Lightning protection — risk assessment and LPL class under IEC 62305-2

The lightning protection for greenhouses guide covers the application of a lightning protection system specifically for greenhouse structures. This article covers the general, preceding step that applies to any building type and that the greenhouse guide assumes as a given: how does IEC 62305-2 determine whether, and with which protection class, a lightning protection system is needed?

Risk comes before design

IEC 62305 is structured in four parts, of which part 2 (risk management) precedes part 3 (the physical design of the LPS itself — conductors, air-termination systems, earthing system). Without a completed risk assessment there is no substantiated basis for determining whether a building needs lightning protection, and if so, in which protection class (Lightning Protection Level, LPL I–IV) it must be implemented.

The risk components (R1–R4)

IEC 62305-2 distinguishes four types of risk, each linked to a different type of loss:

R1 — risk of loss of human life (or permanent injury).
R2 — risk of loss of a public service.
R3 — risk of loss of cultural heritage.
R4 — risk of economic loss (building, contents, business continuity).

Each risk is calculated as a sum of partial risks per damage source: direct lightning strike to the structure (S1), a strike near the structure (S2), a strike to a connected service line (S3), or a strike near a connected service line (S4).

Checking against the tolerable risk

The calculated risk is compared against a standardised tolerable risk (RT) — for R1 (human life) a tolerable risk of RT = 10⁻⁵ per year applies (a probability of at most 1 in 100,000 per year of a lightning-related fatality or permanent injury). As long as the calculated risk stays below this threshold, additional lightning protection is not normatively required. If the calculated risk exceeds the tolerable threshold, protection is needed, and the required LPL class (I through IV, from the heaviest to the lightest protection level) is chosen based on how much risk reduction is needed to get back below RT.

Note: this is a quantitative risk calculation, not a rule of thumb — the outcome depends on building dimensions, location, surroundings (open terrain versus sheltered urban location), the number of people present, the nature of the activities within the building, and the connected services (power, data). Two seemingly comparable buildings can lead to a different required LPL class.

Relation to the greenhouse-specific guide

The lightning protection for greenhouses guide addresses practical aspects specific to greenhouse structures (the metal greenhouse frame as a natural conductor, main bonding with the greenhouse structure — see also the main bonding for greenhouse structures guide). The risk assessment from this article is the step that precedes it: only once it has been established that protection is needed and in which LPL class, does the greenhouse-specific implementation become relevant.

Practical relevance

When assessing whether a building needs a lightning protection system, a risk calculation (R1 through R4, depending on which type of loss is relevant) must first be performed and checked against the tolerable risk — installing an LPS without this substantiation (or, conversely, assuming no LPS is needed without performing the calculation) is not normatively defensible.

Common mistakes

Choosing an LPL class directly without first performing the risk calculation (R1–R4) — the class follows from the required risk reduction, not from a fixed assumption per building type.
Assessing only the risk to human life (R1) while economic loss (R4) or loss of a public service (R2) may also be relevant to the building in question — each relevant risk component must be checked separately against its own tolerable threshold.
Treating the risk assessment as a one-off exercise — a significant change to the building, its surroundings, or its use (for example an extension, more people present, new connected services) can change the outcome of the risk calculation and thus the required LPL class.