Design Calculations of Lightning Protection Systems – Part Two


In Article 
 " Design Process for Lightning Protection Systems ", I indicated that the Design Process for 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.

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


Today, I will Continue explaining Design Calculations of Lightning Protection Systems.






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: Lightning Protection Design Process





Step#1: Characteristics of the Structure to Be Protected





A- The Characteristics of the Structure

When Lightning strikes affecting a structure, the Characteristics of the structure will determine the damage level to the structure itself and to its occupants and contents, including failure of internal systems. The damages and failures may also extend to the surroundings of the structure and even involve the local environment. The Characteristics of the structure include:

  1. The design of the building,
  2. The environment around the building,
  3. The material in the building,
  4. The number of lightning strikes to earth in the area of the building,
  5. The value of the building and its contents,
  6. Sensitive electronics in the building,
  7. Loss of revenue In the event of breakdown,
  8. Escape facilities and number of staff in the building,
  9. Fire protection in the building,
  10. The historical and cultural value of the building,
  11. The social function of the building,
  12. Cable laying up to the building,
  13. Conductivity of the ground in the area of the building,
  14. The risk to the surroundings.



1- The Design Of The Building:

  • A building that is tall and has a large footprint has a greater likelihood of being struck by lightning. IEC Technical Report No 61662 "Assessment of the Risk of Damage due to Lightning" contains a method for calculating how often a building may be expected to be struck by lightning.



2- The Environment Around The Building:

  • The environment affects the probability that the building will be struck by lightning. If there are nearby buildings or if the buildings situated in a hollow, the risk of the building being struck is reduced.
  • The method in IEC Technical Report No 61661 'Assessment of the Risk of Damage due to Lighting" can take into consideration these factors and the way they influence the likelihood of the building being struck by lightning.



3- The Material In The Building:

  • Ira material used in the building has an effect on the seriousness of the consequences of a lightning strike. If the material on the outside is electrically conductive, e.g. sheeting or reinforced concrete, there is a certain natural lightning protection. These buildings tolerate a lightning strike better than buildings comprising non-conductive material such as timber or brick. A non-conductive material can be blown apart by the lightning strike.



4- The Number Of Lightning Strikes To Earth In The Area Of The Building:

  • Certain areas have a larger mean number of lightning strikes to earth annually than others. The probability that the building will be exposed to a lightning strike is larger in areas with a larger mean number of strikes.



5- The Value Of The Building And Its Contents:

  • If lightning protection is to be installed merely to protect property, the cost of the lightning protection must be compared with the value of the building's content. Consideration must also be given to how unique these are Also the contents of the building must be reviewed to proive the adequate protection measure, if any, like for presence of combustible or noncombustible materials and presence explosive or non-explosive materials.



6- Sensitive Electronics In The Building:

  • Sensitive electronics n the building may be destroyed or cease to function as a result of a direct strike, overvolrages that are conducted into the bilking, or by voltage induced into the building by the lightning's electromagnetic pulse.
  • Here it is important to investigate how important the electronics are for continued function of the activity in the building, and how serious the consequences of a failure of the electronics would be. The cost of repairs to the electronics also affects the need for lightning protection.



7- Loss Of Revenue In The Event Of Breakdown:

  • If the effect of lightning on the building would cause a breakdown in operations, it is important to investigate how long and how expensive such a breakdown would be. Such an investigation should also consider whether such breakdown could also entail loss of market shares.



8- Escape Facilities And The Number Of Persons In The Building:

  • For the safety of persons it is important to consider how many persons are regularly present in the building and if they have limited freedom of movement or reduced physical mobility. Statistically speaking, it is relatively improbable to be killed by lightning. This does not, however; mean that lightning cannot strike a place of assembly, in which case the consequences can be very serious.



9- Fire Protection In The Building:

  • Good fire protection in the building is important because it can alleviate the consequences of a fire started by a lightning strike.
  • A lightning strike can destroy or disrupt fire alarm installations and in this way negatively affect fire protection.
  • Lightning often triggers an automatic fire alarm without the outbreak of fire. A correctly installed lightning protection considerably reduces the risk of this happening.



10- The Historic And Cultural Value Of The Building:

  • For a building where lightning protection is being considered only because of the high historic and cultural value of the building, the probability of the building being affected by lightning should be investigated.



