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Top of FormBottom of FormDesign 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 commonlybroken into discrete phases, allowing the lightning protection designer to present an integrated design package. These phases can be listed as follows:Planning phase,Consultation phase,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 ContinuedThird: 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- TheCharacteristics 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:

The design of the building,The environment around the building,The material in the building,The number of lightning strikes to earth in the area of the building,The value of the building and its contents,Sensitive electronics in the building,Loss of revenue In the event of breakdown,Escape facilities and number of staff in the building,Fire protection in the building,The historical and cultural value of the building,The social function of the building,Cable laying up to the building,Conductivity of the ground in the area of the building,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 areAlso 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 ContentsEffects Of Lightning

Dwelling-housePuncture of electrical installations, fire and material damageDamage normally limited to structures exposed to the point of strike or to the lightning current pathFailure of electrical and electronic equipment and systems installed (e.g. TV sets, computers, modems, telephones, etc.)

Farm buildingPrimary risk of fire and hazardous step voltages as well as material damageSecondary 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.

TheatreHotelSchoolDepartment storeSports area

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

BankInsurance companyCommercial company, etc.

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

HospitalNursing homePrison

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

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

Museums and archaeological siteChurch

Loss of irreplaceable cultural heritage

TelecommunicationPower plants

Unacceptable loss of services to the public

Firework factoryMunitions works

Consequences of fire and explosion to the plant and its surroundings

Chemical plantRefineryNuclear plantBiochemical 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 countrys 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 perIEC 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 = 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 forEvaluatingNeed 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 measuresRARBRCRMRURVRWRZ

Collection areaXXXXXXXX

Surface soil resistivityX

Floor resistivityXX

Physical restrictions, insulation, warning notice, soil equipotentializationXX

LPSXXXXaXbXb

Bonding SPDXXXX

Isolating interfacesXcXcXXXX

Coordinated SPD systemXXXX

Spatial shieldXX

Shielding external linesXXXX

Shielding internal linesXX

Routing precautionsXX

Bonding networkX

Fire precautionsXX

Fire sensitivityXX

Special hazardXX

Impulse withstand voltageXXXXXX

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:

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

HomePDF CoursesElectrical CoursesBeginners' Electrical Design CourseBasic Electrical Design Course Level IBasic Electrical Design Course Level IIBasic Electrical Design Course Level IIIGrounding System Design Calculations CourseLightning-1: Introduction to Lightning Protection System DesignLightning-2: Lightning Protection System Design and CalculationsIntroduction to Lighting Design CourseAdvanced Course for Lighting Design - Level IIntroduction to Electrical Motors Basics CourseIntroduction to Elevators CourseIntroduction to HVAC Systems CourseTransformers CourseElectrical Drawings CourseTender Documents Preparation CourseUnderstanding NFPA 70 CourseBasic Sound System Design CourseDownload LibraryBooksSoftware ProgramsElectrical Calculations SpreadsheetsElectrical Drawings DetailsCheck ListsElectrical Work's Method StatementsDesign CriteriaInspection CoursesGrounding System Inspection CourseConductors Inspection CourseConduits and Boxes Inspection CourseFluorescent and Incandescent Light Fixtures Inspection CourseQuiz and AnswerElectricity Today

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Top of FormBottom of FormDesign Calculations of Lightning Protection Systems Part One

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:Planning phase,Consultation phase,Detailed Design phase.

Today, I will explain Design Calculations of Lightning Protection Systems.

Design Calculations of Lightning Protection Systems

1- 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:

Sources and Types of Damage to a Structure,Types of Loss,Types of Risks Associated with Losses,Lightning Protection Levels (LPL),Lightning Protection Zones (LPZ),Class of LPS,Protection Measures.

1.1 Sources and Types of Damage to a Structure

There is an initial focus on the damage that can be caused by lightning. Damage to a structure is subdivided into:

Sources of Damage,Types of Damage.

