Lightning Protection System
The purpose of a lightning protection system is to protect buildings from direct lightning strikes and possible fire or from the consequences of lightning currents (non-igniting flash). If national regulations such as building regulations, special regulations or special directives require lightning protection measures, they must be implemented.
If these regulations do not specify a class of LPS, a lightning protection system which meets the requirements of the class of LPS III according to IEC 62305-3 (EN 62305-3) is recommended as a minimum. In principle, a risk analysis, which is described in the IEC 62305-2 (EN 62305-2) standard should be performed for an overall assessment.
A lightning protection system must be installed even if only one of the requirements is met. A lightning strike can have particularly serious consequences for structures due to their location, type of construction or use.
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Places where a Lightning Protection System is Required
Different types of lightning protection systems should be designed for different types of locations. Requirements for different locations are not same. Structures where a lightning protection system must be typically installed because, in these cases, the law has affirmed the need, are:
- Places of public assembly with stages or covered areas and places of public assembly for showing films if the associated assembly rooms, individually or together, accommodate more than 200 visitors;
- Places of public assembly with assembly rooms which, individually or together, accommodate more than 200 visitors; in case of schools, museums and similar buildings, this regulation only applies to the inspection of technical installations in assembly rooms which individually accommodate more than 200 visitors and their escape routes;
- Sales areas with salesrooms of more than 2000 square meters of floor space;
- Shopping streets with several sales areas which are connected to each other either directly or via escape routes and whose salesrooms individually have less than 2000 square meters of floor space and have a total floor space of more than 2000 square meters;
- Exhibition areas whose exhibition rooms, individually or together, have more than 2000 square meters of floor space;
- Restaurants with more than 400 seats or hotels with more than 60 beds;
- High-rise buildings (depending on the federal state);
- Hospitals and other structures of a similar purpose;
- Medium-sized and large-scale garages (depending on the federal state);
- Containing explosives, such as ammunition factories, ammunition and explosive stores,
- Containing hazardous locations such as varnish and paint factories, chemical factories, large warehouses containing flammable liquids and large gas tanks,
- Particularly at risk of fire such as
– Large woodworking factories,
– Buildings with thatched roofs,
– Warehouses and production facilities with a high fire load,
- For a large number of persons such as
– Homes for the elderly and children´s homes,
– Correctional facilities
– Railway stations,
- With cultural heritage such as
– Buildings of historic interest,
– Museums and archives,
- Protruding above their surroundings such as
– High chimneys,
– High buildings.
Lightning Formation and Discharge
When warm air masses containing sufficient moisture are transported to great altitudes, then the thunderstorms come into existence.
This transport can occur in a number of ways. In the case of heat thunderstorms, the ground is heated up locally by intense insolation. The air layers near the ground heat up and rise.
For frontal thunderstorms, the invasion of a cold air front causes cooler air to be pushed below the warm air, forcing it to rise.
Orographic thunderstorms are caused when warm air near the ground is lifted up as it crosses rising ground. Additional physical effects further increase the vertical upsurge of the air masses.
This forms up draught channels with vertical speeds of up to 100 km/h, which create towering cumulonimbus clouds with typical heights of 5 to 12 km and diameters of 5 to 10 km. Electrostatic charge separation processes, e.g. friction and sputtering, are responsible for charging water droplets and particles of ice in the cloud.
Positively charged particles accumulate in the upper part and negatively charged particles in the lower part of the thundercloud. In addition, there is again a small positive charge center at the bottom of the cloud.
This originates from the corona discharge which emanates from sharp-pointed objects on the ground underneath the thundercloud (e.g. plants) and is transported upwards by the wind.
If the space charge densities, which happen to be present in a thundercloud, produce local field strengths of several 100 kV/m, leader discharges are formed which initiate a lightning discharge.
Cloud-to-cloud flashes result in charge neutralization between positive and negative cloud charge centers and do not directly strike objects on the ground in the process.
The lightning electromagnetic impulses (LEMP) they radiate must be taken into consideration, however, because they endanger electrical and electronic systems. Flashes to earth lead to neutralization of charge between the cloud charges and the electrostatic charges on the ground.
We distinguish between two types of lightning flashes to earth:
- Downward flash (cloud-to-earth flash)
- Upward flash (earth-to-cloud flash)
Downward and Upward Flashes
In case of downward flashes, leader discharges pointing towards the ground guide the lightning discharge from the cloud to the earth. Such discharges usually occur in flat terrain and near low buildings.
Cloud-to-earth flashes can be recognized by the branching which is directed to the earth. The most common type of lightning is a negative downward flash where a leader filled with a negative cloud charge pushes its way from the thundercloud to earth.
This leader propagates as a stepped leader with a speed of around 300 km/h in steps of a few 10 m. The interval between the jerks amounts to a few 10 μs.
When the leader has drawn close to the earth (a few 100 m to a few 10 m), it causes the strength of the electric field of objects on the surface of the earth in the vicinity of the leader (e.g. trees, gable ends of buildings) to increase.
The increase is great enough to exceed the dielectric strength of the air. These objects involved reach out to the leader by growing positive streamers which then meet up with the leader, initiating the main discharge.
Positive downward flashes can arise out of the lower, positively charged area of a thundercloud. The ratio of the polarities is around 90 % negative lightning to 10 % positive lightning. This ratio depends on the geographic location.
On very high, exposed objects (e.g. wind turbines, radio masts, telecommunication towers, steeples) or on the tops of mountains, upward flashes (earth-to-cloud flashes) can occur. It can be recognized by the upwards-reaching branches of the lightning discharge.
In the case of upward flashes, the high electric field strength required to trigger a leader is not achieved in the cloud, but rather by the distortion of the electric field on the exposed object and the associated high strength of the electric field.
