Design Calculations of Lightning Protection Systems – Part Eight

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



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



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




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:
  • Method#1: The simplified Risk assessment,
  • Method#2: The detailed Risk assessment.



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.





First: Manual Method (Equations And Tables Method) as per NFPA-780 - Continued




Method#2: The detailed Risk assessment

The detailed Risk assessment Method includes the following steps:

  • Step#2-1: Identify The Structure to be Protected
  • Step#2-2: for each Loss to be considered, identify the Tolerable Level Of Risk RT
  • Step#2-3: Identify the Types of Risk Due to Lightning (Rn)
  • Step#2-4: For each type of loss to be considered , identify and calculate the risk components Rx that make up Primary risk Rn which are: RA, RB, RC, RM, RU, RV, RW, RZ.
  • Step#2-5: Calculate the Total Risk R = Σ Rx = R1+R2+R3
  • Step#2-6: Comparing the calculated actual risk R of each loss to a tolerable level of risk (RT), then we have (2) cases:

  1. Case#1: If the calculated Total risk R is equal or less than the respective tolerable risk Ri.e. R ≤ R, then Structure is adequately protected for this type of loss and no lightning protection is required for this type of loss,
  2. Case#2: If the calculated Total risk R is higher than the tolerable risk RT i.e.  R > RT, then Install lightning protection measures in order to reduce Total Risk R.


  • Step#2-7: go back to step#2-2 and make a series of trial and error calculations until the risk R is reduced below that of RT (R ≤ RT).






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:

  1. Identification of the Risk Components Rx,
  2. Calculations of the Risk Components Rx.


I explained the first part: Identification of the Risk Components Rx in Article Design Calculations of Lightning Protection Systems – Part Seven ".




Second Part: Calculations of the Risk Components Rx

Each component of risk Rx depends on (3) parameters as follows:

  1. The average annual threat of occurrence, Nx (strikes in the area of interest),
  2. The probability of damage, Px (or step and touch voltages to humans),
  3. The expected loss related to the event, Lx.


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.

Risk Component
Descriptor
RA = NdPALA
Risk of injury due to direct strike to structure
RB = NdPBLB
Risk of physical damage to structure due to a direct strike to the structure
RC = NdPCLC
Risk of failure of internal systems due to direct strike to structure
RM = NMPMLM
Risk of failure of internal systems due to strike near structure
RU = (NL+Nda)PULU
Risk of injury due to strike to incoming service
RV = (NL+Nda)PVLV
Risk of physical damage due to direct strike to incoming service
RW = (NL+Nda)PWLW
Risk of failure of internal systems due to direct strike to incoming service
RZ = (NINL)PZLZ
Risk of failure of internal systems due to strike near incoming service

Table-1: Risk Components Formulas




Calculations of First Parameter: NX = The average annual threat of occurrence




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 ".




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:

  • The collection area (Am) for flashes near the structure includes the area extending a distance of 250 m (820 ft) around the perimeter of the structure.
  • For cases where NM is negative, a value of 0 is assigned to NM.





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

Collection Area
Aerial
Buried
Al
6 Hc(lc – 3(Ha + Hb))
(lc – 3(Ha + Hb)) √ρ

Ai

1000 lc

25 lc √ρ
Al = collection area of flashes striking incoming service (m2)
Ai = collection area of flashes to ground near incoming service (m2)
Hc = height of incoming service conductors above ground (m)
lc = length of incoming service section from structure to first point of transition (m) (a maximum value of lc of 1 km should be used)
Ha = height of structure connected at end “a” of incoming service (m)
Hb = height of structure connected at end “b” of incoming service (m)
ρ= resistivity of soil where service is buried (m) (a maximum value for ρ is 500 Ωm).

Table-2: Values of Collection Areas Al and Ai


Notes:

  • Where the value of lc (used in the determination of Al) is not known, a value of 1 km is assumed for the assessment.
  • A default value of 500 Ωm can be used for soil resistivity (ρ) where this value cannot be determined.
  • If the installation incorporates underground cables run underneath a ground mesh, Al could be assumed to be 0 for that cable set (NL = 0).
  • Ct applies to line sections between the transformer and the structure. A value of 0.2 is applicable for installations having a transformer located between the strike and the structure. Otherwise, a value of 1 is assigned to this variable.