1.1.A Sources of Damage (see Fig.1)

Fig.1: Sources of Damage

The lightning current is the source of damage. The following situations shall be taken into account, depending on the position of the point of strike relative to the structure considered:

S1: flashes to the structure;S2: flashes near the structure;S3: flashes to the lines connected to the structure;S4: flashes near the lines connected to the structure.

S1: Flashes to the structure can cause:

Immediate mechanical damage, fire and/or explosion due to the hot lightning plasma arc itself, due to the current resulting in ohmic heating of conductors (over-heated conductors), or due to the charge resulting in arc erosion (melted metal);Fire and/or explosion triggered by sparks caused by overvoltages resulting from resistive and inductive coupling and to passage of part of the lightning currents;Injury to living beings by electric shock due to step and touch voltages resulting from resistive and inductive coupling;Failure or malfunction of internal systems due to LEMP.

S2: Flashes near the structure can cause:

Failure or malfunction of internal systems due to LEMP.

S3: Flashes to a line connected to the structure can cause:

Fire and/or explosion triggered by sparks due to overvoltages and lightning currents transmitted through the connected line;Injury to living beings by electric shock due to touch voltages inside the structure caused by lightning currents transmitted through the connected line;Failure or malfunction of internal systems due to overvoltages appearing on connected lines and transmitted to the structure.

S4: Flashes near a line connected to the structure can cause:

Failure or malfunction of internal systems due to overvoltages induced on connected lines and transmitted to the structure.

Notes:

Only the sparks carrying lightning current (total or partial) are regarded as able to trigger fire.Lightning flashes, direct to or near the incoming pipelines, do not cause damages to the structure, and provided that they are bonded to the equipotential bar of the structure (see IEC 62305-3).

1.1.B Types of damage

Each source of damage may result in one or more of three types of damage as follows:

D1: injury to living beings by electric shock;D2: physical damage (fire, explosion, mechanical destruction, chemical release) due to lightning current effects, including sparking;D3: failure of internal systems due to LEMP.

The damage to a structure due to lightning may be limited to a part of the structure or may extend to the entire structure. It may also involve surrounding structures or the environment (e.g. chemical or radioactive emissions).

1.2 Types of Loss

Loss LX mean amount of loss (humans and goods) consequent on a specified type of damage due to a dangerous event, relative to the value (humans and goods) of the structure to be protected.

While a Dangerous event means lightning flash to or near the structure to be protected, or to or near a line connected to the structure to be protected that may cause damage

Each type of damage relevant to structure to be protected, alone or in combination with others, may produce different consequential loss. The type of loss that may appear depends on the characteristics of the structure itself.The following types of loss, which may appear as consequence of damages relevant to structure, are considered:

L1: loss of human life (including permanent injury);L2: loss of service to the public;L3: loss of cultural heritage;L4: loss of economic value (structure, its content, and loss of activity).

Notes:

For the purposes of IEC 62305, only utilities such as gas, water,TV, TLC and power supply are considered service to the public.Losses of type L1, L2 and L3 may be considered as loss of social values, whereas a loss of type L4 may be considered as purely an economic loss.L4 relates to the structure and its contents; to the service and the loss of activity, due to the loss. Typically, loss of expensive and critical equipment that may be irretrievably damaged due to the loss of the power supply or data/telecom line. Similarly the loss of vital financial information for example that could not be passed onto clients of a Financial institution due to damage, degradation or disruption of internal IT hardware caused by lightning transients.

The relationship between source of damage, type of damage and loss is reported in Fig.2.

Fig.2:Relationship between Source of damage, Type of damage and Loss

1.3 Types of Risks Associated with Losses

Risk R: is the value of probable average annual loss (humans or goods) due to lightning, relative to the total value (humans or goods) of the structure to be protected.

Fig.3:Types of Loss and corresponding Risks resulting

For each type of loss which may appear in a structure, the relevant risk shall be evaluated corresponding to their equivalent type of loss. The risks to be evaluated in a structure may be as follows: (see Fig.3)

R1: risk of loss of a human life (including permanent injury),R2: risk of loss of service to the public,R3: risk of loss of cultural heritage,R4: risk of loss of economic value.

