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Home / Technical Articles / 5 most important aspects of external protection against the effects of lightning

Analysis of the lightning risk

Analysis of the lightning risk takes numerous factors into consideration. This article will shed the light on one of factors – protection of the structures and its five most important aspects: lightning conductors, electrogeometric model, capture surface areas, downconductors and of course earthing system.

5 most important aspects of external protection against the effects of lightning
5 most important aspects of external protection against the effects of lightning

The protection of installations and the electrical or electronic equipment is not considered here, only the most important aspects of external protection of structures:

Contents:

  1. Protection systems (lightning conductors)
    1. Single rod lightning conductors (Franklin rods)
    2. Lightning conductors with sparkover device
    3. Lightning conductors with meshed cage
    4. Lightning conductors with earthing wires
  2. The electrogeometric model
  3. Capture surface areas
  4. Downconductors
  5. Earthing system

1. Protection systems (lightning conductors)

The purpose of these is to protect structures against direct lightning strikes. By catching the lightning and running the discharge current to earth, they avoid
damage connected with the lightning strike itself and circulation of the associated current.

Lightning conductors are divided into four categories:

Single rod lightning conductors (Franklin rods)
Single rod lightning conductors (Franklin rods)

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1.1. Single rod lightning conductors (Franklin rods)

These consist of one or more tips, depending on the size of the structure and the downconductors.

They are connected either directly to the earthing electrode of the installation (foundation), or, depending on the type of protection and national work practices, to a special earthing electrode (lightning conductor earthing electrode) which is itself connected to the earth of the installation.

Franklin rod
Figure 2 – Franklin rod

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1.2. Lightning conductors with sparkover device

These are a development of the single rod. They are equipped with a sparkover device which creates an electric field at the tip, helping to catch the lightning and improving their effectiveness.

Several lightning conductors can be installed on the same structure. They must be interconnected as well as their earthing electrodes.

Lightning conductors with sparkover device
Figure 3 – Lightning conductors with sparkover device

For buildings more than 60 m high which are protected by single rod lightning conductors or lightning conductors with sparkover devices, the protection system is completed by a metal ring at the top to avoid the risk of lateral lightning strikes.

Another lightning conductor with spark-over device on top
Figure 4 – Another lightning conductor with sparkover device on top

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1.3. Lightning conductors with meshed cage

The meshed cage consists of a network of conductors arranged around the outside of the building so that its whole volume is circumscribed. Catcher rods (0.3 to 0.5 m high) are added to this network at regular intervals on projecting points (rooftops, guttering, etc.).

All the conductors are interconnected to the earthing system (foundation) by downconductors.

Lightning conductors with a meshed cage complete the meshing systems
Figure 5 – Lightning conductors with a meshed cage complete the meshing systems to protect buildings against the radiated electromagnetic fields to which they must be interconnected

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1.4. Lightning conductors with earthing wires

This system is used above certain buildings, outdoor storage areas, electric lines (overhead earth wire), etc. The electrogeometric model of the sphere applies to these.

Transmission tower with ground conductor
Figure 6 – Transmission tower with ground conductor

As the the installation of lightning conductors considerably increases the risk of overvoltages, voltage surge protectors must also be used. According to standard IEC 60364, a class I voltage surge protector (min. Imp 12.5 kA – waveform 10/350) is required at the origin of the installation.

This value can be specified by a risk analysis if necessary (IEC 62305 or similar).

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2. The electrogeometric model

The choice and positioning of lightning capture devices requires a specific study of each site, the objective being to ensure that the lighting will preferably “fall” at one of the predefined points (lightning conductors) and not some other part of the building.

There are various methods for doing this, depending on the type of capture device (lightning conductor) and national work practices (see IEC 62305).

One of these, called the “electrogeometric model” (or imaginary sphere model) method, defines the spherical volume that is theoretically protected by a lightning conductor according to the intensity of the discharge current of the first arc.

General principle of the electrogeometric model
Figure 7 – General principle of the electrogeometric model

The higher this current, the higher the probability of capture and the wider the protected area.

The tip of the leader stroke (or precursor) is deemed to represent the center of an imaginary sphere, with a radius D. This sphere follows the random path of the leader stroke.

The first element to come into contact with this sphere will determine the point at which the lightning will strike: a tree, a roof, the ground or a lightning conductor, if there is one. Beyond the points of tangency of this sphere, protection is no longer provided by the lightning conductor.