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:

  • Ct applies to line sections between the transformer and the structure. A value of 0.2 is applicable for installations having a transformer located between the strike and the structure. Otherwise, a value of 1 is assigned to this variable.






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


Service Environment
Ce
Urban with buildings exceeding 20 m high
0.01
Urban—population greater than 50,000
0.1
Suburban—residential on outskirts of cities
0.5
Rural—settled areas outside of towns and cities
1

Table-3: Service Environmental Coefficient Ce


Notes:

  • The collection area of the service (Ai) is related to the length lc (see Table-2) at which a flash near the service could cause induced overvoltages not lower than 1.5 kV.
  • Ct applies to line sections between the transformer and the structure. A value of 0.2 is applicable for installations having a transformer located between the strike and the structure. Otherwise, a value of 1 is assigned to this variable.





Calculations of Second Parameter: PX = Probabilities of Damage




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:

Protection Measure
PA
No protection measures
1
Warning notices
0.1
Electrical insulation/isolation of exposed down conductor
0.01
Effective soil equipotentialization
0.01
Structural steel frame is used as the down conductor system
10-6
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





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:

Type of protection provided
PB
No protection provided
1
LPS based on 46 m (150 ft) striking distance
0.1
LPS based on 30 m (100 ft) striking distance
0.05
Structure with a metal roof and continuous metal or reinforced concrete frame serving as a natural down conductor system with bonding and grounding in accordance with NFPA 780
0.001
Table-5: Values of Probability (PB) of Physical Damage to a Structure Due to Flashes to the Structure

Note:

  • Values other than those given in this table can be used when justified by a detailed analysis of the protection provided.





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:

SPD Protection Provided
PC
No SPD protection
1
SPDs provided in accordance with Section 4.18
0.03
Table-6: Values of Probability (PC) as a Function of SPD Protection Provided

Notes:

  • SPD protection is effective to reduce PC only in structures protected by an LPS or in structures with a continuous metal or reinforced concrete frame where bonding and grounding requirements of Section 4.18 are met.
  • Shielded internal systems fed by wiring in lightning protective cable ducts or metallic conduit can be used in lieu of SPD protection.
  • Smaller values of PC can be used where SPDs above and beyond those required by Section 4.18 and SPDs having better protection characteristics (higher current withstand capability, lower protective level, etc.) than the minimum specified in Section 4.18. See IEC 62305-2, Protection Against Lightning, Annex B, for additional information.





4- The Probability that a strike near a structure will cause failure of internal systems (PM)

  • The probability that a strike near a structure will cause failure of internal systems (PM) depends on the lightning protection measures implemented. These measures are characterized by a factor KS that takes into consideration protective measures such as the shielding effectiveness of the structure, any internal shielding provided, characteristics of internal wiring, and the withstand voltage of the system to be protected.
  • Where SPDs are not installed at utilization equipment, or the SPDs at the utilization equipment are not properly coordinated with those installed at the service entrances, the value of PM to be used in the equation for the risk of failure of internal systems due to a strike near a structure (PM) can be taken from Table-7:


KS
PM
>0.4
1
0.15
0.9
0.07
0.5
0.035
0.1
0.021
0.01
0.016
0.005
0.015
0.003
0.014
0.001
<0.013
0.0001

Table-7: Values of Probability (PM) as a Function of KS

  • Where coordinated SPDs are installed at the utilization equipment, the value of PM used in the computation of PM is the lower value between PC and PM.
  • For internal systems with equipment having withstand voltage levels that are unknown or are less than 1.5 kV, a value of PM = 1 should be used in the assessment.
  • The value of KS is calculated using the following equation:


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.

