Design Calculations of Lightning Protection Systems – Part seventeen

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.

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
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.

    Today, I will explain in detail the (3) Positioning Methods for Air Termination system which were:

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

    For more information, please review the following Articles:

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

    1- The Rolling Sphere Method (RSM)


    1.1 The striking distance approach

    • For lightning flashes to earth, a downward leader grows step-by-step in a series of jerks from the cloud towards the earth. When the leader has got close to the earth within a few tens, to a few hundreds of metres, the electrical insulating strength of the air near the ground is exceeded.
    • A further “leader” discharge similar to the downward leader begins to grow towards the head of the downward leader: the upward leader.
    • Upward leader will be launched at points of greatest electric field intensity (see Fig.1) and can move in any direction towards the approaching downward leader. It is for this reason that lightning can strike the side of tall structures rather than at their highest point.

    Fig.1:Development of downward leader striking distance

    • The distance of the last step of a downward leader is termed the striking distance and is determined by the amplitude of the lightning current. This striking distance can be represented by a sphere with a radius equal to the striking distance (see Fig.2). The striking distance r is given by:

    r = 10 I 0.65

    Where I is the peak current of the resulting stroke.

    Fig.2: striking distance

    Notes to the above formulas:

    • The larger the amount of charge carried by the lightning leader, the larger the resulting lightning current, the greater will be the distance at which this happens.
    • The head of the downward leader approaches the objects on the ground, unaffected by anything, until it reaches the final striking distance.
    • It is more difficult for an air-terminal to intercept a smaller lightning flash than a larger flash, as the smaller flash must approach closer to the air-terminal before the upward leader is launched.
    • To protect the structure against smaller lightning flashes, air-terminals must be spaced closer together. For smaller lightning flashes there is a risk that an air terminal may not be close enough to intercept the down leader, thus a closer structural point releases an upward leader which intercepts the flash (i.e. the building is struck).

    1.2 Relation between Lightning Protection Levels and Rolling Sphere Radius

    The below Table#1 indicates the following:

    • The Lightning Protection Levels LPL,
    • Minimum current level to be protected against,
    • Probability percentages that lightning may be greater than these levels,
    • The rolling sphere radius used in the rolling sphere design method.


    Also, The above Table#1 explains the relation between Lightning protection levels and rolling sphere radius as in the following examples:


    Suppose that a lightning protection system to provide LPL I such that 99% of all lightning flashes are intercepted (all those of 3 kA or greater). There is only a 1% probability that lightning may be smaller than the 3 kA minimum, and may not be close enough to an air-terminal to be intercepted. It should be noted that flashes of less than 3 kA are rare, and typically would not be expected to cause damage to the structure. Protection greater than LPL I (99%) would require significantly more material, is not covered by the standard and generally is not required for commercial construction.


    The lower lightning protection levels (LPL II, III & IV) each increase the air-terminal spacing, reducing their ability to capture smaller lightning flashes, thus reducing overall the percentage of lightning events they can protect against.


    Suppose that a lightning protection system to provide LPL IV, designed using the rolling sphere method, would use air-terminals placed using a rolling sphere radius of 60 m.
    These air-terminals would be positioned such that they would capture all lightning flashes of 16 kA or greater, thus offering protection to at least 84% of the lightning (the term “at least” is used to indicate that the percentage of lightning captured might be greater, since smaller lightning flashes could be captured if they were closer to the air-terminal). 


    To offer a greater lightning protection level (e.g. LPL I, II or III) a smaller rolling sphere radius would be used. This would result in a reduced spacing between air-terminals (more air-terminals), thus positioning the air-terminals to capture smaller lightning flashes, and increasing the total percentage of lightning flashes captured.

    1.3 The Rolling Sphere Method Protection Applications

    The rolling sphere methods can be used for the following applications:

    1. Rolling sphere method with rod air-terminations,
    2. Rolling sphere method and mesh/catenary conductors,
    3. Rolling sphere method and Tall structures.

    1.3.1 Rolling Sphere Method With Rod Air-Terminations

    When rods are to be used as the air-termination for the protection of plane surfaces (see Fig.3), the following formula can be used:

    d = 2 √ (2rh – h2)


    d = distance between two rods (m)
    r = radius of the rolling sphere (m)
    h = height of the rods (m)

    Fig.3: Rolling Sphere Method With Rod Air-Terminations

    The following Table#2 shows some examples of rolling sphere protection distance (distance between Air terminals) according to the Air terminals height and the Rolling Sphere Radius according to lightning protection level LPL.


