Design Calculations of Lightning Protection Systems – Part Fifteen

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
Explained in above Article




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


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


    For more information, please review the following Articles:


    Step#3: Selection Of External LPS Type and Material




    1- The Correct Choice Of Lightning Protection Components (LPC)


    • The correct choice of material, configuration and dimensions of the lightning protection components is essential when linking the various elements of an LPS together. 
    • The designer/user needs to know that the components, conductors, earth electrodes etc will meet the highest levels when it comes to durability, long term exposure to the environmental elements and perhaps most importantly of all, the ability to dissipate the lightning current safely and harmlessly to earth. 
    • Various standards series have been compiled with this very much in mind. At present these standards are as follows:



    Standards for Lightning Protection Systems

    1- Within Europe:

    Various standards series have been issued by (2) National Committees which are:

    1. The European Committee for Electrotechnical Standardisation (CENELEC).
    2. The International Electrotechnical Commission (IEC)


    The CENELEC has released the EN 50164 series of standards. The EN 50164 series are component standards to which the manufacturers and suppliers of lightning protection components should test their products to verify design and quality.

    The EN 50164 series currently comprises of:

    • EN 50164-1 Lightning protection components (LPC) – Part 1: Requirements for connection components
    • EN 50164-2 Lightning protection components (LPC) – Part 2: Requirements for conductors and earth electrodes
    • EN 50164-3 Lightning protection components (LPC) – Part 3: Requirements for isolating spark gaps
    • EN 50164-4: Lightning Protection Components (LPC) – Part 4: Requirements for conductor fasteners
    • EN 50164-5: Lightning Protection Components (LPC) – Part 5: Requirements for earth electrode inspection housings and earth electrode seals
    • EN 50164-6: Lightning Protection Components (LPC) – Part 6: Requirements for lightning strike counters
    • EN 50164-7: Lightning Protection Components (LPC) – Part 7: Requirements for earthing enhancing compounds


    Notes:

    • The standards generally have an IEC prefix to their number (CEI for French versions). IEC standards are produced in English and French languages.
    • IEC and CENELEC generally work in parallel, and CENELEC members vote to adopt new IEC standards as CENELEC standards. The committees of CENELEC may choose to make some alterations to the IEC version.
    • Additionally, CENELEC produce their own standards to which IEC have no counterpart. CENELEC documents are produced in English, French and German and an approved CENELEC standard will have an EN prefix (or NE in the French language versions).


    For example:

    IEC 62305-1 (IEC version) is parallel to EN 62305-1 (CENELEC adopted copy of the above)
    And both are parallel to BS EN 62305-1 (British National Standard adoption ofthe above)


    2- Within USA:

    Various standards series have been issued such as:

    1. Underwriters Laboratory (UL96 & 96A),
    2. The National Fire Protection Association (NFPA 780),
    3. The Lightning Protection Institute (LPI-175).


    Note:

    For heavy fault conditions, Conductor Size should be calculated in accordance with IEEE Std 80.





    2- Material Requirements for Conductors and Air Terminations

    • All lightning protection materials should conform to EN 50164-1 and EN 50164-2 Lightning Protection Components requirements. The exceptions to these requirements are non-current carrying devices such as down-conductor fixings (clips), anti-vandal guards and mechanical supports.
    • The IEC and EN standards prescribe the minimum material requirements as summarized in the following Table:


    Table#1: Material Requirements for Conductors and Air Terminations


    Note:

    • The standards do not prescribe any relative performance advantages between these choices; all are adequate to conduct the lightning current. But, 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.






    3- Comparison between Copper and Aluminum
    as a Lightning Conductor Material

    A conductor material should be chosen that is compatible with the surface it is to be located upon and that which it is to connect to. As a typical lightning protection system requires frequent bonds to nearby metallic items, compatibility with this should also be assessed.

    Comparison between Copper and Aluminum Conductor Materials is indicated in the following table:
      
    Aluminum
    Copper
    lower cost

    More expensive than Aluminum
    lighter weight

    More heavy than Aluminum
    Less compatible with many building materials

    More compatible with many building materials
    Cannot be buried in the ground

    Can be buried in the ground
    the aesthetics of eventual white corrosion of aluminum

    the aesthetics of green verdigris of copper
    Aluminum is prone to corrosion when in contact with limestone, plaster, mortar and cement. For this reason, aluminum conductors should not be placed in direct contact with such surfaces.Stand-off fixings can be used (see below figure), or PVC covered aluminum conductors used

    care should be taken in areas with
    sulphurous atmospheres (e.g. smoke stacks), where stainless steel or lead covered copper may be more appropriate


    Aluminum is prone to corrosion in marine or sea-side environments


    Aluminum should not be installed where it will be exposed to water run-off from copper (or copper alloy) surfaces

    Copper should not be installed above galvanized, zinc or aluminum parts where water run-off occurs (unless parts are protected such as by PVC covering). Water run-off from the copper surface carries fine copper corrosion particles, which when deposited on lower galvanized, zinc or aluminum parts may results in severe corrosion.

    Aluminum should not be installed on surfaces coated with alkaline based paint.


    Aluminum should not be installed in locations subject to excessive moisture (i.e. in gutters, or on surfaces where water may be retained).




