When people think of lightning protection, an air terminal on the roof usually comes to mind first. In the modern approach, however, the air terminal is only the visible part of a much broader engineering process. Which protection level a facility actually needs, which measures make economic sense, and which risks can be considered acceptable — none of this can be determined without a systematic risk assessment.

Internationally, the framework for that assessment is set by the IEC 62305 series. In Türkiye the same series has been adopted as TS EN 62305. This article outlines how a risk assessment proceeds at an industrial facility and which decisions are made along the way.

The structure of the standard

IEC 62305 consists of four parts, each answering a different question.

Part 1 — General principles. Defines the physical effects of lightning, the damage mechanisms and the basic logic of protection. It is the conceptual ground the other three parts are built on.

Part 2 — Risk management. Provides the method for calculating a structure's lightning risk. This is what protection decisions rest on.

Part 3 — Physical damage to structures and life hazard. Contains the design rules for external lightning protection systems, air terminations, down conductors and earthing.

Part 4 — Electrical and electronic systems within structures. Covers surge protection, shielding, equipotential bonding and the protection zone approach.

In practice a project starts with Part 2. Moving on to Parts 3 and 4 without a risk calculation means investing in a system whose requirement is unknown.

Source and type of damage

A risk calculation begins by separating out how lightning could affect the facility. The standard groups the sources of damage under four headings: lightning striking the structure directly, lightning striking near the structure, lightning striking a line connected to the facility, and lightning striking near that line.

The distinction matters, because each calls for a different protective measure. A direct strike creates physical damage and fire risk; that is managed with an external lightning protection system. A nearby strike, on the other hand, induces voltage in the internal wiring through the magnetic field without ever touching the structure; here the external system alone is not enough, and surge protection and shielding come into play.

Types of damage are separated in the same way: harm to living beings through step and touch voltages, physical damage to the structure, and failure of electrical and electronic systems. In a data center the third category dominates, while on an open-air construction site the first comes to the fore.

Types of loss and tolerable risk

The standard addresses risk through four types of loss: loss of human life, interruption of a service provided to the public, damage to cultural heritage, and economic loss.

At industrial facilities the first and fourth categories are usually decisive. When power generation and distribution plants, hospitals or communications infrastructure are involved, however, continuity of service also becomes an independent item of loss.

For each type of loss, the calculated risk is compared against a tolerable threshold. The threshold commonly referenced for loss of human life is in the order of 10⁻⁵ per year; that value and the others, however, should be confirmed against the edition of the standard in force and the relevant national annex before being applied to a project. The economic loss assessment works on a different logic: here the question is not whether a threshold has been exceeded, but whether the protection investment is meaningful relative to the expected loss.

If the calculated risk falls below the threshold, no additional measures may be required. If it is above, measures are selected that bring the risk below the threshold and the calculation is repeated. The process is iterative by nature.

The data that feeds the calculation

A risk calculation works with data specific to the facility. The inputs most often required in practice are:

  • Lightning density. The annual ground flash density for the region is the fundamental input. It is obtained from meteorological data sources or lightning detection networks.
  • Structure geometry. Height, footprint area and shape determine the structure's collection area.
  • Environmental conditions. Taller nearby structures can have a shielding effect, while a structure standing alone in open terrain carries higher risk.
  • Structure material and contents. The presence of flammable material directly affects fire risk and therefore the calculation.
  • Connected lines. The type, length and underground or overhead routing of power, communication and data lines.
  • Human presence. How many people are present at the facility, and for how long.

Collecting this data correctly from the site is more critical than the calculation itself. A report produced from incomplete or assumed inputs may look formally compliant with the standard, but it will not support a decision.

From protection level to design

The risk calculation results in a protection level. The standard defines four levels; the highest provides protection across the widest range of lightning currents and naturally carries the highest cost.

The selected level then translates into concrete design parameters: the spacing of air terminations, the number and spacing of down conductors, the layout of the earthing arrangement, and the class and position of surge protective devices. In other words, the answer to "which level" largely determines the rest of the project.

This is where the protection zone approach comes in. The facility is divided into zones of decreasing electromagnetic influence from the outside in. Equipotential bonding is applied at each zone boundary and a protective device of the appropriate class is positioned there. The aim is to reduce the surge energy reaching the innermost zone, where sensitive equipment sits, step by step.

Commonly encountered gaps

The recurring problems we see in the field fall under a few headings.

No risk assessment at all. The protection system is chosen according to what similar facilities have installed, or on a supplier's recommendation. The result is either excessive investment or inadequate protection.

Focusing only on the external system. An air terminal is installed, but internal protection is neglected. Yet at modern facilities the bulk of the economic loss comes not from physical damage but from the failure of electronic systems and the downtime that follows.

Forgetting earthing after installation. A commissioning measurement is taken, and then nothing is checked for years. Earth resistance, meanwhile, can degrade over time through seasonal variation and corrosion.

Changes not reflected in the calculation. A new building is added to the facility, a line is run, or the intended use changes; the risk assessment is not updated.

Conclusion

A lightning risk assessment is not an exercise in producing a compliance document; it is the process of making a protection decision based on the real conditions of the facility. Done properly, it achieves two things at once: it avoids unnecessary investment, and it establishes adequate protection at the points that genuinely matter.

Running the process with engineering discipline, gathering site data carefully, and updating the results as the facility's conditions change are what make protection work in the field rather than on paper.

This article is for general information. The parameters applied on a project basis should be determined on the basis of the edition of the standard in force and the relevant national annexes.