R1: Risk of loss of human life:

It is by far the most important risk to consider, and as such the examples and subsequent discussions relating to IEC 62305-2 Risk management will focus largely on R1.

R2: Risk of loss of service to the public:

It may initially be interpreted as the impact/implications of the public losing its gas, water or power supply. However the correct meaning of loss of service to the public lies in the loss that can occur when a service provider (whether that be a hospital, financial institution, manufacturer etc) cannot provide its service to its customers, due to lightning inflicted damage.For example, a financial institution whose main server fails due to a lightning overvoltage occurrence will not be able to send vital financial information to all its clients. As such the client will suffer a financial loss due to this loss of service as they are unable to sell their product into the open market.

R3: Risk of loss of cultural heritage:

It covers all historic buildings and monuments, where the focus is on the loss of the structure itself.

R4: risk of loss of economic value:

It evaluates the economic benefits of providing protection to establish if lightning protection is cost effective.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.

Notes:

To evaluate risks, R, the relevant risk components (partial risks depending on the source and type of damage) shall be defined and calculated. (This will be explained later).Each risk, R, is the sum of its risk components. When calculating a risk, the risk components may be grouped according to the source of damage and the type of damage.Protection against lightning is required if the risk R (whether this be R1, R2 or R3) is greater than the tolerable risk RT. Conversely if R is lower than RT then no protection measures are required.

1.4 Lightning protection zones (LPZ)

Lightning protection zone LPZ are used to define the lightning electromagnetic environment. The zone boundaries of an LPZ are not necessarily physical boundaries (e.g. walls, floor and ceiling). The zones are areas characterized according to threat of direct or indirect lightning flashes and full or partial electromagnetic field. Protection measures such as LPS, shielding wires, magnetic shields and SPD determine lightning protection zones (LPZ).

Fig.4:LPZ defiend by an LPS -IEC 62305-3

Fig.5:LPZ defiend by an LPMS -IEC 62305-4

With respect to the threat of lightning, the following LPZs are defined (see Figures 4 and 5):

LPZ 0A zone where the threat is due to the direct lightning flash and the full lightning electromagnetic field. The internal systems may be subjected to full or partial lightning surge current;LPZ 0B zone protected against direct lightning flashes but where the threat is the full lightning electromagnetic field. The internal systems may be subjected to partial lightning surge currents;LPZ 1 zone where the surge current is limited by current sharing and by isolating interfaces and/or SPDs at the boundary. Spatial shielding may attenuate the lightning electromagnetic field;LPZ 2, ..., n zone where the surge current may be further limited by current sharing and by isolating interfaces and/or additional SPDs at the boundary. Additional spatial shielding may be used to further attenuate the lightning electromagnetic field.

A comparisonbetweenthe exposurethreatsfor eachLightningZone can be listed in Fig.6 in below:

Fig.6:Comparisonbetweenthe exposurethreatsfor eachLightningZone

Notes:

In general, the higher the number of an individual zone, the lower the electromagnetic environment parameters. LPZ 0 (Zero) is considered the lowest zone, LPZ 1, 2, 3, being respectively higher.It is the design and placement of the LPS that ensures the structure and internal contents are within an LPZ 0B zone.Internal systems are required be located within an LPZ 1 (or higher) zone. As seen fromFig.5, electrical/electronic equipment located in LPZ 1 (or higher) and connecting to external services (located in LPZ 0B or LPZ 0A) require surge protective devices to limit energy being conducted from zones exposed to direct lightning or full/partial electromagnetic fields or surge current.Non electrical services (e.g. water, gas, etc) meet this requirement by the application of the bonding requirements.As a general rule for protection, the structure to be protected shall be in an LPZ whose electromagnetic characteristics are compatible with the capability of the structure to withstand stress causing the damage to be reduced (physical damage, failure of electrical and electronic systems due to overvoltages).For most electrical and electronic systems and apparatus, information about withstand level can be supplied by manufacturer.