  • In those structures where it is ensured that steel reinforcing bars are interconnected and terminated by approved grounding electrodes, w is the spacing between the reinforcing bars.
  • If the equipment is located closer to the applicable boundary than the distance, w, the values of KS1 and KS2 should be doubled. In those cases where multiple internal boundaries exist, the resulting value of KS2 is the product of each individual value of KS2.


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.

Type of Internal Wiring
KS3
Unshielded cable—no routing precaution to avoid loops
1
Unshielded cable—routing precaution to avoid large loops
0.2
Unshielded cable—routing precaution to avoid loops up to 10 m2
0.02
Shielded cable with shield resistance of 20 > RS > 5 Ω/km
0.001
Shielded cable with shield resistance of 5 > RS > 1 Ω/km
0.0002
Shielded cable with shield resistance of 1 > RΩ/km
0.0001
Table-8: Values of Factor (KS3) as a Function of Internal Wiring

Note:

  • Shielded cable includes those conductors installed within a metallic raceway.



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.




5- The Probability, PU, that a lightning flash will result in injury to living beings

  • The probability, PU, that a lightning flash will result in injury to living beings due to touch voltage by a flash to a service entering the structure depends on the characteristics of the service shield, the impulse withstand voltage of internal systems connected to the service, typical protection measures (physical restrictions, warning notices), and SPDs provided at the entrance of the service.
  • Where SPDs are not provided for equipotential bonding, PU is characterized by the probability of failure of internal systems due to a flash to the connected service as shown in Table-9:


Uw
(kV)
RS >5
(/km)
5 >RS >1
(/km)
1 >RS
(/km)
1.5
1
0.8
0.4
2.5
0.95
0.6
0.2
4
0.9
0.3
0.04
6
0.8
0.1
0.02

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:

  • RS is the resistance of the cable shield.
  • Where SPDs are provided for equipotential bonding, the value of PU to be used in the equation for the risk of injury to humans due to flashes to a service is the lower value between PC and PU. For unshielded services, a value of PU = 1 is used. 
  • Where physical restrictions, warning notices, etc., are used, the value of PU can be further reduced by multiplying it by PA.





6- The Probability of physical damage due to a strike to a service entering a structure (PV)

  • The probability of physical damage due to a strike to a service entering a structure (PV) depends on the service line shielding characteristics, the impulse withstand voltage of internal systems connected to the service, and any SPDs provided.
  • Where SPDs are not provided, the value of PV is equal to the value of PU
  • Where SPDs are provided, the value of PV to be used in the equation for the risk of physical damage due to a strike to a service is the lower value between PC and PU.





7- The Probability of a failure of internal systems due to a strike to a service entering a structure (PW)

  • The probability of a failure of internal systems due to a strike to a service entering a structure (PW) depends on the service line shielding characteristics, the impulse withstand voltage of internal systems connected to the service, and any SPDs provided.
  • Where SPDs are installed, the value of PW is the lower value of PC or PU
  • Where SPDs are not installed, the value of PW to be used in the equation for the risk of failure of internal systems due to a strike to a service is equivalent to the value of PU.





8- The Probability of a failure of internal systems due to a strike near a service entering the structure under consideration (PZ)

  • The probability of a failure of internal systems due to a strike near a service entering the structure under consideration (PZ) depends on the service line shielding characteristics, the impulse withstand voltage of internal systems connected to the service, and the protection measures provided.
  • Where SPDs are not installed, the probability of failure of internal systems due to a flash near the connected service (PZ) can be taken from Table-10.
  • Where SPDs are installed, the value of PZ can be taken to be the lower value of PC or PZ.


Uw
(kV)

No
Shield


Shield and
Equipment
Not Bonded
to Same
System

Shield and
Equipment
Bonded to
Same
System
RS >5
(/km)
Shield and
Equipment
Bonded to
Same
System
5 >RS >1
(/km)
Shield and
Equipment
Bonded to
Same
System
1 >RS
(/km)
1.5
1
0.5
0.15
0.04
0.02
2.5
0.4
0.2
0.06
0.02
0.008
4
0.2
0.1
0.03
0.008
0.004
6
0.1
0.05
0.02
0.004
0.002

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:

  • RS is the resistance of the cable shield.





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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