Design 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:

  1. Planning phase,
  2. Consultation phase,
  3. 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:

  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.





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:

  1. Sources of Damage,
  2. 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 comparison between the exposure threats for each Lightning Zone can be listed in Fig.6 in below:


Fig.6: Comparison between the exposure threats for each Lightning Zone


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 from Fig.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 explain Fig.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 in Fig.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 in Fig.10, are defined in this standard corresponding 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)

  1. LPS Protection Measures,
  2. LPMS Protection Measures,
  3. 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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