11- The Social Function Of The Building:

  • If the building has an important social function, e.g. hospital, nuclear plant water, gas or electricity installation, major telecommunications installation and radio stations, alarm and surveillance centers, important installations for the police, military, rescue services and traffic control, a lightning protection may be needed.
  • Other social functions of the building are dwelling house, office, farm, theatre, hotel, school, church, prison, department store, bank, factory, industry plant and sports area, a lightning protection should be determined by risk assessment.
  • An assessment should be made of the consequences for the pubic if the installation is knocked out by lightning. It should also be assessed whether the function which these buildings have are especially important during thundery weather Or whether a breakdown then can be accepted.



12- Cable Laying Up To The Building:

  • If electric and telecommunications cables are completely laid in the ground, the risk that lightning current will be led into the building is less than if the cables are placed wholly or partly above ground. "Assessment of the Risk of Damage due to Lightning" contains a method for calculating how often a building will be exposed to over-voltages.



13- Conductivity Of The Ground In The Area Of The Building:

  • If the ground has good conductivity, the voltage due to the lightning decreases over some tens of meters from the site of the strike. If the conductivity of the ground is low, large voltages may arise along the ground surface over up to several kilometers from the site of strike. Voltages can then enter the building via the ground, electric or telecommunications networks or some other metallic conductor.
  • In some areas the soil layer is relatively thin, and at times of powerful storms high voltages can therefore arise over a distance of several kilometers from the site of strike.
  • Clay-like materials have good conductivity, while sand, fine sand and stone have lower conductivity.



14- The Risk To The Surroundings:

  • The risk to the surroundings should be considered if lightning protection is to be installed. This mainly applies to industries. For installations which must conduct a hazard analysis, lightning and also the effect of Lightning on the security system must be included as a hazard.
  • The risk to the surroundings should be considered also for connected lines to the building (power lines, telecommunication lines, pipelines).






B- Effects Of Lightning On A Structure

  • Lightning affecting a structure can cause damage to the structure itself and to its occupants and contents, including failure of internal systems. The damages and failures may also extend to the surroundings of the structure and even involve the local environment.
  • The scale of this extension depends on the characteristics of the structure and on the characteristics of the lightning flash.


Table-1 reports the effects of lightning on various types of structures as follows:

Type Of Structure According To Function and/or Contents
Effects Of Lightning
Dwelling-house
Puncture of electrical installations, fire and material damage
Damage normally limited to structures exposed to the point of strike or to the lightning current path
Failure of electrical and electronic equipment and systems installed (e.g. TV sets, computers, modems, telephones, etc.)

Farm building
Primary risk of fire and hazardous step voltages as well as material damage
Secondary risk due to loss of electric power, and life hazard to livestock due to failure of electronic control of ventilation and food supply systems, etc.

Theatre
Hotel
School
Department store
Sports area

Damage to the electrical installations (e.g. electric lighting) likely to cause panic
Failure of fire alarms resulting in delayed fire fighting measures

Bank
Insurance company
Commercial company, etc.

As above, plus problems resulting from loss of communication, failure of computers and loss of data

Hospital
Nursing home
Prison

As above, plus problems of people in intensive care, and the difficulties of rescuing immobile people

Industry
Additional effects depending on the contents of factories, ranging from minor to unacceptable damage and loss of production

Museums and archaeological site
Church

Loss of irreplaceable cultural heritage

Telecommunication
Power plants

Unacceptable loss of services to the public

Firework factory
Munitions works

Consequences of fire and explosion to the plant and its surroundings

Chemical plant
Refinery
Nuclear plant
Biochemical laboratories and plants

Fire and malfunction of the plant with detrimental consequences to the local and global environment

Table-1: Effects of lightning on typical structures






Step#2: Risk Assessment Study




A- What are the benefits from performing risk assessments study?

The benefits from performing the risk assessment study are to:

  • It provides the basis on which decisions can be made in order to limit the risks for a given structure.
  • It makes clear which risks should be covered by insurance.
  • It is used to Objectify and quantify the risk to buildings and structures, and their contents, as a result of direct and indirect lightning strikes.
  • Determine if lightning protection is required or not.
  •  if required, to select the appropriate lightning class which determines the minimum lightning protection level (LPL) that is used within the lightning protection design.


Important Notes:

  • There are some minor differences to the procedures, parameters and parameters’ values between national standards of different countries like IEC 62305-2, BS EN 62305-2 and NFPA 780 due to different lightning activity from country to country coupled with each country’s interpretation and perception of risk.
  • These differences occurred to better reflect the localized conditions and acceptable local tolerable risk. These differences will be highlighted in next articles.
  • The decision to provide lightning protection may be taken regardless of the outcome of risk assessment where there is a desire that there be no avoidable risk. Lightning protection can be installed even when the risk management process may indicate that it is not required. A greater level of protection than that required may also be selected.
  • Local regulations requirements, if any, may be applicable and have to be taken into account.





