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.
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:
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:
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:
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#2-4: 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 ".
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
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)
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.
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
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)
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
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)
Table-3: Values of Reduction Factor (rp) as a Function of Provisions Taken to Reduce the Consequences of Fire
Table-4: Values of Reduction Factor (rf) as a Function of Risk of Fire of a Structure
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
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.
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
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:
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,
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.