Design Calculations of Lightning Protection Systems – Part Five


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:

  1. Planning phase,
  2. Consultation phase,
  3. Detailed Design phase.


A Quality assurance is required in each phase in above.




Also, in Article Design Calculations of Lightning Protection Systems – Part One ", I explained an Introduction to design calculations of lightning protection systems as follows:




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:

  1. Sources and Types of Damage to a Structure,
  2. Types of Loss,
  3. Types of Risks Associated with Losses,
  4. Lightning Protection Levels (LPL),
  5. Lightning Protection Zones (LPZ),
  6. Class of LPS,
  7. Protection Measures.




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 as per IEC 62305



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:
  • IEC 62305-2,
  • NFPA780.

2-Software Method,
3- Excel Sheets Method,
4-Online Calculators Method.





First: Manual Method (Equations And Tables Method) as per IEC 62305-2




Procedure For Performing The Risk Assessment Study By Manual Method

Procedure for performing the risk assessment study includes three parts as follows:

  1. Part#1: evaluating Need for lightning protection,
  2. Part#2: Determination of Required Protection Level,
  3. Part#3: evaluating the cost-effectiveness of protection measures.





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.


Fig.2: Procedure For Evaluating Need For Lightning Protection





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.


Also, in Article " Design Calculations of Lightning Protection Systems – Part Four ", I explained Step#2-4 Second Part: Calculations of the Risk Components Rx - Calculation of the first Parameter Nx =  Number of dangerous events per year a
nd I indicated that:





Each of the risk components Rx 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, ...

So, The task of the risk assessment therefore involves the determination of the three parameters NX, PX and LX.




Today, I will explain how to calculate the second Parameter: PX = probability of damage to structure.





Step#2-4 Second Part: Calculations of the Risk Components Rx




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





Probability of damage to structure PX

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 Table-1:


PX
Source of
Damage S
Type of
Damage D
Nature of damage corresponding with each Probability
PA
S1
D1
Electric shock suffered by living beings as a result of a direct lightning strike to the building or structure
PB
S1
D2
physical damage (Fire, explosion, mechanical and chemical reactions) as a result of a direct lightning strike to the building or structure
PC
S1
D3
Failure of Internal (electrical / electronic) systems as a result of a direct lightning strike to the building or structure
PM
S2
D3
Failure of Internal (electrical / electronic) systems as a result of a lightning strike to the ground next to the building or structure;
PU
S3
D1
Electric shock suffered by living beings as a result of a direct lightning strike to the utility lines entering the building or structure
PV
S3
D2
physical damage (Fire, explosion, mechanical and chemical reactions) as a result of a direct lightning strike to a utility line entering the building or structure
PW
S3
D3
Failure of Internal (electrical / electronic) systems as a result of a direct lightning strike to a utility line entering the building or structure
PZ
S4
D3
Failure of Internal (electrical / electronic) systems as a result of a lightning strike to the ground next to a utility line entering the building or structure
Table-1: The Eight Damage Probabilities






1- Probability PA that a flash to a structure will cause injury to living beings by electric shock

The values of probability PA 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:

PA = PTA x PB

Where:

  • PTA depends on additional protection measures against touch and step voltages, such as those listed in Table B.1. Values of PTA are given in Table-2.
  • PB depends on the lightning protection level (LPL) for which the LPS conforming to IEC 62305-3 is designed. Values of PB are given in Table-3.



Additional Protection Measure
PTA
No protection measures
1
Warning notices
10–1
Electrical insulation (e.g. at least 3 mm cross-linked polyethylene) of exposed parts (e.g. down-conductors)
10–2
Effective soil equipotentialization
10–2
Physical restrictions or building framework used as a down-conductor system
0
Table-2: Values of probability PTA that a flash to a structure will cause shock to living beings due to dangerous touch and step voltages


Notes:

  • If more than one Protection provision has been taken, the value of PTA is the product of the corresponding values.
  • Protection measures are effective in reducing PA only in structures protected by an LPS or structures with continuous metal or reinforced concrete framework acting as a natural LPS, where bonding and earthing requirements of IEC 62305-3 are satisfied.






2- Probability PB that a flash to a structure will cause physical damage

The values of probability PB of physical damage by a flash to a structure, as a function of lightning protection level (LPL) are given in Table-3.


