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:

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

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

And In Article " Design Calculations of Lightning Protection Systems – Part Three ", I explained the following:
 Step#21: Identify the structure to be protected,
 Step#22: Identify the types of loss relevant to the structure to be protected Rn,
 Step#23: For each loss to be considered, identify the tolerable level of risk RT,
 Step#24 First Part: Identification of the Risk Components Rx.
Also, in Article " Design Calculations of Lightning Protection Systems – Part Four ", I explained Step#24 Second Part: Calculations of the Risk Components Rx  Calculation of the first Parameter Nx = Number of dangerous events per year and I indicated that:
Each of the risk
components Rx is obtained using further calculations, subcalculations and
reference tables based on the general equation:
R_{X} = N_{X} x P_{X} x L_{X}
Where
N_{X} = number of dangerous
events per year,
P_{X} = probability of
damage to structure,
L_{X} = amount of consequent loss.
X = A, B, ...
So, The task
of the risk assessment therefore involves the determination of the three
parameters N_{X}, P_{X} and L_{X}.

Today, I will explain how to calculate the second Parameter: PX = probability of damage to structure.
Step#24 Second Part: Calculations of the Risk Components Rx

Calculations of second
Parameter: PX = probability of damage to
structure

Probability of damage
to structure P_{X}
The damage probability
parameter gives the probability that a supposed lightning strike will cause a
quite specific type of damage. It is therefore assumed that there is a
lightning strike on the relevant area; the value of the damage probability
can then have a maximum value of (1). We differentiate between the following
eight damage probabilities in Table1:
Table1: The Eight Damage Probabilities

1 Probability P_{A} that a flash to a structure
will cause injury to living beings by electric shock
The values of probability P_{A }of shock to living beings due to
touch and step voltage by a lightning flash to the structure, depend on the
adopted LPS and on additional protection measures provided:
P_{A} = P_{TA} x P_{B}
Where:
Table2: Values of probability P_{TA} that a flash to a structure
will cause shock to living beings due to dangerous touch and step voltages
Notes:

2 Probability P_{B} that a flash to a structure
will cause physical damage
The values of probability P_{B} of physical damage by a flash to
a structure, as a function of lightning protection level (LPL) are given in Table3.
Table3: Values of probability P_{B} depending on the protection
measures to reduce physical damage
Notes:

3 Probability P_{C} that a flash to a structure
will cause failure of internal systems
The probability P_{C} that a flash to a structure will
cause a failure of internal systems is given by:
P_{C} = P_{SPD} x C_{LD}
Where:
Table4: Value of the probability P_{SPD} as a function of LPL for
which SPDs are designed
Notes:
Table5: Values of factors C_{LD} and C_{LI }depending on shielding,
grounding and isolation conditions
Notes:

4 Probability P_{M} that a flash near a
structure will cause failure of internal systems
P_{M} = P_{SPD} x P_{MS}
Notes:
The values of PMS are obtained from the product:
P_{MS} = (K_{S1} x K_{S2} x K_{S3} x K_{S4})^{2}
Where:
Note:
Inside an LPZ, at a safety
distance from the boundary screen at least equal to the mesh width w_{m}, factors K_{S1} and K_{S2} for LPS or spatial gridlike
shields may be evaluated as:
K_{S1} = 0,12 x w_{m1}
K_{S2 }= 0,12 x w_{m2}
Where:
w_{m1} (m) and w_{m2} (m) are the mesh widths of
gridlike spatial shields, or of mesh type LPS downconductors or the spacing
between the structure metal columns, or the spacing between a reinforced
concrete framework acting as a natural LPS.
Notes:
Table6: Value of factor K_{S3} depending on internal
wiring
The factor K_{S4 }is evaluated as:
K_{S4} = 1/U_{W}
Where:
U_{w} is the rated impulse withstand
voltage of system to be protected, in kV.
Notes:

5 Probability P_{U} that a flash to a line will
cause injury to living beings by electric shock
The values of probability P_{U} of injury to living beings
inside the structure due to touch voltage by a flash to a line entering the
structure depends on the characteristics of the line shield, the impulse
withstand voltage of internal systems connected to the line, the protection
measures like physical restrictions or warning notices and the isolating
interfaces or SPD(s) provided for equipotential bonding at the entrance of
the line according to IEC 623053.
Note:
A coordinated SPD system
according to IEC 623054 is not necessary to reduce P_{U}; in this case SPD(s) according
to IEC 623053 are sufficient.
The value of P_{U} is given by:
P_{U} = P_{TU} x P_{EB} x P_{LD} x C_{LD}
Where:
Note:
When SPD(s) according to IEC
623053 are provided for equipotential bonding at the entrance of the line,
earthing and bonding according to IEC 623054 may improve protection.
Table7: Values of probability P_{TU} that a flash to an entering
line will cause shock to living beings due to dangerous touch voltages
Note:
If more than one provision has
been taken, the value of P_{TU} is the product of the corresponding values.
Table8: Value of the probability P_{EB} as a function of LPL for
which SPDs are designed
Note:
The values of P_{EB} may be reduced for SPDs having
better protection characteristics (higher nominal current I_{N}, lower protective level U_{P}, etc.) compared with the
requirements defined for LPL I at the relevant installation locations.
Table9: Values of the probability P_{LD} depending on the resistance
R_{S} of the cable screen and the
impulse withstand voltage U_{W }of the equipment
Note:
In suburban/urban areas, an LV
power line uses typically unshielded buried cable whereas a telecommunication
line uses a buried shielded cable (with a minimum of 20 conductors, a shield
resistance of 5 Ω/km, a copper wire diameter of 0,6 mm). In rural areas an LV
power line uses an unshielded aerial cable whereas a telecommunication line
uses an aerial unshielded cable (copper wire diameter: 1 mm). An HV buried
power line uses typically a shielded cable with a shield resistance in the
order of 1Ω/km to 5 Ω/km. National committees may improve this information in
order to better meet national conditions of power and telecommunication
lines.

6 Probability P_{V} that a flash to a line will
cause physical damage
The values of probability P_{V }of physical damage by a flash to
a line entering the structure depend on the characteristics of the line
shield, the impulse withstand voltage of internal systems connected to the
line and the isolating interfaces or the SPDs provided for equipotential bonding
at the entrance of the line according to IEC 623053.
Note:
A coordinated SPD system
according to IEC 623054 is not necessary to reduce P_{V}; in this case, SPDs according to
IEC 623053 are sufficient.
The value of P_{V} is given by:
P_{V} = P_{EB }x P_{LD} x C_{LD}
Where:

7 Probability P_{W} that a flash to a line will
cause failure of internal systems
The values of probability P_{W} that a flash to a line entering
the structure will cause a failure of internal systems depend on the
characteristics of line shielding, the impulse withstand voltage of internal
systems connected to the line and the isolating interfaces or the coordinated
SPD system installed.
The value of P_{W} is given by:
P_{W} = P_{SPD} x P_{LD} x C_{LD}
Where:

8 Probability P_{Z} that a lightning flash near
an incoming line will cause failure of internal systems
The values of probability P_{Z} that a lightning flash near a
line entering the structure will cause a failure of internal systems depend
on the characteristics of the line shield, the impulse withstand voltage of
the system connected to the line and the isolating interfaces or the
coordinated SPD system provided.
The value of P_{Z} is given by:
P_{Z} = P_{SPD} x P_{LI} x C_{LI}
Where:
Table10: Values of the probability P_{LI} depending on the line type
and the impulse withstand voltage U_{W} of the equipment

The Probability P of damage in a structure with zones
ZS
For the evaluation of risk
components and the selection of the relevant parameters involved, the
following rules apply for the probability P of damage:
P_{C} = 1 – (1 – P_{C1}) x (1 – P_{C2}) x (1 – P_{C3})
P_{M} = 1 – (1 – P_{M1}) x (1 – P_{M2}) x (1 – P_{M3})
Where:
P_{Ci}, and P_{Mi} are parameters relevant to
internal system i =1, 2, 3,…
With the exception made for P_{C} and P_{M}, if more than one value of any
other parameter exists in a zone, the value of the parameter leading to the
highest value of risk is to be assumed.

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