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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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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Fig.1: The Lightning Protection Design Process
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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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Procedure For Performing The Risk Assessment Study By Manual Method
Procedure for performing the risk assessment
study includes three parts as follows:
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Part#1: Evaluating Need For Lightning
Protection
To evaluate the need
for lightning protection, the following steps need to be carried out a
follows:
Step#2-1: Identify the structure to
be protected.
Step#2-2: Identify the types of
loss relevant to the structure to be protected Rn, where:
R1 risk of loss of
human life,
R2 risk of loss of
services to the public,
R3 risk of loss of
cultural heritage.
Step#2-3: For each loss to be
considered, identify the tolerable level of risk RT (tolerable means still
acceptable).
Step#2-4: For each type of
loss to be considered , identify and calculate the risk components Rx that
make up risk Rn which are: RA, RB, RC, RM, RU, RV, RW, RZ.
Step#2-5: Calculate Rn = Σ Rx
Step#2-6: Comparing the
calculated actual risk Rn of each loss to a tolerable level of risk (RT),
then we have (2) cases:
Case#1: If the calculated
risk Rn is equal or less than the respective tolerable risk RT i.e.
Rn ≤ RT , then Structure is adequately protected for this type of loss and
no lightning
protection is required for this type of loss,
Case#2: If the calculated
risk Rn is higher than the tolerable risk RT i.e. Rn > RT, then Install lightning
protection measures in order to reduce Rn.
Step#2-7: go back to step#2-4
and make a series of trial and error calculations until the risk Rn is
reduced below that of RT (Rn ≤ RT).
Note:
In cases where the
risk cannot be reduced to a tolerable level, the site owner should be
informed and the highest level of protection provided to the installation.
The following flow
diagram in Fig.2 shows this procedure for evaluating Need for lightning
protection.
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And In Article " Design Calculations of Lightning Protection Systems – Part Three ", I explained the following:
- Step#2-1: Identify the structure to be protected,
- Step#2-2: Identify the types of loss relevant to the structure to be protected Rn,
- Step#2-3: For each loss to be considered, identify the tolerable level of risk RT,
- Step#2-4 First Part: Identification of the Risk Components Rx.
Today, I will explain Step#2-4 Second Part: Calculations of the Risk Components Rx.
Step#2-4: For each type of loss to be considered ,
identify and calculate the risk components Rx that make up Primary risk Rn
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Second Part: Calculations of the Risk Components Rx
Each of the risk
components Rx ( see Fig.3) is obtained using further calculations,
sub-calculations and reference tables based on the general equation:
RX = NX x PX x LX
Where
NX = number of dangerous
events per year,
PX = probability of
damage to structure,
LX = amount of consequent loss.
X = A, B, ...
1- The number NX of dangerous events:
2- The probability of damage PX:
3- The consequent loss LX:
For evaluation of risk components
RX related to lightning flashes to
the structure and based on the general equation, the following relationships
for each Risk Component are illustrated in the following Table-1 with their
corresponding source and type of damage:
Table-1: Risk Component Equations
Note: In many cases NDJ may be neglected.
So, The task
of the risk assessment therefore involves the determination of the three
parameters N, P and L for all relevant risk components. This involves
establishing and determining of many Other individual parameters for each
Risk Components and this will be done in next Articles.
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1- Calculations Of Number Of Dangerous Events Per Year
NX
Number of dangerous
events per year NX depends on:
To calculate the number N use the following general
equation:
NX = NG x AD x Correction Factors
Where:
NG: The lightning ground flash
density which is the number of lightning flashes per km2 per year.
AD: An equivalent collection area
of the structure, taking into account correction factors for the structure’s
physical characteristics.
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1.1 Calculation Of The Lightning Ground Flash Density NG
The value of NG is available from
ground flash location networks Map in many areas of the world (see Fig.4).
Note:
If a map of NG is not available, in temperate
regions it may be estimated by:
NG = 0,1 TD
Where TD is the thunderstorm days per
year (which can be obtained from isokeraunic maps in Fig.5).
Note:
The NG Map and isokeraunic maps
may vary each year and you must get the most recent one as possible for
better results.
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1.2 Determination of the collection area AD
Case#1: For Isolated Structures On Flat
Ground
The collection area AD is the area defined by the
intersection between the ground surface and a straight line with 1/3 slope
which passes from the upper parts of the structure (touching it there) and
rotating around it. Determination of the value of AD may be performed graphically or
mathematically as will be seen in sub-cases A and B.
A- Rectangular structure
For an isolated rectangular
structure with length L, width W, and height H on flat ground, the collection area is then equal to:
AD = L x W + 2 x (3 x H) x (L + W) + Π x (3 x H)2
Where L, W and H are expressed in meters (see Fig.6).