1.5 Lightning protection levels (LPL)

Lightning protection level LPL: is a number related to a set of lightning current parameters values relevant to the probability that the associated maximum and minimum design values will not be exceeded in naturally occurring lightning.

In the IEC 62305 series, (4) lightning protection levels are introduced and the design rules are based on the LPS being able to protect against maximum values (sizing efficiency) and minimum values (interception efficiency) of current.

The four lightning protection levels are:

LPL I, LPL II, LPL III and LPL IV.

LPL I offers the highest protection level (greatest level of protection), with LPL IV offering the lowest level of protection.

Fig.7 indicates for these lightning protection levels the maximum current expected and the probability that this may be exceeded. The probability of occurrence of lightning with minimum or maximum current parameters outside the range of values defined for LPL I is less than 2 %.

Fig.7

Note:The design must ensure that air-termination, conductor and earth termination size are sufficient to withstand the expected maximum current.

As the lightning downward leader approaches the ground or structure, the electric field increases to the point that the ground or structure launches an upward leader that may eventually intercept the downward leader. This is termed the striking distance (see fig.8). The larger the amount of charge carried by the lightning leader, the greater will be the distance at which this happens. The larger the charge of the leader, the larger the resulting lightning current. It is generally accepted that the striking distance r is given by:

r = 10 I 0.65

Where I is the peak current of the resulting stroke.

Fig.8:Striking Distance

For each of the lightning protection levels, a minimum current level to be protected against has been determined (selected). Fig.9 details these current levels, together with probability percentages that lightning may be greater than these levels.

Fig.9

For example:

LPL I positions terminals such that 99% of all lightning flashes are intercepted (all those of 3 kA or greater). There is only a 1% probability that lightning may be smaller than the 3 kA minimum, and may not be close enough to an air-terminal to be intercepted. It should be noted that flashes of less than 3 kA are rare, and typically would not be expected to cause damage to the structure. Protection greater than LPL I (99%) would require significantly more material, is not covered by the standard and generally is not required for commercial construction.

To further explainFig.9, a lightning protection system to provide LPL IV, designed using the rolling sphere method, would use air-terminals placed using a rolling sphere radius of 60 m.

These air-terminals would be positioned such that they would capture all lightning flashes of 16 kA or greater, thus offering protection to at least 84% of the lightning (the term at least is used to indicate that the percentage of lightning captured might be greater, since smaller lightning flashes could be captured if they were closer to the air-terminal).

To offer a greater lightning protection level (e.g. LPL I, II or III) a smaller rolling sphere radius would be used. This would result in a reduced spacing between air-terminals (more air-terminals), thus positioning the air-terminals to capture smaller lightning flashes, and increasing the total percentage of lightning flashes captured.

Notes:

The lower lightning protection levels (LPL II, III & IV) each increase the air-terminal spacing, reducing their ability to capture smaller lightning flashes, thus reducing overall the percentage of lightning events they can protect against.The maximum values of lightning current parameters for the different lightning protection levels are given in Fig.7 and are used to design lightning protection components (e.g. cross-section of conductors, thickness of metal sheets, current capability of SPDs, separation distance against dangerous sparking) and to define test parameters simulating the effects of lightning on such components.Lightning protection level is used to design protection measures according to the relevant set of lightning current parameters.The minimum values of lightning current amplitude for the different LPL are used to derive the rolling sphere radius in order to define the lightning protection zone LPZ 0B which cannot be reached by direct strike. The minimum values of lightning current parameters together with the related rolling sphere radius are given inFig.9.They are used for positioning of the air-termination system and to define the lightning protection zone LPZ 0B.The protection measures specified in IEC 62305-3 and IEC 62305-4 are effective against lightning whose current parameters are in the range defined by the LPL assumed for design. Therefore the efficiency of a protection measure is assumed equal to the probability with which lightning current parameters are inside such range. For parameters exceeding this range, a residual risk of damage remains.