The theoretical radius (D) of the sphere is defined by the relationship: D = 10 × I2/3, where D is in metres and I is in kA.

Table 1 – Theoretical radius (D) of the sphere and lightning current values

D (m) 15 29 46 96 135 215
I (kA) 2 5 10 30 50 100

For optimum protection incorporating the probable lowest lightning current values (protection level I), a 20 m (I = 2.8 kA) sphere must be considered.


Protection levels (IEC 62305)

The model must be adapted according to the type of protection device (single rod lightning conductor, meshed cage, earthing wires) and structure to be protected.

Standard IEC 62305 defines protection volumes according to four protection levels based on the probability of capture:

Table 2 – Protection volumes according to four protection levels

Level I II II IV
Probability of capture (%) 99 97 91 84
Min. capture current (kA) 3 5 10 16
Max. sparkover distance (m) 20 30 45 60

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3.Capture surface areas

When the site to be protected consists of several buildings or extends beyond the range of a single capture device (lightning conductor), a protection plan must be drawn up for the area, juxtaposing the different theoretical capture surface areas.

It is always difficult to achieve total coverage of a site when it is made up of structures of different heights.

Superimposing the protection plan over the layout of the area makes it possible to see areas that are not covered, but above all it must assist in-depth consideration taking account of:

  1. The probability of lightning strikes by determining the main strike points (towers, chimneys, antennae, lamp posts, masts, etc.)
  2. The sensitivity of the equipment housed in the buildings (communication and computer equipment, PLCs, etc.)
  3. The potential risk linked to the business or the types of material stored (fire, explosion, etc.)
It must also be remembered that the numerous links between the buildings (computer networks, remote monitoring, communications, alarms and power) can create interference as a result of the effect of the lightning’s electromagnetic field or that of the voltage gradient generated in the ground.

There are two ways in which these links can be protected:

WAY #1 – Shielding or use of Faraday cages which will, as well as protecting against these fields, primarily maintain the equipotentiality of the link (adjacent earthing conductor, twisting, conductor screen, etc.)

WAY #2 – Galvanic decoupling, which will separate buildings electrically (optocouplers, fibre optics, isolation transformers, etc).

The protection plan must take the buildings and structures to be protected against direct lightning strikes into consideration, but it must also take into account elements or non-built areas for which lightning strikes may cause destructive effects.

Example of a protection plan
Figure 8 – Example of a protection plan

On this (imaginary) site we can see that the sensitive areas: manufacturing, storage, processing etc., have been protected effectively by lightning conductors or by a meshed cage, but that two areas are not protected, as they are considered to be low-risk: reception area and car park.

Further consideration shows that the lamp posts lighting the car park could be struck by lightning and transmit the lightning strike to the installation, and that the reception area which houses the telephone switchboard and the paging aerial (beep) represents an area which is both vulnerable and sensitive.

The pumping station is theoretically protected by the silo lightning conductors which are much higher. A situation which must not however allow us to forget that in this case a sideways lightning strike is possible.

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4. Down-conductors

These provide the link between the lightning conductor itself (rod, cage, wire) and the earthing electrode. They are subjected to intense currents and must therefore be of an adequate cross-section (min. 50 mm2 copper), flat (HF current), firmly fixed and follow the shortest possible route.

They must have no rises or sharp angles. The conductors can be fitted with lightning strike counters.

It is advisable to increase the number of downconductors in order to reduce the currents in each one and the associated thermal, electrodynamic and inductive effects. Downconductors must end in a meshed, equipotential earth circuit.

The consequences in the installation of the effects caused by circulation of the lightning current in the downconductors can be minimized by:

  • Increasing the number of downconductors in order to divide the current and limit the effects caused.
  • Ensuring that the downconductors are interconnected with the bonding systems on all floors in the building.
  • Creating equipotential bonding systems incorporating all conductive elements, including those that are inaccessible:
    • fluid pipes,
    • protection circuits,
    • reinforcements in concrete,
    • metal frames, etc.
  • Avoiding placing downconductors near sensitive areas or equipment (computing, telecommunications, etc.).
Interconnection of downconductors with the bonding systems in buildings
Figure 9 – Interconnection of downconductors with the bonding systems in buildings

In buildings with several floors, it is recommended that the lightning conductor downconductor(s) are connected to the bonding systems on each floor.