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:

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






Part#1: Evaluating Need of Lightning Protection

To evaluate the need of 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 of lightning protection.


Fig.2: Procedure for Evaluating Need of Lightning Protection









Part#2: Determination of Required Protection Measure


Repeat from step#2-1 to step#2-6.

Step#2-7 in above procedure: ignored.

Step#2-8: if the lightning protection measure is needed, then we have (3) cases:

Case#1: check if the risk components RA+RB +RU+RV> RT. if yes, Install an adequate type of LPS. Otherwise, install adequate type of LPMS. Then go back to step#2-4 to calculate new values of risk components and make a series of trial and error calculations until the risk Rn is reduced below that of RT (Rn ≤ RT). Otherwise go to case#2.


Case#2: If the structure under study had LPS installed but the Risks still need to be reduced, you will need to install LPMS. Then go back to step#2-4 to calculate new values of risk components and make a series of trial and error calculations until the risk Rn is reduced below that of RT (Rn ≤ RT). Otherwise go to case#3.


Case#3: If the structure under study had both LPS and LPMS installed but the Risks still need to be reduced, you will need to install other protection measures. Then go back to step#2-4 to calculate new values of risk components and make a series of trial and error calculations until the risk Rn is reduced below that of RT (Rn ≤ RT).

The other protection measures that can reduce and influence the values of the risk components are shown in Table-2:


Characteristics of structure or of internal systems Protection measures
RA
RB
RC
RM
RU
RV
RW
RZ
Collection area
X
X
X
X
X
X
X
X
Surface soil resistivity
X







Floor resistivity
X



X



Physical restrictions, insulation, warning notice, soil equipotentialization
X



X



LPS
X
X
X
Xa
Xb
Xb


Bonding SPD
X
X


X
X


Isolating interfaces


Xc
Xc
X
X
X
X
Coordinated SPD system


X
X


X
X
Spatial shield


X
X




Shielding external lines




X
X
X
X
Shielding internal lines


X
X




Routing precautions


X
X




Bonding network


X





Fire precautions

X



X


Fire sensitivity

X



X


Special hazard

X



X


Impulse withstand voltage


X
X
X
X
X
X
a Only for grid-like external LPS.
b Due to equipotential bonding.
c Only if they belong to equipment.
Table-2: Factors influencing the Risk Components

Notes:

  • In step#2-8, case#1: If RA+ RB < RT, a complete LPS is not necessary; in this case SPD(s) according to IEC 62305-3 are sufficient.
  • 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.3 shows this procedure for Determination of Required Protection measure.


Fig.3: Procedure for Determination of Required Protection Measure


Notes:

  • In any case, the installer or planner should identify the most critical risk components and reduce them, also taking into account economic aspects.
  • Selected Protection measures shall be considered effective only if they conform to the requirements of the national relevant standards which may be:

  1. IEC 62305-3 or BS EN 62305-3 for protection against injury to living beings and physical damage in a structure,
  2. IEC 62305-4 or BS EN 62305-4 for protection against failure of electrical and electronic systems,
  3. NFPA 780.


  • Where protection against lightning is required by the authority having jurisdiction for structures with a risk of explosion, at least a class (II) LPS should be adopted. Exceptions to the use of lightning protection level (II) may be allowed when technically justified and authorized by the authority having jurisdiction. For example, the use of lightning protection level (I) is allowed in all cases, especially in those cases where the environments or contents within the structure are exceptionally sensitive to the effects of lightning. In addition, authorities having jurisdiction may choose to allow lightning protection level (III) systems where the infrequency of lightning activity and/or the insensitivity of the contents of the structure warrants it.
  • When the damage to a structure due to lightning may also involve surrounding structures or the environment (e.g. chemical or radioactive emissions), additional protection measures for the structure and measures appropriate for these zones may be requested by the authorities having jurisdiction.






Part#3: evaluating the cost-effectiveness of protection measures

  • It may be beneficial to evaluate the economic benefits of providing a specified protection measure to establish if lightning protection is cost effective. This can be assessed by evaluating R4: risk of loss of economic value. R4 is not equated to a tolerable level risk RT but compares, amongst other factors, the cost of the loss in an unprotected structure to that with protection measures applied (There is no tolerable risk RT, but rather a cost-benefit analysis).
  • The procedure for performing Part#3: evaluating the cost-effectiveness of protection measures, will be explained later in next Articles after finishing parts#1 &2.





In the next Article, I will continue explaining Step#2: Risk Assessment Study. Please, keep following.




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