1.6 Class of LPS

Class of LPS is a number denoting the classification of an LPS according to the lightning protection level for which it is designed power line or telecommunication line connected to the structure to be protected.

Four classes of LPS (I to IV), as shown inFig.10, are defined in this standardcorresponding to lightning protection levels defined in IEC 62305-1.

Fig.10:Relation between Lightning Protection Level (LPL) and Class of LPS

Each class of LPS is characterized by the following:

A- Data dependent upon the class of LPS:

Lightning parameters (see Tables 3 and 4 in IEC 62305-1:2010);Rolling sphere radius, mesh size and protection angle;Typical preferred distances between down-conductors;Separation distance against dangerous sparking;Minimum length of earth electrodes.

B- Factors not dependent upon the class of LPS:

Lightning equipotential bonding,Minimum thickness of metal sheets or metal pipes in air-termination systems,LPS materials and conditions of use,Material, configuration and minimum dimensions for air-terminations, down-conductors and earth-terminations,Minimum dimensions of connecting conductors.

The choice of what Class of LPS shall be installed is governed by the result of the risk assessment calculation. Thus it is prudent to carry out a risk assessment every time to ensure a technical and economic solution is achieved.

1.7 Protection Measures

Protection Measures are measures to be adopted for the structure to be protected in order to reduce the risk, according to the type of damage, in the event of a lightning strike to or near a structure or connected service.

For each type of loss, there is a number of protection measures which, individually or in combination, make the condition R RT. Lightning protection science include (3) types of protection measures as follows:(see fig.11)

LPS Protection Measures,LPMS Protection Measures,Other Protection Measures.

Fig.11:Protection Measures

Where:

LEMP: Lightning Electromagnetic Pulse,LPMS: LEMP Protection Measures System,LPS: Lightning Protection Measures System.

1- LPS Protection Measures:

It used to reduce physical damage, Protection is achieved by the lightning protection system (LPS) which includes the following features:

Air-termination system;Down-conductor system;Earth-termination system;Lightning equipotential bonding (EB);Electrical insulation (and hence separation distance) against the external LPS.

Notes:

When an LPS is installed, equipotentialization is a very important measure to reduce fire and explosion danger and life hazard.Provisions limiting the development and propagation of the fire such as fireproof compartments, extinguishers, hydrants, fire alarms and fire extinguishing installations may reduce physical damage.Protected escape routes provide protection for personnel.

2- LPMS Protection Measures:

It used to reduce failure of electrical and electronic systems, Possible protection measures (LPMS) include:

Earthing and bonding measures,Magnetic shielding against induced Lightning Electromagnetic Impulse (LEMP) effects,Careful planning in the routing of internal cables and the suitable location of sensitive equipment,Isolating interfaces,The correct installation of coordinated Surge Protection Devices (SPDs) which will additionally ensure continuity of operation.

Notes:

These measures in total are referred to as an LEMP Protection Measures System (LPMS).These LPMS Protection measures may be used alone or in combination.When source of damage S1 is considered, protection measures are effective only in structures protected by an LPS.The use of storm detectors and the associated provision taken may reduce failures of electrical and electronic systems.

3- Other Protection Measures:

It used to reduce injury of living beings by electric shock, other Possible protection measures include:

Adequate insulation of exposed conductive parts;Equipotentialization by means of a meshed earthing system;Physical restrictions and warning notices;Lightning equipotential bonding (EB).

Notes:

Equipotentialization and an increase of the contact resistance of the ground surface inside and outside the structure may reduce the life hazard.Protection measures are effective only in structures protected by an LPS.The use of storm detectors and the associated provision taken may reduce the life hazard.

Details of the methodology and criteria for deciding the most suitable protection measures are given in the Risk management study which will be explained in next Articles.

In the next Article, I will continue explaining Design Calculations of Lightning Protection System. Please, keep following.

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Course Lightning-2: Lightning Protection System Design and Calculations

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