    Therefore, most lightning protection systems are entirely copper or utilize an upper aluminum portion connecting to a copper earth termination system. As aluminum and copper are not compatible, a bimetallic joint should be used to interconnect these two materials.

    Note:

    As aluminum and copper are not compatible, a bimetallic joint should be used to interconnect these two materials.


    Bimetallic Connector






    4- Use of Dissimilar Metals

    • Galvanic corrosion occurs when two dissimilar metals are in contact with each other in the presence of an electrolyte. In this situation, one metal becomes the anode and the other the cathode. The anode will tend to go into solution and therefore corrode. The electrolyte can be water with impurities from the air, other surfaces or from the metal itself
    • The following Table shows the potential difference between dissimilar metals. Combinations of metals with potential differences above 0.5 V should be rejected to avoid excessive corrosion.




    For example:

    A bare copper conductor should not be directly connected to steel, as the electrochemical potential difference is 0.53 V (≥ 0.5 V). However, if the copper is tin plated then the difference becomes that of tin (0.24 V), which is acceptable.

    • One method of reducing the effects of corrosion is to use plating of one or both of the metals to reduce the electrochemical potential difference. Commonly, tin plated copper conductors are used for this purpose. Tin plating also has the advantage of stopping the appearance of green verdigris coating and reducing the chance of theft (as the conductor no longer looks like copper). Tin plated copper should be used for connections to:


    1. Lead,
    2. Grey cast iron,
    3. Steel (stainless steel connections do not need to be tinned),
    4. Aluminum,
    5. Cadmium.


    The following Table shows the material of structure and its LPS compatible material:








    5- Temperature Rise Criteria


    In non-isolated lightning protection system, the down-conductors are primarily mounted directly onto the structure (with no distance).




    But, sometimes a separation distance is required between the down conductors and the structure. This is governed by the criteria of temperature rise in the event of lightning striking the lightning protection system.

    Temperature Rise Criteria:

    The temperature rise criterion is based on the level of flammability for the structure walls; we have two cases as follows:

    Case#1: If the wall is made of flame-resistant material or material with a normal level of flammability

    • In this case, the down-conductor systems may be installed directly on or in the wall.
    • For example, Wood with a bulk density greater than 400 kg/m2 and a thickness greater than 2 mm is considered to have a normal level of flammability. Hence the down-conductor system can be mounted on wooden poles.


    Case#2: If the wall is made of highly flammable material

    In this case, we have two sub-cases according to considering the temperature rise (when lightning currents flow) is a hazard or not, this is can be known from table as follows:
    • The below Table states the maximum temperature rise ΔT in K of the various conductors for each class of lightning protection system. These values mean that, generally, it is even permissible to install down conductors underneath heat insulation because these temperature rises present no fire risk to the insulation materials. This ensures that the fire retardation measure is also provided.


    • So, the two sub-cases for case#2: If the wall is made of highly flammable material, are:


    Case#2-A: The temperature rise of the down-conductor systems is not hazardous

    • In this case, the down conductors can be installed directly on the surface of the wall.


    Case#2-B:  The temperature rise of the down-conductor systems presents a hazard

    • In this case, the down conductors must be mounted (by using Standoff brackets for example) to ensure that the separation distance between the down-conductor and the wall is greater than 0.1 m. The mounting elements may touch the wall.


    Standoff brackets


    Notes:

    • When installing the down-conductor system in or underneath heat insulation, the temperature rise (on the surface) is reduced if an additional PVC sheath is used. Aluminum wire sheathed in PVC can also be used.
    • The erector of the structure must state whether the wall, where a down-conductor is to be installed, is made of flammable material.






    6- Natural Components

    • Natural conductive components can be used as an integral part of the lightning protection system. Natural components are typically metallic structural items that will not be modified during the life of the structure, such as reinforcing steel, metal framework and roofing/cladding.
    • Natural components must meet minimum material requirements which listed in below Table and be electrically continuous with secure interconnections between sections such as brazing, welding, clamping, seaming, screwing or bolts.


    Table#2

    • The above Table provides the thickness requirements for natural air terminations. 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. If the materials do not meet these requirements, then they must be protected by the lightning protection system.
    • Metal pipes and tanks on roofs can be used, provided they meet the requirements of Tables#1 and 2. Refer to the standard for requirements of tanks and piping that contain combustible or explosive mixtures. It is not desirable to use vessels and pipe work which contains gas or liquids under high pressure or flammable gas or liquids.
    • The requirements for natural air-terminations differ from natural down-conductors. Down-conductors and air-terminations need to withstand the ohmic heating and electromechanical/magnetic forces, but air-terminations also need to withstand the heat of the lightning plasma arc.
    • The following parts of a structure can be used as “natural components” of the lightning protection system:


    1. Metal Installations
    2. Facade Elements, Mounting Channels and the Metal Substructures of Facades
    3. Metal downpipes
    4. Rebar in Reinforced Concrete
    5. Rebar in Precast Concrete
    6. Rebar in Prestressed Concrete



     Facade Elements as Down Conductors


    For more information about using Natural Components as down conductors, please review the Article " Conventional Lightning Protection System Components – Part Four ".




    In the next Article, I will explain Step#4: Sizing of Air Termination System Components. Please, keep following.