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.
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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:
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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
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The lightning protection design process involves a number of design steps as in Fig.1.
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Step#1: Characteristics of the Structure to Be Protected
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Step#2: Risk Assessment Study
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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:
2-Software Method,
3- Excel Sheets Method,
4-Online Calculators Method.
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First: Manual Method (Equations And Tables Method) as per IEC 62305-2
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The Manual Method (Equations and Tables Method) for Calculations of Risk Assessment Study as per IEC 62305-2 can be reviewed in the following Articles:
First: Manual Method (Equations
And Tables Method) as per NFPA 780
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In Article " Design Calculations of Lightning Protection Systems – Part Seven ", I indicated that:
To evaluate the need
for lightning protection, We have two methods to perform this
as per NFPA 780, which are:
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In this Article, I explained Method#1: The Simplified Risk Assessment and Some Steps from Method#2: The Detailed Risk Assessment.
Today, I will continue explaining the Steps of Method#2: The Detailed Risk Assessment as per NFPA-780.
In the next Article, I will continue explaining the steps of Method#2: The Detailed Risk Assessment as per NFPA 780. Please, keep following.
Today, I will continue explaining the Steps of Method#2: The Detailed Risk Assessment as per NFPA-780.
First: Manual Method (Equations
And Tables Method) as per NFPA-780 - Continued
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Method#2: The detailed Risk assessment
The detailed Risk assessment Method includes the following steps:
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Step#2-4: For each type of Risk to be considered,
identify and calculate the risk components Rx that make up Primary risk Rn
Step#2-4 includes
two main parts as follows:
I explained the first part: Identification
of the Risk Components Rx in Article " Design Calculations of Lightning Protection Systems – Part Seven ".
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Second Part: Calculations of the Risk Components Rx
Each component of
risk Rx
depends
on (3) parameters as follows:
The value of each
component of risk Rx can be calculated using the
following expression:
RX = NX x PX x LX
Where:
Nx = Number of
Lightning Strikes affecting the Structure or Service
Px = Probability Of
Damage
Lx = Loss Factor
Specific formulas
for the calculation of the risk components are given in Table-1.
Table-1: Risk Components Formulas
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Calculations of First
Parameter: NX = The average annual
threat of occurrence
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1- Annual Threat of Occurrence resulting from a
direct strike to a structure (Nd)
The calculation of
the annual threat of occurrence resulting from a direct strike to a structure
(Nd) is calculated as
in Step#2-3: Calculate Annual Threat of Occurrence (Nd) of method#1: the
simplified risk assessment In Article " Design Calculations of Lightning Protection Systems – Part Seven ".
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2- The annual threat of occurrence due to strikes near
a structure (NM)
The annual threat of
occurrence due to strikes near a structure (NM) is given by the
following equation:
NM = Ng (Am – Ae)(C1)
10-6 events/yr
Where:
Ng = lightning ground
flash density in flashes/km2/year
Am = collection area of
flashes near the structure (m2)
Ae = equivalent
collection area of the structure (m2)
C1 = environmental
coefficient
Notes:
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3- The annual threat of occurrence due to a strike to
an incoming service (NL)
The annual threat of
occurrence due to a strike to an incoming service (NL) is characterized
by the following formula:
NL = Ng Al C1 Ct 10-6 events/yr
Where:
Ng = lightning ground
flash density in flashes/km2/year
Al = collection area of
flashes striking the service (m2) (see Table-2)
C1 = environmental
coefficient of the incoming service
Ct = correction factor
for the presence of an HV/LV transformer located between the point of strike
and the structure
Table-2: Values of Collection Areas Al and Ai
Notes:
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4- The annual threat of occurrence due to flashes to an
adjacent structure (Nda)
The annual threat of
occurrence due to flashes to an adjacent structure (Nda) can be estimated
by using the following equation:
Nda = Ng Ae C1 Ct 10-6 events/yr
Where:
Ng = lightning ground
