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
Explained in Article " Design Calculations of Lightning Protection Systems – Part Two "

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 623052
 
First: Manual Method (Equations And Tables Method) as per NFPA 780
 
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
Explained in Article " Design Calculations of Lightning Protection Systems – Part Fifteen "

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 623053 and Based on BS EN 623053,
 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:
 The Rolling Sphere Method (RSM),
 The Protective Angle Method (PAM),
 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)

1.2 Relation between Lightning Protection Levels and
Rolling Sphere Radius
The below Table#1
indicates the following:
Also, The above Table#1
explains the relation between Lightning protection levels and rolling sphere
radius as in the following examples:
Example#1:
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 airterminal 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.
Result:
The lower lightning
protection levels (LPL II, III & IV) each increase the airterminal
spacing, reducing their ability to capture smaller lightning flashes, thus
reducing overall the percentage of lightning events they can protect against.
Example#2:
Suppose that a lightning
protection system to provide LPL IV, designed using the rolling sphere
method, would use airterminals placed using a rolling sphere radius of 60 m.
These airterminals
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 airterminal).
Result:
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 airterminals
(more airterminals), thus positioning the airterminals 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:

1.3.1 Rolling Sphere Method With Rod AirTerminations
When rods are to be
used as the airtermination for the protection of plane surfaces (see Fig.3),
the following formula can be used:
d = 2 √ (2rh – h^{2})
Where:
d = distance between
two rods (m)
r = radius of the
rolling sphere (m)
h = height of the rods
(m)
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 airtermination for protection of roof top items/structures (see
Fig.4) and The arrangement of the airtermination 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 – √ (r^{2} –d^{2}/4)
Where:
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
airtermination rods or two parallel airtermination conductors (m)
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.
Note:
The height of the
airtermination 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

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