equipotential bonding of roof top fixtures requirements will be as follows:

    1. The casing of the equipment should be bonded directly to the LPS.
    2. If the equipment has metallic services entering the structure (gas, water etc) that can be bonded directly, then these should be bonded to the nearest equipotential bonding bar.
    3. If the service cannot be bonded directly (power, telecom, cables) then the ‘live’ cores should be bonded to the nearest equipotential bonding bar, via suitable Type I lightning current SPDs.


    Scenario#2: If the roof mounted equipment cannot withstand a direct lightning strike and there is sufficient space on the roof for achieving a separation distance.

    In this case, Equipotential bonding of roof top fixtures requirements will be as follows:

    1. An air rod or suspended conductor should be installed (see Figure 8.A). This should offer sufficient protection and is so spaced from the equipment, such that it complies with the separation distance. This air rod/suspended conductor should form part of the air termination system.
    2. If the equipment has metallic services entering the structure (gas, water etc) that can be bonded directly, then these should be bonded to the nearest equipotential bonding bar.
    3. If the other electrical services do not have an effective outer core screen, then consideration should be given to bonding to the nearest equipotential bonding bar, via Type II overvoltage SPDs.
    4. If the electrical services are effectively screened but are supplying electronic equipment, then again due consideration should be given to bonding, via Type II overvoltage SPDs.
    5. If the electrical services are effectively screened but are not supplying electronic equipment, then no additional measures are required.



    Fig.8: Location of air-terminal and bonding requirement for conductive items



    Scenario#3: If the roof mounted equipment cannot withstand a direct lightning strike and there isn’t sufficient space on the roof for achieving a separation distance.

    In this case, Equipotential bonding of roof top fixtures requirements will be as follows:

    1.  An air rod or suspended conductor should be installed and there should be a direct bond to the casing of the equipment, the air rod/suspended conductor should be connected into the air termination system.
    2. If the equipment has metallic services entering the structure (gas, water etc) that can be bonded directly, then these should be bonded to the nearest equipotential bonding bar.
    3. If the service cannot be bonded directly, (power, telecom, cables) then the ‘live’ cores should be bonded to the nearest equipotential bonding bar, via suitable Type I lightning current SPDs.


    These (3) scenarios are summarized in the below flow chart:


    Fig.9: Flowchart showing requirements for roof top items.


    However, to eliminate the need to bond, it may be possible to select air-termination location and height so the fixture is protected by the air-termination, but positioned far enough distance away so that bonding is not required (see Fig.8B).





    In the next Article, I will explain Step#5: the Design of Down-Conductor System. Please, keep following.



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