In Article " Design Process for Lightning Protection Systems ", I indicated the (3) phases of the Design Process for Lightning Protection Systems as follows:
Design Process For Lightning Protection Systems
The design process of lightning protection systems is commonly broken into discrete phases, allowing the lightning protection designer to present an integrated design package. These phases can be listed as follows:
A Quality assurance is required in each phase in above.

Introduction To Design Calculations Of Lightning Protection Systems
It is very important before explaining the design calculations of lightning protection systems to highlight some important topics or expressions that will be used in these calculations. These topics can be listed as follows:

And in Article " Design Calculations of Lightning Protection Systems – Part Two ", I explained the following:
Design Calculations of Lightning Protection Systems – Continued
Third: Detailed Design Phase

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

Methods Of Calculations
For Risk Assessment Study
The risk assessment study can be done by (4) different
methods as follows:
1 Manual Method (equations and tables method),which will be
explained as per:
2Software Method,
3 Excel Sheets Method,
4Online Calculators Method.

First: Manual Method (Equations
And Tables Method) as per IEC 623052

The Manual Method (Equations and Tables Method) for Calculations of Risk Assessment Study as per IEC 623052 can be reviewed in the following Articles:
Today, I will explain The Manual Method (Equations and Tables Method) for Calculations of Risk Assessment Study as per NFPA780.
First: Manual Method (Equations
And Tables Method) as per NFPA780

Procedure For Performing The Risk Assessment Study By Manual Method
Procedure for performing the risk assessment
study includes three parts as follows:

General notes for the Risk Assessment
Study as per NFPA780

Part#1: Evaluating Need For Lightning
Protection
To evaluate the need
for lightning protection, We have two methods to perform this
as per NFPA 780, which are:

Method#1: The Simplified Risk Assessment

Method#1: The Simplified Risk Assessment
The simplified risk
assessment method includes the following steps:

Step#22: Determine the Value of Lightning Flash
Density (N_{g})
Lightning flash
density, the yearly number of flashes to ground per square kilometer, can be
found from Fig.4.
Note:

Step#23: Calculate Annual Threat of
Occurrence (N_{d})
The yearly annual
threat of occurrence (lightning strike frequency) (Nd) to a structure is
determined by the following equation:
N_{d}=(N_{g})(A_{e})(C_{1})( 10^{6}) potential events/yr
Where:
The location factor C_{1}
accounts for the topography of the site of the structure and any objects
located within the distance 3H from the structure
that can affect the collection area. Location factors are given in Table1.
Table1: Location Factor C_{1}

Step#24: Calculate the Tolerable Lightning Frequency (N_{c})
The tolerable
lightning frequency (N_{c}) is a measure of the risk of
damage to the structure, including factors affecting risks to the structure,
to the contents, and of environmental loss.
The tolerable
lightning frequency (N_{c}) is calculated by dividing the
acceptable frequency of property losses by various coefficients relating to
the structure, the contents, and the consequence of damage.
The tolerable
lightning frequency is expressed by the following formula:
N_{c} = default value of tolerable frequency of
property losses / The coefficient C
N_{c} = (1.5×10−3) / C events/yr
Where:
The default value of
tolerable frequency of property losses is 1.5 x 10–3.
The coefficient (C) is the product of
structural coefficients C_{2 }through C_{5} and it is expressed
by the following formula:
C = (C_{2})(C_{3})(C_{4})(C_{5})
The structural
coefficients C_{2} through C_{5} are obtained from
Table2 through Table5 as follows:
Table2: Determination of Construction Coefficient C_{2}
Table3: Determination of Structure Contents Coefficient
C_{3}
Table4: Determination of Structure Occupancy Coefficient C_{4}
Table5: Determination of Lightning Consequence
Coefficient C_{5}

Step#25: Comparing the Annual Threat of Occurrence (N_{d}) to the Tolerable
Lightning Frequency (N_{c})
The result of this
comparison is used to decide if a lightning protection system is needed or
not. So, we have two cases:

Procedure for Method#1: Simplified Risk Calculation
Table6 provides a
simple method of calculating and using the simplified Risk assessment method
described in above.
Table6: Procedure for Method#1: Simplified Risk
Calculation

Method#2: The Detailed Risk Assessment

Method#2: The detailed Risk assessment
The detailed Risk assessment Method
includes the following steps:

Step#21: Identify The Structure to be Protected
The procedure for
the risk assessment is to first define the extent of the facility or
structure being assessed, The structure to be protected includes:
One must then determine all relevant physical,
environmental, and service installation factors applicable to the structure.

Step#22: for each Loss to be considered, identify the
Tolerable Level Of Risk RT
Values of tolerable
levels of loss could be selected by the owner, the owner’s representative, or
the authority having jurisdiction. Default values that can be used where
risk levels are not provided by other sources are included in Table
The tolerable risk is
expressed in the form of number of events per year and is given in
engineering units (e.g. 10x) as in the following table which includes the
values of RT from different Standards and codes.
Notes:

Step#23: identify the Types of Risk Due to
Lightning (Rn)
The following risks have
been identified, corresponding to their equivalent type of loss:
Hereafter the primary
risks will be referred to collectively as Rn where the subscript n indicates 1, 2 or
3 as described above.

Step#24: For each type of Risk to be considered ,
identify and calculate the risk components Rx that make up Primary risk Rn
Step#24 includes
two main parts as follows:
First: identification of
the Risk Components Rx
These risk
categories are composed of risk components that are summed to determine the
overall risk of the loss in a given application. The risk components are
categorized according to the type of loss and source of the damage ( see Fig.5)as
follows:
1 Risk Components due to Direct Strikes to a Structure
includes:
2 Risk Components due to Strikes near a Structure includes:
3 Risk Components due to Strike to a Service Connected to a Structure
includes:
4 Risk Components due to Strikes Near a Service Connected to
the Structure includes:

In the next Article, I will continue explaining the steps of Method#2: The Detailed Risk Assessment as per NFPA 780. Please, keep following.
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