Protection against effects of lightning

The principle consists in creating one or more preferred impact points for a lightning strike using low impedance, conductor elements. These then conduct and dissipate the lightning current into the ground. This coherent system enables the lightning to be captured and dissipated whilst providing protection to the structure. There are five types of lightning protection systems for protecting structures against lightning:

• Rods,
• Meshed conductors,
• Catenary wires,
• Early streamer emission lightning protection system,
• Protection by «natural» components,

Different types of protection

Typical example

Protection of a pylon using an Air-Termination Rod

Air-Termination Lightning Protection System

Benjamin Franklin invented the Lightning Rod in 1753. This lightning conductor is made up of a 2 to 8 m high tapered metal rod that dominates the structure to be protected and which is connected to minimum two down conductors and two earthing system.

As the protection radius of this type of Air-Termination Rod is limited to around 30 meters environ (Lightning Protection Level = IV, height = 60 meters), it is normally only used to protect small structures or zones such as pylons, chimneys, tanks, water towers, aerial masts, etc...

Meshed Conductors Lightning Protection System

Typical example

Protection of a building using a mesh cage lightning conductor

This lightning protection, derived from the Faraday cage, consists of meshed conductors that cover the roof and walls of the structure to be protected.

Air terminals are positioned around the edge of the roof and on high points. A network of conductors follows the external perimeter of the roof. This network is completed with transverse elements. The mesh size is between 5 and 20 meters according to the effectiveness required.

The top of the down conductors fitted to the walls are connected to the roof mesh, and the bottom to dedicated earthing systems. The distance between two down conductors is between 10 and 20 meters according to lightning protection level required.

The largest part of lightning current is conducted and dissipated through the conductors and earthing systems closest to the point of impact of the lightning strike.

Catenary wire lightning conductor

Typical example

Protection of an external explosive storage area by a catenary wire lightning conductor

This lightning protection system, using a similar principle to that of the mesh cage, consists of a mesh of conductors, but at a distance from the structure to be protected. The aim is to avoid the lightning current coming directly into contact with the structure.

Catenary wire conductors placed above the structure to be protected are connected to down conductors and dedicated earthing systems. The size of the mesh and the distance between down conductors are subjected to the same rules as for the meshed conductors lightning protection system.

This protection requires that additional mechanical studies (resistance of materials for masts, qualifying ground pressure, resistance to wind and weather condition, etc.) be carried out and insulation distances defined.

The catenary wire lightning conductor is particularly used to protect open areas when there is no architectural support or hazardous storage.

Protection using «natural» components

Components that have a lightning protection function but that were not installed for this purpose.
Comment: these are conducting parts of a structure or building that are able to participate in the external protection through their capacity to capture a lightning strike or to conduct lightning current. They can be used to replace all or part of a down conductor or in addition to an external installation.

These components may be made up of:

• The frame of metal constructions;
• Metal coatings of walls or metal cladding
• Sheet metal covering the volume to be protected, provided there is no risk of them being perforated by an impact
• Metal components of a roof structure (interconnected steel frames, etc.), even if covered with non-metallic materials, provided that these may be excluded from the volume to be protected
• Metal rods in reinforced concrete, provided that there is electrical interconnection between them, and particularly with the capture means and earthing system
• Metal parts such as gutters, decoration, guardrails, etc, provided that their cross-section is not less than that specified for normal components
• Metal pipes and tanks, provided they are at least 2.5 mm thick and if perforated, do not cause a dangerous or unacceptable situation

These elements must comply with thickness, cross-section and continuity requirements, thus making their use a difficult matter.

Early streamer emission lightning conductor

The principle of an early streamer emission lightning conductor is to artificially generate, with the aid of an ionization device, an early upward leader occurring before the other «natural» upward leaders, in order to establish a privileged impact point of the lightning strike.

Diagram showing the operating principle for capturing a lightning strike using an early streamer emission conductor

Typical example

Protection of an industrial site using an early streamer emission air terminal:

Because the capture of a lightning strike is quicker than with a lightning rod, this technology can be used to protect zones spread over a wider area, thus ensuring the protection of large structures.

The protection radius generated depends on the lightning conductor's triggering advance value (Δt in μs), its height and the effectiveness of the protection, the maximum value being 120 metres (Level III, height = 60 metres).

Advantages and disadvantages of the different protection types

• Rods
• Meshed conductors
• Catenary wires
• Protection by «natural» components
• Early streamer emission lightning protection system

Rods

Meshed Conductors

Catenary wires

Protection by «natural» components

Early streamer emission air terminal