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.
- Protection systems (lightning conductors)
- The electrogeometric model
- Capture surface areas
- Earthing system
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:
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.
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.
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.
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.
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.
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).
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.
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 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
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
|Probability of capture (%)||99||97||91||84|
|Min. capture current (kA)||3||5||10||16|
|Max. sparkover distance (m)||20||30||45||60|
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:
- The probability of lightning strikes by determining the main strike points (towers, chimneys, antennae, lamp posts, masts, etc.)
- The sensitivity of the equipment housed in the buildings (communication and computer equipment, PLCs, etc.)
- The potential risk linked to the business or the types of material stored (fire, explosion, etc.)
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.
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.
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.
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.).
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.
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.
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.
Source: Protection against lightning by Legrand