From this location, the leader and its charge channel propagate towards the cloud. Upward flashes occur with both negative polarities and with positive polarity.
Since, with upward flashes, the leaders propagate from the exposed object on the surface of the earth to the cloud, high objects can be struck several times by one lightning discharge during a thunderstorm.
Depending on the type of flash, each lightning discharge consists of one or more partial lightning strikes. We distinguish between short strokes with a duration of less than 2 ms and long strokes with a duration of more than 2 ms.
Further, distinctive features of partial lightning strikes are their polarity (negative or positive) and their temporal position in the lightning discharge (first, subsequent or superimposed). The possible combinations of partial lightning strikes are shown in images above for downward flashes and upward flashes.
Lightning Surge Protection Devices
SPDs (Surge Protection Devices) and UPS devices are common protection devices used in industry. But, truth is no device can provide full guarantied protection from lightning strike. All these devices can provide protection from regular power surges and a far stroked lightning surge. But, when lightning is very close to the SPD these devices can’t tolerate. They go off.
SPDs are connected as one end to be ground and other end to the circuit. Operation of an SPD is, during normal operations it act as a high resistant device and when a huge current comes in to circuit it reduces it resistance. So low resistive path make major part of current to flow through ground.
Lightning current are in ranges of 100k. S0 say 99% of current were sent off to ground and only 1% left through circuit. Still it is 100Amps where it is considerable high current on circuit. So, SPDs won’t provide guarantee full circuit protection all the time. Not only that, but not even a full-fledged lightning protection system with rods, cables and grounds will guarantee against damage.
Sources of Lightning Damages
The actual sources of damage are lightning strikes that are subdivided into four groups depending on the point of strike,
S1 – Direct lightning strike to a structure
S2 – Lightning strike near a structure
S3 – Direct lightning strike to an incoming line
S4 – Lightning strike near an incoming line
Types of Lightning Damages
We have seen sources of damages may result in different types of damage which cause the loss. The standard specifies three types of damage:
D1 – Injury to living beings by electric shock as a result of touch and step voltage
D2 – Fire, explosion, mechanical and chemical reactions as a result of the physical effects of the lightning discharge
D3 – Failure of electrical and electronic systems as a result of surges
Types of Lightning Losses
Depending on the type of construction, use and substance of the structure, the relevant loss can be very different.
IEC 62305-2 (EN 62305-2) specifies the following four types of loss:
L1 – Loss of human life (injury to or death of persons);
L2 – Loss of service to the public;
L3 – Loss of cultural heritage;
L4 – Loss of economic value.
Table 1: Sources of damage, types of damage and types of loss depending on the point of strike
Components of Lightning Protection System
As per IEC/EN 62305 following are the components of lightning protection system
- Air Termination System
- Down Conductor System
- Earth Termination System
- Separation Distances
- Lightning Equipotential Bonding
A lightning protection system consists of an external and an internal lightning protection system
- The functions of the external lightning protection system are:
- To intercept direct lightning strikes via an air-termination system
- To safely conduct the lightning current to the ground via a down-conductor system
- To distribute the lightning current in the ground via an earth-termination system
- The function of the internal lightning protection system is:
- To prevent dangerous sparking inside the structure. This is achieved by establishing equipotential bonding or maintaining a separation distance between the components of the lightning protection system and other electrically conductive elements inside the structure.
Lightning equipotential bonding reduces the potential differences caused by lightning currents. This is achieved by connecting all isolated conductive parts of the installation directly by means of conductors or surge protective devices (SPDs).
What does the Lightning protection system do?
The main objective of the Lightning protection system is to facilitate the shortest, low impedance and safe path such that lightning voltage or current successfully discharge through it. Hence, building or structure safety is ensured when lightning hits it directly.
In addition to this, there are some points to consider which a lightning protection system do and do not do:
- They don’t attract lightning.
- They don’t and cannot dissipate or prevent lightning by ‘draining’ a storm of its charge.
- Most of them don’t offer surge protection for sensitive electronics.
- They offer fire protection and structural damage protection by preventing a hot, explosive lightning channel from passing through building materials.
Working of Lightning Protection System
The nature of lightning strike is it always find shortest impedance path to discharge to the ground. If that structure or building is not applied with a proper lightning protection system then lightning tends to discharge through any conductor available on the structure or building.
The discharge path may be arial cables, electrical lines, steel bars inside the structure, water pipes or gas pipes. Sometimes lightning will jump through the air via a side flash to reach a better-grounded conductor.
If lightning discharge through one or more of the above paths, this may lead to severe damages. Sometimes life losses will occur.
- Damage to building materials: The steel bars inside the concrete structure may conduct lightning through them. They can blow out concrete poles, cement slabs, walls, fragment concrete and plaster, and shatter nearby glass.
- Fire: Lightning may introduce fire that can start from anywhere it contacts, penetrates or comes near flammable material (wood, paper, gas pipes, etc) in a building – including structural lumber or insulation inside walls and roofs. If lightning travels through electric conductors in the building it may get overheated and get burn. This may introduce fire hazards on buildings.
- Side flashes: Side flashes may jump across air, possibly injuring any living body (Animal or Human). They can also lit gasoline which may be in a garage.
- Burn electrical appliances: Any electrical appliances plugged into an affected circuit may be damaged beyond repair. Electronic devices and computers are especially vulnerable. Examples are televisions, VCRs, microwaves, phones, washers and lamps, etc…
Lightning protection system couldn’t prevent the lightning strike, but they can defend the damage which would be occurred. They create a (low resistive,) better, safer path to ground.
The air terminals, cables and ground rods work together to carry the lightning currents away from the structure, preventing fire and most appliance damage.