flash density in flashes/km2/year
Ae = equivalent
collection area of the adjacent structure
C1 = environmental
coefficient
Ct = correction factor
for the presence of an HV/LV transformer located between the point of strike
and the structure
Notes:
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5- The annual threat of occurrence due to flashes near
a service (NI)
The annual threat of occurrence due to
flashes near a service (NI) can be estimated by using the
following equation:
NI = Ng Ai Ce Ct 10-6 events/yr
Where:
Ng = lightning ground
flash density in flashes/km2/year
Ai = equivalent
collection area of flashes to ground near the service (m2) (see Table)
Ce = service
environmental coefficient (see Table-3)
Ct = correction factor
for the presence of an HV/LV transformer located between the point of strike
and the structure
Table-3: Service Environmental Coefficient Ce
Notes:
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Calculations of Second
Parameter: PX = Probabilities of
Damage
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1- The Probability Of Injury (PA)
The factors
associated with the probability of injury (PA) due to a direct strike
to a structure are primarily related to touch and step potentials. Default
values for (PA) are given in Table-4:
Table-4: Values of Probability (PA) That a Flash to a
Structure Will Cause Shock to Living Beings Due to Dangerous Touch and Step
Voltages
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2- The Probability Of Physical Damage (PB)
The factors
associated with the probability of physical damage (PB) due to a direct
strike to a structure are primarily related to the type of protection
provided. Default values for (PB) are given in Table-5:
Table-5: Values of Probability (PB) of Physical Damage
to a Structure Due to Flashes to the Structure
Note:
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3- The Probability of failure of internal systems due
to a direct strike (PC)
The factors
associated with the probability of failure of internal systems due to a
direct strike (PC) are primarily related to the
surge protection measures provided. Default values for PC are given in Table-6:
Table-6: Values of Probability (PC) as a Function of
SPD Protection Provided
Notes:
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4- The Probability that a strike near a structure will
cause failure of internal systems (PM)
Table-7: Values of Probability (PM) as a Function of KS
KS = KS1 x KS2 x KS3 x KS4
Where:
KS1 = factor relating to
the shielding effectiveness of the structure, lightning protection system, or
other shields at the exterior boundary of the structure
KS2 = factor relating to
the shielding effectiveness of shields internal to the structure
KS3 = factor relating
to the characteristics of the internal wiring
KS4 = factor relating
to the withstand voltage of the system to be protected
Values of KS1 and KS2:
For continuous metal
shields with a thickness of 0.1 to 0.5 mm, KS1 and KS2 should be assigned
the value of 10-4 to 10-5 (scaled linearly). Where not
otherwise known, the value of KS1 and KS2 can be evaluated by
the following relationship as long as the equipment is located a distance, w from the boundary
shield:
KS1 = KS2 = 0.12 w
Where:
w = distance measured
in meters and given by a mesh grid spacing, the spacing between down
conductors, or the spacing between structural steel columns.
Values of KS3:
Table-8 provides values
which can be selected for factor KS3 based on the
configuration of internal wiring. For wiring contained in continuous metallic
conduit that is properly bonded to the lightning protection grounding system,
the selected value of KS3 from the table is
multiplied by a factor of 0.1.
Table-8: Values of Factor (KS3) as a Function of
Internal Wiring
Note:
Values of KS4:
The value of factor KS4 is evaluated by the
following formula:
KS4 = 1.5/ UW
Where:
UW = lowest withstand
voltage of the hardware in the system under consideration.
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5- The Probability, PU, that a lightning
flash will result in injury to living beings
Table-9: Values of the Probability (PU) as a Function of
the Resistance of the Cable Shield and the Impulse Withstand Voltage (Uw) of the Equipment
Notes:
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6- The Probability of physical damage due to a strike
to a service entering a structure (PV)
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7- The Probability of a failure of internal systems due
to a strike to a service entering a structure (PW)
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8- The Probability of a failure of internal systems due
to a strike near a service entering the structure under consideration (PZ)
Table-10: Values of the Probability (PZ) as a Function of
the Resistance of the Cable Shield and the Impulse Withstand Voltage (Uw) of the Equipment
Note:
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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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