Design Calculations of Lightning Protection Systems – Part Seven


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:




Today, I will explain The Manual Method (Equations and Tables Method) for Calculations of Risk Assessment Study as per NFPA-780.





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



Procedure For Performing The Risk Assessment Study By Manual Method

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

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



General notes for the Risk Assessment Study as per NFPA-780

  • The lightning risk assessment methodology is provided to assist the building owner, safety professional, or architect/engineer in determining the risk of damage or injury due to lightning.  Once the level of risk has been determined, the development of appropriate lightning protection measures can begin.
  • There are some cases where the need for protection should be given serious consideration regardless of the outcome of the risk assessment. Examples are those applications where the following are factors:

  1. Large crowds,
  2. Continuity of critical services,
  3. High lightning flash frequency,
  4. Tall isolated structure,
  5. Building containing explosive or flammable materials,
  6. Building containing irreplaceable cultural heritage.

  • Statutory, regulatory, and insurance requirements for the installation of a lightning protection system should take precedence over the results of a risk assessment.
  • This risk assessment method is a guide that takes into account the lightning threat parameters and the following factors:

  1. Building environment,
  2. Type of construction,
  3. Structure occupancy,
  4. Structure contents,
  5. Lightning stroke consequences.



Part#1: Evaluating Need For Lightning Protection

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.



Method#1: The Simplified Risk Assessment




Method#1: The Simplified Risk Assessment

The simplified risk assessment method includes the following steps:

  • Step#2-1: Calculate Equivalent Collection Area Ae.
  • Step#2-2: Determine The Value Of Lightning Flash Density (Ng).
  • Step#2-3: Calculate Annual Threat Of Occurrence (Nd).
  • Step#2-4: Calculate The Tolerable Lightning Frequency (Nc).
  • Step#2-5: Comparing the Annual Threat of Occurrence (Nd) To the Tolerable Lightning Frequency (Nc).



Step#2-1: Calculate Equivalent Collection Area Ae

Ae refers to the equivalent ground area having the equivalent lightning flash vulnerability as the structure. It is an area adjusted for the structure that includes the effect of the height and location of the structure.

The equivalent ground collection area of a structure is the area obtained by extending a line with a slope of 1:3 from the top of the structure to ground completely around the structure.

The equivalent collection area of a rectangular structure with length L, width W, and height H (see Fig.1) is as follows:

Ae = LW + 6H(L + W) + Π9H2



Fig.1: Calculation of the Equivalent Ground Collection Area for a Rectangular Structure.

  
The equivalent collection area of complex structures can be developed by numerical or graphical methods (See Fig.2 and Fig.3).


Fig.2: Calculation of the Equivalent Collection Area for a Complex Shape Structure Where a Prominent Part Encompasses All Portions of the Lower Part

Note:
For a structure where a prominent part encompasses all portions of the lower part, Ae = Π9H2.



Fig.3: Graphical Solution of the Equivalent Collection Area for a Structure Where a Prominent Part Encompasses Part of the Lower Structure.

Notes:

  • Where the equivalent collection area of one structure or object totally encompasses another structure, the covered structure is disregarded.
  • When the collection areas of several structures overlap, the corresponding common collection area is considered as a single collection area.



Step#2-2: Determine the Value of Lightning Flash Density (Ng)

Lightning flash density, the yearly number of flashes to ground per square kilometer, can be found from Fig.4.


Fig.4: Average U.S. Lightning Flash Density Map (flashes per square kilometer per year)

Note:

  • The Ng Map may vary each year and you must get the most recent one as possible for better results.



Step#2-3: Calculate Annual Threat of Occurrence (Nd)

The yearly annual threat of occurrence (lightning strike frequency) (Nd) to a structure is determined by the following equation:

Nd=(Ng)(Ae)(C1)( 10-6) potential events/yr

Where:

  • Nd = yearly lightning strike frequency to the structure or object,
  • Ng = lightning ground flash density in flashes/km2/year (from Step#1),
  • Ae = the equivalent collection area of the structure (m2) (from Step#2),
  • C1 = environmental or Location Factor (see Table-1).


The location factor C1 accounts for the topography of the site of the structure and any objects located within the distance 3H from the structure that can affect the collection area. Location factors are given in Table-1.


Relative Structure Location
C1
Structure surrounded by taller structures or trees within a distance of 3H
0.25
Structure surrounded by structures of equal or lesser height within a distance of 3H
0.5
Isolated structure, with no other structures located within a distance of 3H
1
Isolated structure on hilltop
2
Table-1: Location Factor C1



Step#2-4: Calculate the Tolerable Lightning Frequency (Nc)

The tolerable lightning frequency (Nc) is a measure of the risk of damage to the structure, including factors affecting risks to the structure, to the contents, and of environmental loss.

The tolerable lightning frequency (Nc) is calculated by dividing the acceptable frequency of property losses by various coefficients relating to the structure, the contents, and the consequence of damage.