If this is not done, the voltage difference that occurs between the downconductors and the internal exposed conductive parts could cause a sparkover through the walls of the building.

The circulation of the HF lightning current may in fact cause a significant voltage rise in the downconductor (several hundred kV) due to the increase in its high frequency impedance.


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5. Earthing system

This is an essential element in protection against lightning: all the exposed conductive parts, which are themselves interconnected, must be connected, and the system must be capable of discharging the lightning current, avoiding a voltage rise in the earthing system itself and the surrounding ground.

Although it must be low enough (< 10 Ω), the low frequency resistance value of the earthing electrode is less important than its shape and size as far as the discharge of the high frequency lightning current is concerned.

As a general rule, each downconductor must end in an earthing electrode which can consist of conductors (at least three) in a crow’s foot layout buried at least 0.5 m deep, or earth rods, preferably in a triangular layout.

In addition, IEC 62305 implies that the lightning conductor downconductors should be interconnected with the bonding system with the main equipotential link.

When possible, it is always advisable to increase the number of downconductors and linking points (each floor), and thus to increase the overall scale of the equipotential bonding system. At the same time as this, the earthing system must of course be capable of discharging the lightning currents in order to limit the voltage rise of the bonding system as much as possible.

There must only be one earthing system.

Separate, independent circuits (power, computers, electronic, communications) should be prohibited, but this does not exclude multiple earthing electrodes (electrodes) if they are all interconnected.

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Source: Protection against lightning by Legrand

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author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

8 Comments


  1. Shankar Neerukonda
    Mar 07, 2019

    Hello
    In the section of the earthing system(5), you told this “As a general rule, each down conductor must end in an earthing electrode which can consist of conductors (at least three) in a crow’s foot layout buried at least 0.5 m deep, or earth rods, preferably in a triangular layout”.
    You wrote that we can have more than one earthing rods, if that is the case, there can be a potential difference between those three rods buried underneath which result in the increase of step voltage and touch voltage which is not preferable. As we know that when lightning occurs there can be a voltage of at least 10000V in the vicinity of its strike within a space of 1m.

    Why don’t we place an equipotential bonding conductor and connect that to Earthing rod?
    Please explain if my understanding is wrong.
    Thank you.


  2. Walter Linggi
    Mar 01, 2019

    Great article, Thanks for the detailed explanation including reference to interference as a result of the effect of the lightning’s electromagnetic field or that of the voltage gradient generated in the ground (GPR).

    For over 40 years we have been providing state-of-the art line isolation units to protect communication equipment from GPR, EMP and Nuclear EMP.

    Most telephone wiring entering structures, only use surge suppressors for lightning protection, this is opening a reverse path in a GPR event, setting your communication equipment and personnel at risk.

    The situation might be different, when there is something more expensive than ordinary telephone connected to the line. For example, expensive computer systems are usually worth to protect, because the damage caused by the lightning strike can cause very expensive damage. For example in PC case, lightning strike can not only destroy the modem (which is not usually very expensive), but also something else inside of the PC. That can become very expensive if valuable information is lost and the PC is very important at your business.

    Walter, http://www.telbit.ch – telby® Universal Broadband Isolation Transformer 10kV 15kV 20kV


  3. Asamoah Boateng
    Feb 28, 2019

    In fact I don’t know the words I could write to explain how I cherish the way u teach me in electricals I like that keep it up


  4. K THIRUMAL RAO
    Feb 28, 2019

    Thanks for the detailed explanation on Lightning protection.
    I seek some input on the following, in some of IT buildings, the out put neutral of the UPS is grounded to through independent copper earth electrodes ( generally multiple electrodes of various UPS of client are connected together and again connected to general protective earth); As general protective earth and UPS Neutral as well as the lightning earth pits are connected to common ground of the facilty, will it impose higher voltage on the electronic equipment ( UPS) during lightning.


  5. Prabhakar Samal
    Feb 28, 2019

    thanks for information.


  6. Marifatullah Farooqi
    Feb 28, 2019

    Thank you from your useful articles , information and other educational …. , these very beneficial thank you again from your hard work which improves educational background.

    Regards
    Marifatullah Farooqi from Kandahar Afghanistan


  7. James O'Connor
    Feb 27, 2019

    Love your work,

    James

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