In countries applying the IEC 62305, all lightning protection installations must be based on a lightning risk assessment . This analytical approach, defined by Part 2 of the standard (IEC 62305-2), makes it possible to quantify the risks associated with lightning and to select appropriate protection measures.
Therefore, a risk assessment is not a mere administrative formality. It is, in fact, the technical foundation of any lightning protection project. Consequently, understanding and conducting it correctly is essential for engineering firms, installers, and managers of industrial or commercial sites.
Why conduct a lightning risk assessment?
Lightning poses a multifaceted threat to structures and their occupants. It can cause fires, explosions, electronic equipment failures, and even personal injuries. However, not all structures are exposed to the same level of risk.
Therefore, the IEC 62305-2 risk assessment allows us to:
- Objectively quantify the risk of lightning damage to a given structure
- Compare this risk to the tolerable thresholds defined by the standard
- To size the protective measures precisely as needed — neither insufficient nor oversized
- Provide documentary justification for the technical choices made (particularly for regulated sites: ATEX, ICPE, ERP)
However, many installers neglect this step, preferring to apply the maximum level of protection by default. This is a mirod: without a risk assessment, it is impossible to demonstrate compliance with the IEC 62305 standard.
The regulatory framework: IEC 62305-2 and FD C 17-108
IEC 62305-2: the international method
The IEC 62305-2 standard defines the internationally applicable method for calculating lightning risk. It is structured around four main risk components :
- R1 : Risk of loss of life (death or serious injury)
- R2 : risk of public service disruptions (network failures, communications)
- R3 : Risk of loss of irreplaceable cultural heritage
- R4 : Risk of economic losses
In practice, R1 is systematically calculated and compared to the tolerable threshold (RT = 10⁻⁵ for ordinary structures). Consequently, if the calculated risk exceeds this threshold, protective measures are required.
FD C 17-108: the simplified version for France
In addition, FD C 17-108 (Documentation Booklet) proposes a simplified method derived from IEC 62305-2, specifically adapted to the French regulatory context. It notably incorporates ground lightning density data ( Ng) specific to the territory.
However, for complex or high-rods sites, it is preferable to use the complete IEC 62305-2 method directly. FD C 17-108, on the other hand, is particularly well-suited to typical projects in France.
The steps of a lightning risk study IEC 62305-2
Step 1: Identifying the sources and types of damage
First, all potential lightning sources that could affect the structure must be identified :
- Lightning striking the structure directly (S1)
- Lightning strikes near structure (S2)
- Lightning striking the lines connected to the structure (S3)
- Lightning striking near connected lines (S4)
Next, the types of potential damage are classified: physical damage (D1), electrical system failures (D2), and human error (D3). Therefore, the analysis is comprehensive and covers all impact scenarios.
Step 2: Calculation of lightning strike density and site parameters
The ground lightning density Ng (in strikes per km² per year) is the central meteorological parameter of the analysis. It is defined in IEC 62305-2 and in FD C 17-108 for the French territory.
In addition, structural parameters are integrated into the calculation:
- Structure dimensions (length, width, height)
- Geographic location (on a hill, in a plain, in a coastal area…)
- Nature of the immediate environment (other structures, trees)
- Type of construction (roof, materials used)
For example, a tall, isolated structure on a hill presents a much larger collection area than a building of the same size surrounded by other structures. It follows that the calculated risk can vary considerably for seemingly similar structures.
Step 3: Calculation of risk components
Based on the collected data, the predicted frequency of occurrence and associated losses are calculated for each lightning source. These elements then allow the risk components (R1, R2, R3, R4) to be calculated.
In practice, each risk component is expressed as an annual probability. Thus, R1 = 5 × 10⁻⁶ means that we expect 5 deaths per 1,000,000 years of exposure. This figure is then compared to the tolerable threshold RT.
Step 4: Assessing the need for protection and selecting measures
If the calculated risk exceeds the tolerable threshold, protective measures must be implemented. Therefore, the effect of different combinations of measures is simulated until a residual risk below the tolerable threshold is achieved.
Possible measures include, but are not limited to:
- Installation of an external lightning protection system (ESE lightning rod, Faraday cage)
- Installation of surge protectors on electrical and communication installations
- Improved Equipotential Spark Gap and grounding
- Enhanced fire resistance and building safety
Key data: Ng and Nsg
Ground lightning density Ng
Ground lightning density (Ng) is defined and used within the framework of IEC 62305-2 and FD C 17-108 standards. It represents the number of ground lightning strikes per km² per year in a given geographical area. It is the standard parameter for risk analysis.
In Europe and internationally, Ng values are published by national meteorological organizations and integrated into calculation software conforming to IEC 62305-2.
Nsg density: additional data for advanced cases
Furthermore, the lightning strike density (NSG) refers to the density of lightning strikes in clouds and above ground. This data, provided exclusively by the Strike Radar, offers superior geographic accuracy and can justify more precise protection levels in certain cases.
However, Nsg does not replace Ng in the standardized calculations IEC 62305-2 and FD C 17-108: these two notions are complementary and serve distinct uses.
Tools and software for lightning risk assessment
The complexity of the IEC 62305-2 analysis (more than 50 technical variables) makes the use of dedicated software essential. Indeed, manual calculation is not only tedious but also prone to errors.
This is particularly true of LPS Manager, which incorporates an automated calculation engine compliant with IEC 62305-2. Thus, it allows you to:
- Enter the site parameters and get the risk calculation in minutes
- Simulate different combinations of protective measures
- Automatically generate document compliance reports
- Access certified keraunic data and lightning strike certificates via Strike Radar
Ultimately, LPS Manager transforms a complex technical approach into a fluid, traceable and auditable process.
Conclusion
In summary, a lightning risk assessment according to IEC 62305-2 is the essential foundation for any serious lightning protection system. It is the only way to correctly size protective measures and demonstrate compliance with international standards.
For your lightning protection projects—installation of ESE lightning rods, grounding systems, surge arresters— LPS France supports you with its expertise and range of certified products. Contact our team for personalized support, throughout Europe and internationally.
And to manage and document your lightning protection cases over time, LPS Manager is your go-to tool.
