Design Calculations of Lightning Protection Systems – Part Nine

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 performthis 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.



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




In Article " Design Calculations of Lightning Protection Systems – Part Eight ", I explained How to calculate the first two parameters Nx and Px as per NFPA 780.

Today, I will explain How to calculate the Third parameter Lx and I will continue explaining the steps of Method#2: The detailed Risk assessment as per NFPA-780.





Calculations of Third Parameter: LX = Loss Factor





1- Loss Factors Lt, Lf, and Lo

The value of Lt, Lf, and Lo can be determined in terms of the relative number of victims from the following approximate relationship:

LA = (np / nt ) x (tp / 8760)

Where:

LA = value for loss of human life
np = number of possible endangered persons (victims)
nt = expected total number of persons (in the structure)
tp = time in hours per year for which the persons are present in a dangerous place, outside of the structure (Lt only) or inside the structure (Lt, Lf, and Lo)

Typical mean values of Lt, Lf, and Lo for use when the determination of np , nt , and tp is uncertain or difficult, are given in Table-1 which provides typical mean values for loss of life, physical damage to a structure, or failure of an internal system from a strike to or near a structure.

Type of Structure
Loss of Life
(Lt)
Physical Damage
(Lf)
Failure of Systems
(LO)
All types: persons inside building
105


All types: persons outside building
103


Hospitals

101
103
Hotels, civil buildings

101

Industrial, commercial, school

5 × 102

Public entertainment, churches, museums

2 × 102

Others

102

Risk of explosion


101

Table-1: Typical Mean Values of Losses






2- Value Of Loss Due To Injury To Humans

The following equation calculates the value of injury to humans:

LA = LU = ra x Lt

Where:

LA = value for loss of human life
LU = value of loss of living being
ra = reduction factor for type of surface soil or floor (Table-2)
Lt = mean value of loss of life (Table-1)


Type of Surface
Contact Resistance (k*)
ra
Soil, concrete
<1
102
Marble, ceramic
1–10
103
Gravel, carpets
10–100
104
Asphalt, linoleum, wood
> 100
105
*Values measured between a 4000 mm2 electrode compressed with force of 500 N at a point of infinity.

Table-2 Values of Reduction Factor (ra) as a Function of the Type of Surface of Soil or Floor





3- Value Of Loss Due To Physical Damage

The following equation calculates the value of loss from physical damage to the structure:

LB = LV = rp X rf X hZ X Lf

Where:

LB = value of loss due to direct strike to the structure
LV = value of loss due to strike to incoming service
rp = reduction factor for provisions taken to reduce consequences of fire (Table-3)
rf = reduction factor for risk of fire to structure (Table-4)
hZ = factor for the kinds of hazard in the structure (Table-5)
Lf = mean value of physical damage loss (Table-1)

Provisions
rp
No provisions or structure contains risk of explosion
1
Fixed manually operated extinguishing installations, manual alarm installations, hydrants, fire proof compartments, and/or protected escape routes
0.5
Protected against overvoltages and other damages, or firefighters can arrive in less than 10 minutes, or fixed automatically operated extinguishing installations or automatic alarm installed
0.2

Table-3: Values of Reduction Factor (rp) as a Function of Provisions Taken to Reduce the Consequences of Fire

Note:

  • If more than one provision has been taken, the value of rp is the lowest of the relevant values.



Risk of Fire
rf
Explosiona
1
Highb
0.1
Ordinaryc
0.01
Lowd
0.001
Nonee
0
aStructures with risk of explosion or structures that contain explosive mixtures of gases, dusts, or materials.
bStructures with significant quantities of combustible materials and/or storage of significant quantities of flammable and combustible liquids (e.g., large warehouses, shipping terminals, big box stores, industrial facilities with flammable and combustible processes, printing, saw mills, plastics processing, paint dipping and spraying).
cStructures with moderate quantities of combustible materials with minor storage areas that produce significant amounts of smoke, but no flammable or combustible liquids (e.g., small warehouses, mercantile, post offices, electronic plants, ordinary chemical plants, restaurant service areas, wood product assembly).
dStructures with limited quantities of combustible materials and generally noncombustible construction (e.g., residences, churches, educational buildings, institutional, museums, offices, theaters).
eNoncombustible construction with no exposed combustible contents.

Table-4: Values of Reduction Factor (rf) as a Function of Risk of Fire of a Structure



Kind of Hazard
hZ
No special hazard
1
Low level of panic (e.g., structures limited to two floors and the number of people not greater than 100)
2
Average level of panic (e.g., structures designed for cultural or sporting events with a number of people between 100 and 1000)
5
Difficulty of evacuation (e.g., structures with immobilized people, such as hospitals)
5

High level of panic (e.g., structures designed for cultural or sporting events with the number of people greater than 1000)
10

Hazard to surrounding area or environment
20
Contamination of surrounding area or environment
50

Table-5: Values of Hazard Factor (hZ)





4- Value Of Loss Due To Failure Of Internal Systems

 The following equation calculates the value of loss due to failure of internal systems:

LC = LM = LW = LZ = L0

Where:

LC = value of loss due to direct strike to the structure
LM = value of loss due to a strike near the structure
LW = value of loss due to a strike to a service connected to the structure
LZ = value of loss due to a strike near a service connected to the structure
L0 = mean value of loss of internal system (Table-1)






End of Step#2-4

After Calculating the three parameters NX, PX and LX which consisting the risk components Rx. we can calculate each risk components by using Specific formulas given in Table-6.

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





Step#2-5: Calculate R = Σ Rx




the total risk due to lightning (R) can be calculated by using the following relationships:

R = R1 + R2 + R3 + R4

Where:

R1 = RA + RB + RC* + RM*, + RU + RV + RW* + RZ*

R2 = RB + RC + RM + RV + RW + RZ

R3 = RB + RV

R4 = RA** + RB + RC + RM + RU** + RV + RW + RZ


*RC, RM, RW, and RZ in R1 are applicable only for structures with risk of explosion, for structures with life-critical electrical equipment (such as hospitals), or other structures where the failure of internal systems immediately endangers human life.

**RA and RU in R4 are applicable only for structures where animals might be injured.






Step#2-6: Comparing the total risk R with the maximum tolerable risk (RT) for each type of loss relevant to the structure




Comparing the total risk R of each loss with the maximum tolerable risk (RT), then we have (2) cases:

Case#1:
If the total risk R is equal or less than the respective tolerable risk RT i.e. R ≤ 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 total risk R is higher than the tolerable risk RT i.e.  R > RT, then Install lightning protection measures in order to reduce R.





Step#2-7: go back to step#2-4 and make a series of trial and error calculations until the total risk R is reduced below that of RT (R ≤ RT).



In the next Article, I will explain The Software Method for Performing the Risk Assessment Study. Please, keep following.

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