Characteristics of structure
Class of LPS
PB
Structure not protected by LPS
_
1
Structure protected by LPS

IV
0,2
III
0,1
II
0,05
I
0,02
Structure with an air-termination system conforming to LPS I and a continuous metal or reinforced concrete framework acting as a natural down-conductor system
0,01

Structure with a metal roof and an air-termination system, possibly including natural components, with complete protection of any roof installations against direct lightning strikes and a continuous metal or reinforced concrete framework acting as a natural down-conductor system
0,001

Table-3: Values of probability PB depending on the protection measures to reduce physical damage

Notes:

  • An LPS is suitable as a protection measure to reduce PB.
  • Values of PB other than those given in Table-3 are possible if based on a detailed investigation taking into account the requirements of sizing and interception criteria defined in IEC 62305-1.






3- Probability PC that a flash to a structure will cause failure of internal systems

The probability PC that a flash to a structure will cause a failure of internal systems is given by:

PC = PSPD x CLD

Where:

  • PSPD depends on the coordinated SPD system conforming to IEC 62305-4 and to the lightning protection level (LPL) for which its SPDs are designed. Values of PSPD are given in Table-4.
  • CLD is a factor depending on shielding, grounding and isolation conditions of the line to which the internal system is connected. Values of CLD are given in Table-5.


LPL
PSPD
No coordinated SPD system
1
III-IV
0,05
II
0,02
I
0,01
NOTE 2
0,005 – 0,001
Table-4: Value of the probability PSPD as a function of LPL for which SPDs are designed

Notes:

  • A coordinated SPD system is suitable as a protection measure to reduce PC only in structures protected by an LPS or structures with continuous metal or reinforced concrete framework acting as a natural LPS, where bonding and earthing requirements of IEC 62305-3 are satisfied.
  • The values of PSPD may be reduced for SPDs having better protection characteristics (higher nominal current IN, lower protective level UP, etc.) compared with the requirements defined for LPL I at the relevant installation locations.



External line type
Connection at entrance
CLD
CLI
Aerial line unshielded
Undefined
1
1
Buried line unshielded
Undefined
1
1
Multi grounded neutral power line
None
1
0.2
Shielded buried line (power or TLC)
Shield not bonded to the same bonding bar as equipment
1
0.3
Shielded aerial line (power or TLC)
Shield not bonded to the same bonding bar as equipment
1
0.1
Shielded buried line(power or TLC)
Shield bonded to the same bonding bar as equipment
1
0
Shielded aerial line (power or TLC)
Shield bonded to the same bonding bar as equipment
1
0
Lightning protective cable or wiring in lightning protective cable ducts, metallic conduit, or metallic tubes
Shield bonded to the same bonding bar as equipment
0
0
(No external line)
No connection to external lines (stand-alone systems)
0
0
Any type
Isolating interface according to IEC 62305-4
0
0
Table-5: Values of factors CLD and CLI depending on shielding, grounding and isolation conditions

Notes:

  • In the evaluation of probability PC, values of CLD in Table-5 refer to shielded internal systems; for unshielded internal systems, CLD = 1 should be assumed.
  • For non-shielded internal systems:

  1. Not connected to external lines (stand-alone systems), or
  2. Connected to external lines through isolating interfaces, or
  3. Connected to external lines consisting of lightning protective cable or systems with wiring in lightning protective cable ducts, metallic conduit, or metallic tubes, bonded to the same bonding bar as equipment, 

  • a coordinated SPD system according to IEC 62305-4 is not necessary to reduce PC, provided that the induced voltage UI is not higher than the withstand voltage UW of the internal system (UIUW).









4- Probability PM that a flash near a structure will cause failure of internal systems

  • The probability PM that a lightning flash near a structure will cause failure of internal systems depends on the adopted SPM measures.
  • When a coordinated SPD system meeting the requirements of IEC 62305-4 is not provided, the value of PM is equal to the value of PMS.
  • When a coordinated SPD system according to IEC 62305-4 is provided, the value of PM is given by:


PM = PSPD x PMS

Notes:

  • A grid-like LPS, screening, routing precautions, increased withstand voltage, isolating interfaces and coordinated SPD systems are suitable as protection measures to reduce PM.
  • For internal systems with equipment not conforming to the resistibility or withstand voltage level given in the relevant product standards, PM = 1 should be assumed.