    When rods are to be used as the air-termination for protection of roof top items/structures (see Fig.4) and The arrangement of the air-termination rods, over which no cable is normally spanned, means that the sphere does not “roll on rails” but “sits deeper” instead, thus increasing the penetration depth () of the sphere. In this case the following formula of sphere penetration distance can be used:

    p = r – √ (r2 –d2/4)


    p = penetration distance (m)( part of the sphere below the horizontal lines between top of air terminals)
    r = radius of the rolling sphere (m)
    d = Distance between two air-termination rods or two parallel air-termination conductors  (m)

     Fig.4: Penetration distance of rolling sphere

    The following Table#3 shows Rolling sphere penetration distance according to the distance between Air rods and the Rolling Sphere Radius according to lightning protection level LPL.



    The height of the air-termination rods h should always be greater than the value of the penetration depth p determined to ensure that the rolling sphere does not touch the structure to be protected.

    1.3.2 Rolling Sphere Method And Mesh/Catenary Conductors

    • Where the rolling sphere method is to be used to evaluate the protection provided by mesh conductors or network of catenary wires, the mesh must be mounted at some distance above the roof (see Fig.5), to ensure the rolling sphere does not touch its surface in a similar way to the catenary conductors.

    Fig.5: Rolling Sphere Method And Mesh Conductors

    • Also, As with a free standing mast, catenary conductors can be used to keep the rolling sphere away from the structure to be protected (see Fig.6). One or more catenary conductors may be utilised to ensure that the sphere does not come into contact with any part of the structure’s roof.

    Fig.6: Rolling Sphere Method And Catenary Conductors

    • If the system is required to be isolated from the structure then a conductor suspended between two free standing masts may be employed. This arrangement is suitable for small sensitive structures such as explosive stores.
    • In a non isolated system, a catenary conductor may be used to protect larger items of roof mounted equipment from a direct strike (see Fig.7).

    Fig.7: catenary conductors used to protect larger
    items of roof mounted equipment

    • The two formulas in the case of rod air-terminations can be used also in case of using mesh/catenary conductors. The distance/height of the mesh/catenary replaces the rod distance/height. As in fig.4 Note that the distance for penetration or protection distance is the diagonal of the grid (distance between points A & B).

    1.3.3 Rolling Sphere Method And Tall Structures

    Research shows that it is the upper 20% of the Tall structure that is most vulnerable to side strikes and potential damage (see Fig.8).

    Fig.8: Rolling Sphere Method And Tall Structures

    Case#1: Buildings Above 60 m High

    In the IEC standards, for buildings above 60 m, 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.

    Case#2: 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.

    Case#3: Buildings Taller Than 120 m High

    For structures taller than 120 m, the standard recommends that all parts above 120 m 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.

    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.

    1.4 How To Apply The Rolling Sphere Method for
    Lightning Protection Design?

    The basic concept of applying the rolling sphere to a structure is as follows:

    Step#1: Scale The building / structure to be protected (e.g. on a scale of 1:100) (see Fig.9) Depending on the location of the building under design, it is also necessary to include the surrounding structures and objects with the same scale of the building, since these could act as “natural protective measures” for the building under design.

    Fig.9: Scaled Building and Scaled Rolling Sphere of LPL I

    Step#2:  calculate The radius of the sphere which must be equal to the striking distance associated with the minimum current level for the chosen lightning protection level.

    Step#3: Scale the radius r of the “rolling sphere” calculated from Step#2 with the same scale of the building (see Fig.9). (For example, if the building with scale 1:100, from Table#1 for a lightning protection levels I, the rolling sphere radius will be 20 cm and for LPL II will be 30 cm and for LPL III will be 45 cm).

    Step#4: Make a circular path around the building under design with distance apart equal to the scaled rolling sphere radius (see Fig.10). This circular path will terminate on the corner of the building.

    Fig.10: Circular path around the building

    Step#5: Roll an imaginary sphere over the surface of the structure in all directions (see Fig.11).

    Fig.11: imaginary sphere rolled over the surface
    of the structure 
    in all directions

    Note: the rolling process of the imaginary sphere is controlled by the distance between Air terminals as given in part#3 in this Article i.e. each roll is far from the previous one by the allowable distance between air terminals calculated from part#3.

    Step#6: Where the sphere touches the building, A lightning protection would be needed by placing Air Terminal. Using the same logic, the areas where the sphere does not touch the Building (see shaded area in Fig.11) would be deemed to be protected and would not require protection.

    Note: Generally a lightning protection system is designed such that the rolling sphere only touches the lightning protection system and not the structure i.e. The air termination system is placed such that the sphere only touches the air-terminations, and not the structure.

    In the next Article, I will explain other Positioning Methods for Air Termination system: The Protective Angle Method (PAM) and The Mesh Method. Please, keep following.