B- Complex shaped structure
If the structure has a complex
shape such as elevated roof protrusions (see Fig.7), a graphical
method should be used to evaluate AD (see Fig.8).
An acceptable approximate value
of the collection area is the greater between the collection area ADMIN evaluated with above Equation
(in case of Rectangular structure) taking the minimum height HMIN of the structure, and the
collection area attributed to the elevated roof protrusion AD’. AD’ may be calculated by:
AD’ = Π x (3 ×HP)2
Where HP is the height of protrusion.
Case#2: Structure as a Part of
a Building
Where the structure S to be
considered consists of only a part of a building B, the dimensions of structure S may
be used in evaluation of AD provided that the following conditions are fulfilled (see Fig.9):
Note:
Where these conditions are not
fulfilled, the dimensions of the whole building B should be used.
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1.3 The Correction Factors
The Correction Factors
include the following:
A- The Location Factor CD (Relative Location Of The
Structure)
The relative location of the
structure, compensating for surrounding structures or an exposed location,
will be taken into account by a location factor CD (see Table-2).
A more precise evaluation of the
surrounding objects' influence can be obtained considering the relative
height of the structure with respect to the surrounding objects or the ground
within a distance of 3 x H from the structure and assuming CD = 1.
Table-2: Structure location factor CD
B- Line Installation Factor CI (see Table-3)
Table-3: Line Installation Factor CI
C- Line Type Factor CT (see Table-4)
Table-4: Line Type Factor CT
D- Line Environmental Factor
CE (see Table-5)
Table-5: Line Environmental Factor CE
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2- Calculation of the average annual number of dangerous events
ND due to flashes to a
structure and NDJ to an adjacent structure
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2.1 Number of dangerous events ND for the structure
ND may be evaluated as the
product:
ND = NG x AD x CD x 10–6
Where:
NG is the lightning ground flash
density (1/km2 x year);
AD is the collection area of the
structure (m2) (see Fig.10);
CD is the location factor of the
structure (see Table-2).
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2.2 Number of dangerous events NDJ for an adjacent structure
The average annual number of
dangerous events due to flashes to a structure connected at the far end of a
line, NDJ (see Table-1 and Fig.10) may be
evaluated as the product:
NDJ = NG x ADJ x CDJ x CT ×10–6
Where:
NG is the lightning ground flash
density (1/km2 x year);
ADJ is the collection area of the
adjacent structure (m2) (see Fig.10);
CDJ is the location factor of the
adjacent structure (see Table-2);
CT is the line type factor (see Table-4);
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2.3 Calculation of the average annual number of dangerous
events NM due to flashes near a structure
NM may be evaluated as the
product:
NM = NG x AM x 10-6
Where:
NG is the lightning ground flash
density (1/km2 x year);
AM is the collection area of flashes
striking near the structure (m2).
The collection area AM extends to a line located at a
distance of 500 m from the perimeter of the structure (see Fig.10):
AM = 2 x 500 x (L + W) + Π x 5002
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2.4 Calculation of the average annual number of dangerous
events NL due to flashes to a line
A line may consist of several
sections. For each section of line, the value of NL may be evaluated by:
NL = NG x AL x CI x CE x CT x 10–6
Where:
NL is the number of overvoltages of
amplitude not lower than 1 kV (1/year) on the line section)
NG is the lightning ground flash
density (1/km2 x year);
AL is the collection area of
flashes striking the line (m2) (see Fig.10);
CI is the installation factor of
the line (see Table-3);
CT is the line type factor (see Table-4);
CE is the environmental factor (see
Table-5);
With the collection area for
flashes to a line:
AL = 40 x LL
LL is the length of the line
section (m).
Where the length of a line
section is unknown, LL = 1 000 m is to be assumed.
Notes:
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2.5 Calculation of average annual number of dangerous events NI due to flashes near a line
A line may consist of several
sections. For each section of line, the value of NI may be evaluated by
NI = NG x AI x CI x CE x CT x 10–6
Where:
NI is the number of overvoltages of
amplitude not lower than 1 kV (1/year) on the line section;
NG is the lightning ground flash
density (1/km2 x year);
AI is the collection area of
flashes to ground near the line (m2) (see Fig.10);
CI is the installation factor (see Table-3);
CT is the line type factor (see Table-4);
CE is the environmental factor (see
Table-5).
With the collection area for
flashes near a line
AI = 4 000 x LL
Where LL is the length of the line
section (m).
Where the length of a line
section is unknown, LL =1 000 m is to be assumed.
Notes:
National committees can improve
this information in order to better meet national conditions of power and
telecommunication lines.
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