The values of PMS are obtained from the product:

PMS = (KS1 x KS2 x KS3 x KS4)2

Where:

  • KS1 takes into account the screening effectiveness of the structure, LPS or other shields at boundary LPZ 0/1;
  • KS2 takes into account the screening effectiveness of shields internal to the structure at boundary LPZ X/Y (X>0, Y>1);
  • KS3 takes into account the characteristics of internal wiring (see Table-6);
  • KS4 takes into account the impulse withstand voltage of the system to be protected.


Note:

  • When equipment provided with isolating interfaces consisting of isolation transformers with earthed screen between windings, or of fiber optic cables or optical couplers is used, PMS = 0 should be assumed.


Inside an LPZ, at a safety distance from the boundary screen at least equal to the mesh width wm, factors KS1 and KS2 for LPS or spatial grid-like shields may be evaluated as:

KS1 = 0,12 x wm1
KS2 = 0,12 x wm2

Where:

wm1 (m) and wm2 (m) are the mesh widths of grid-like spatial shields, or of mesh type LPS down-conductors or the spacing between the structure metal columns, or the spacing between a reinforced concrete framework acting as a natural LPS.


Notes:
  • For continuous metal shields with thicknesses not lower than 0,1 mm, KS1 = KS2 = 10–4. 
  • Where a meshed bonding network is provided according to IEC 62305-4, values of KS1 and KS2 may be halved.
  • Where the induction loop is running closely to the LPZ boundary screen conductors at a distance from the shield shorter than the safety distance, the values of KS1 and KS2 will be higher. For instance, the values of KS1 and KS2 should be doubled where the distance to the shield ranges from 0,1 wm to 0,2 wm.
  • For a cascade of LPZs the resulting KS2 is the product of the relevant KS2 of each LPZ.
  • The maximum value of KS1 and KS2 is limited to 1.



Type of internal wiring
KS3
Unshielded cable – no routing precaution in order to avoid loopsa
1
Unshielded cable – routing precaution in order to avoid large loopsb
0.2
Unshielded cable – routing precaution in order to avoid loopsc
0.01
Shielded cables and cables running in metal conduitsd
0.0001
a Loop conductors with different routing in large buildings (loop area in the order of 50 m2).
b Loop conductors routed in the same conduit or loop conductors with different routing in small buildings (loop area in the order of 10 m2).
c Loop conductors routed in the same cable (loop area in the order of 0,5 m2).
d Shields and the metal conduits bonded to an equipotential bonding bar at both ends and equipment is connected to the same bonding bar.
Table-6: Value of factor KS3 depending on internal wiring


The factor KS4 is evaluated as:

KS4 = 1/UW

Where:

Uw is the rated impulse withstand voltage of system to be protected, in kV.

Notes:

  • The maximum value of KS4 is limited to 1.
  • If there is equipment with different impulse withstand levels in an internal system, the factor KS4 relevant to the lowest impulse withstand level should be selected.







5- Probability PU that a flash to a line will cause injury to living beings by electric shock

The values of probability PU 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 62305-3.

Note:

A coordinated SPD system according to IEC 62305-4 is not necessary to reduce PU; in this case SPD(s) according to IEC 62305-3 are sufficient.

The value of PU is given by:

PU = PTU x PEB x PLD x CLD

Where:

  • PTU depends on protection measures against touch voltages, such as physical restrictions or warning notices. Values of PTU are given in Table-7;
  • PEB depends on lightning equipotential bonding (EB) conforming to IEC 62305-3 and on the lightning protection level (LPL) for which its SPDs are designed. Values of PEB are given in Table-8;
  • PLD is the probability of failure of internal systems due to a flash to the connected line depending on the line characteristics. Values of PLD are given in Table-9.
  • CLD is a factor depending on shielding, grounding and isolation conditions of the line. Values of CLD are given in Table-5.


Note:

When SPD(s) according to IEC 62305-3 are provided for equipotential bonding at the entrance of the line, earthing and bonding according to IEC 62305-4 may improve protection.