The tolerable lightning frequency is expressed by the following formula:

Nc = default value of tolerable frequency of property losses / The coefficient C

Nc = (1.5×10−3) / C    events/yr

Where:

The default value of tolerable frequency of property losses is 1.5 x 10–3.
The coefficient (C) is the product of structural coefficients C2 through C5 and it is expressed by the following formula:

C = (C2)(C3)(C4)(C5)

The structural coefficients C2 through C5 are obtained from Table-2 through Table-5 as follows:


Structure
Metal Roof
Nonmetallic Roof
Combustible Roof
Metal
0.5
1.0
2.0
Nonmetallic
1.0
1.0
2.5
Combustible
2.0
2.5
3.0
Table-2: Determination of Construction Coefficient C2


Structure Contents
C3
Low value and noncombustible
0.5
Standard value and noncombustible
1.0
High value, moderate combustibility
2.0
Exceptional value, flammable liquids, computer or electronics
3.0
Exceptional value, irreplaceable cultural items
4.0
Table-3: Determination of Structure Contents Coefficient C3


Structure Occupancy
C4
Unoccupied
0.5
Normally occupied
1.0
Difficult to evacuate or risk of panic
3.0
Table-4: Determination of Structure Occupancy Coefficient C4


Lightning Consequence
C5
Continuity of facility services not required, no environmental impact
1.0
Continuity of facility services required, no environmental impact
5.0
Consequences to the environment
10.0
Table-5: Determination of Lightning Consequence Coefficient C5



Step#2-5: Comparing the Annual Threat of Occurrence (Nd) to the Tolerable Lightning Frequency (Nc)

The result of this comparison is used to decide if a lightning protection system is needed or not. So, we have two cases:

  • Case#1: If Nd ≤ Nc , an LPS could be optional.
  • Case#2: If Nd > Nc , an LPS is recommended.



Procedure for Method#1: Simplified Risk Calculation

Table-6 provides a simple method of calculating and using the simplified Risk assessment method described in above.

Step#
Data Input Equations
Computation
Result
Step#1
Equivalent collection area

Ae = LW + 6H(L + W) + Π9H2
L =
W =
H =
H2 =
Ae =
Step#2
Step#3
Expected annual threat occurrence

Nd = (Ng)(Ae)(C1) (10–6)
Ng =
Ae =
C1 =
Nd =
Step#4
Tolerable lightning frequency to the structure

Nc = (1.5 × 10–3)/C

Where:
C = (C2)(C3)(C4)(C5)
C2 =
C3 =
C4 =
C5 =
C =
Nc =
Step#5
If Nd ≤ Nc , an LPS could be optional.
If Nd > Nc , an LPS is recommended.
Table-6: Procedure for Method#1: Simplified Risk Calculation





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 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,
  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#2-1: Identify The Structure to be Protected

The procedure for the risk assessment is to first define the extent of the facility or structure being assessed, The structure to be protected includes:

  • The structure itself;
  • associated outbuildings or equipment support structures.


One must then determine all relevant physical, environmental, and service installation factors applicable to the structure.



Step#2-2: for each Loss to be considered, identify the Tolerable Level Of Risk RT

Values of tolerable levels of loss could be selected by the owner, the owner’s representative, or the authority having jurisdiction. Default values that can be used where risk levels are not provided by other sources are included in Table-

The tolerable risk is expressed in the form of number of events per year and is given in engineering units (e.g. 10-x) as in the following table which includes the values of RT from different Standards and codes.

Types Of Loss
RT(y-1)
IEC 62305-2
BS EN 62305-2
NFPA-780
Loss Of Human Life
10-5
(risk of 1 in 100,000)
10-5
(risk of 1 in 100,000)
10-6
(risk of 1 in 1000,000)
Loss Of Service To The Public
10-3
(risk of 1 in 1,000)
10-4
(risk of 1 in 10,000)
10-3
(risk of 1 in 1,000)
Loss Of Cultural Heritage
10-4
(risk of 1 in 10,000)
10-4
(risk of 1 in 10,000)
10-3
(risk of 1 in 1,000)

Notes:

  • Use the national standard appropriate to the country of installation, for example don’t use NFPA 780 in UK.
  • If your country didn’t have a national standard or local regulation which better reflect the localized conditions and acceptable local tolerable risk in your country, so select a national standard for a country which experiences similar lightning risk (ground flash density/ thunderdays) and similar social/economic values.



Step#2-3: identify the Types of Risk Due to Lightning (Rn)

The following risks have been identified, corresponding to their equivalent type of loss:

  • R1 risk associated with loss of life or injury.
  • R2 risk associated with loss of service.
  • R3 risk associated with loss of historical significance.


Hereafter the primary risks will be referred to collectively as Rn where the subscript n indicates 1, 2 or 3 as described above.



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.



First: identification of the Risk Components Rx

These risk categories are composed of risk components that are summed to determine the overall risk of the loss in a given application. The risk components are categorized according to the type of loss and source of the damage ( see Fig.5)as follows:

  1. Risk Components due to Direct Strikes to a Structure
  2. Risk Components due to Strikes near a Structure
  3. Risk Components due to Strike to a Service Connected to a Structure
  4. Risk Components due to Strikes Near a Service Connected to the Structure




Fig.5: Types of loss and corresponding risks resulting


1- Risk Components due to Direct Strikes to a Structure includes:

  • RA indicates injuries caused by strikes to a structure (touch and step potentials).
  • RB indicates damage to a structure due to a direct strike.
  • RC indicates failure of internal systems due to a strike to a structure.


2- Risk Components due to Strikes near a Structure includes:

  • RM indicates failure of internal systems due to a strike near a structure.


3- Risk Components due to Strike to a Service Connected to a Structure includes:

  • RU indicates injury due to strikes to a service connected to the structure.
  • RV indicates damage to a structure due to strikes to a service connected to the structure.
  • RW indicates failure of internal systems or equipment due to a strike to a service connected to the structure.


4- Risk Components due to Strikes Near a Service Connected to the Structure includes:

  • RZ indicates failure of internal systems or equipment due to strikes near a service connected to the structure.


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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