Africa is the lightning capital of the world! According to NASA’s lightning tracking technology, which has recorded every lightning strike to hit the earth for the past 18 years, some regions in Africa have the largest lightning flash indexes in the world. These statistics have serious consequences for lightning protection designers who design lightning protection systems (LPSes) for structures in high lightning flash regions of Africa.
By Trevor Manas, national director, Earthing and Lightning Protection Association (ELPA) in South Africa
The evaluation and management of the risk of life, hazard and damage to structures by lightning must take the higher frequency of lightning into account. The proper risk evaluation in accordance with SANS / IEC 62305 Part 2 is mandatory for all structures in high lightning areas and the resultant lightning protection level must be properly designed in accordance with Part 3 and 4 of the standard.
The correct design of the Lightning Protection Air Termination System is essential, as this is the interception system, which ensures that the structure is not penetrated by a lightning strike. Once the lightning flash is intercepted by the air termination system, it is then guided towards the ground via the Down Conductor System and then safely dissipated into the ground by means of a properly designed Earth Termination System. The air termination system, down conductor system and earth termination system are the three elements that form the external lightning protection system.
Air Termination System (ATS) Design
There are three protection methods that can be used for the design and installation of an ATS:
1) Mesh Method: Used for flat roof type structures.
2) Protection Angle Method: Used for single masts and finials (has height limitations for its use).
3) Rolling Sphere Method: Used universal method for all types of structures.
The rolling sphere method of protection is the preferable method because it can be applied to all types of structures with even the most complicated geographical arrangement.
The use of 3D modelling to determine the zones of protection over the structure gives clear visualisation of the air termination system and how the lightning protection measures are integrated into the building’s architecture.
Implementation of the rolling sphere method of protection
The radius of the rolling sphere is determined by the lightning protection level (LPL), which is derived from the LPS risk assessment, as follows:
The sphere is rolled up to and over the structure in all possible directions. Wherever the sphere touches the structure, it represents a potential point of strike on the structure. Air terminals (finials, masts or conductors) are placed at all of the potential points of strike.
The sphere is then once again rolled up to and over the structure, this time ensuring that the sphere only touches the air terminals and not the structure.
Sag of rolling sphere
As the sphere is rolled over the air terminals, it is natural that there will be a sag of the sphere between the two air terminals. The penetration depth of the sphere between the air terminals must be calculated to ensure that the sphere does not touch the structure or any rooftop plant or equipment. The height (▲h) of the air terminals must always be greater than the penetration depth. The conventional way of determining the height of the air terminals is by calculating the penetration depth for the largest distance between the air terminals.
The following formula is used to calculate the sag/penetration depth between air terminals:
Conventional method vs. 3D modelling
Conventional or 2D simulations have limitations and it is extremely difficult to identify protection zones correctly, as shown above. Whereas the use of 3D modelling clearly identifies areas which are not protected, resulting in an accurate LPS design process.
Separation distance concept
When designing air termination systems for structures that are equipped with rooftop plant and equipment, it is vitally important that an ‘Isolated Air Termination System’ be designed for these types of structures. The purpose of creating an isolated ATS is to prevent lightning currents from entering the building via the rooftop plant. Uncontrolled entry of lightning currents into the building increases the risk of fire (uncontrolled flashovers) and can be devastating to electronic systems located inside the structure. In order to create an isolated ATS, the separation distance between the air terminals and the rooftop plant must be calculated and maintained so as not to induce the lightning currents onto the rooftop plant.
Use of drone technology
The use of drones to assist in the assessment of rooftop structures has become an invaluable tool in devising more accurate analysis of lightning protection installations or site evaluations for design purposes. Drones are particularly valuable in assessing inaccessible areas on roofs. [Refer to page 48 for more on the use of drones – Ed]
The proper design of lightning protection air termination systems is an extremely complex undertaking and should only be carried out by competent LPS designers.
The use of modern technologies, like 3D modelling of the LPS and the use of drones as an inspection tool, are valuable in providing extremely accurate design processes and thereby more effective lightning protection solutions.
About the association
ELPA, a non-profit organisation, is focused on communicating holistically around the potentially deadly dangers presented by lightning; and assists with advice on overarching legislation to guide all concerned parties. Contact Claudelle Pillay on +27 11 704 1487 or visit the website. www.elpasa.org.za