Protection measure
PTU
No protection measures
1
Warning notices
10–1
Electrical insulation
10–2
Physical restrictions
0
Table-7: Values of probability PTU 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 PTU is the product of the corresponding values.


LPL
PEB
No SPD
1
III-IV
0.05
II
0.02
I
0.01
NOTE 3
0,005 – 0,001
Table-8: Value of the probability PEB as a function of LPL for which SPDs are designed

Note:

The values of PEB may be reduced for SPDs having better protection characteristics (higher nominal current IN, lower protective level UP, etc.) compared with the requirements defined for LPL I at the relevant installation locations.

Line
type
Routing, shielding and bonding conditions
Withstand voltage UW in kV
1
1,5
2,5
4
6
Power lines
or
Telecom lines

Aerial or buried line, unshielded or shielded whose shield is not bonded to the same bonding bar as equipment
1
1
1
1
1
Shielded aerial or buried whose shield bonded to the same bonding bar as equipment

5W/km < RS ≤ 20 W/km
1
1
0,95
0,9
0,8
1W/km < RS ≤5 W/km
0,9
0,8
0,6
0,3
0,1
RS ≤ 1 W/km
0,6
0,4
0,2
0,04
0,02
Table-9: Values of the probability PLD depending on the resistance RS of the cable screen and the impulse withstand voltage UW 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 PV that a flash to a line will cause physical damage

The values of probability PV 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 62305-3.

Note:

A coordinated SPD system according to IEC 62305-4 is not necessary to reduce PV; in this case, SPDs according to IEC 62305-3 are sufficient.

The value of PV is given by:

PV = PEB x PLD x CLD

Where:

  • PEB depends on lightning equipotential bonding (EB) conforming to IEC 62305-3 and on the lightning protection level (LPL) for which its SPDs are designed. Values of PEB are given in Table-8;
  • PLD is the probability of failure of internal systems due to a flash to the connected line depending on the line characteristics. Values of PLD are given in Table-9;
  • CLD is a factor depending on shielding, grounding and isolation conditions of the line. Values of CLD are given in Table-5.







7- Probability PW that a flash to a line will cause failure of internal systems

The values of probability PW 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 PW is given by:

PW = PSPD x PLD x CLD

Where:

  • PSPD depends on the coordinated SPD system conforming to IEC 62305-4 and the lightning protection level (LPL) for which its SPDs are designed. Values of PSPD are given in Table-4;
  • PLD is the probability of failure of internal systems due to a flash to the connected line depending on the line characteristics. Values of PLD are given in Table-9;
  • CLD is a factor depending on shielding, grounding and isolation conditions of the line. Values of CLD are given in Table-5.







8- Probability PZ that a lightning flash near an incoming line will cause failure of internal systems

The values of probability PZ 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 PZ is given by:

PZ = PSPD x PLI x CLI

Where:

  • PSPD depends on the coordinated SPD system conforming to IEC 62305-4 and the lightning protection level (LPL) for which its SPDs are designed. Values of PSPD are given in Table-4;
  • PLI is the probability of failure of internal systems due to a flash near the connected line depending on the line and equipment characteristics. Values of PLI are given in Table-10;
  • CLI is a factor depending on shielding, grounding and isolation conditions of the line. Values of CLI are given in Table-5.



Line type

Withstand voltage UW in kV
1
1,5
2,5
4
6
Power lines
1
0,6
0,3
0,16
0,1
TLC lines
1
0,5
0,2
0,08
0,04
Table-10: Values of the probability PLI depending on the line type and the impulse withstand voltage UW 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:


  • Parameters relevant to the probability P of damage shall be evaluated according to the above rules of the eight probabilities P of damage.
  • For components RC and RM, if more than one internal system is involved in a zone, values of PC and PM are given by:


PC = 1 – (1 – PC1) x (1 – PC2) x (1 – PC3)
PM = 1 – (1 – PM1) x (1 – PM2) x (1 – PM3)

Where:

 PCi, and PMi are parameters relevant to internal system i =1, 2, 3,…

With the exception made for PC and PM, 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.




In the next Article, I will continue explaining Step#2-4 Second Part: Calculations of the Risk Components Rx – Calculation of the Third Parameter: LX = = amount of consequent loss. Please, keep following.


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