IEC 62305-2: the risk assessment method

No risk is a single number you read off a chart. Each one is the sum of components, and every component describes a specific way a strike can harm the building. A component pairs a source of damage, meaning where the strike lands, with a type of damage, meaning what that strike then does. Splitting the problem this way is what lets the method credit a protection measure precisely: a surge arrester helps against an induced surge on an incoming line but does nothing for a person standing next to a down conductor, and the component structure keeps those effects apart.

The four sources of damage (where the strike lands). A direct flash to the structure itself (S1), a flash to the ground near the structure (S2), a flash directly to a service line that enters the building such as power or telecoms (S3), and a flash to the ground near such a line (S4). The direct and near hits threaten the structure in different ways, and the two line sources matter because a surge travels in along the cable from well outside the building.

The three types of damage (what the strike does). Injury to living beings from touch and step voltages (D1), physical damage such as fire, explosion or mechanical destruction (D2), and failure of internal electrical and electronic systems caused by the electromagnetic pulse the strike radiates, known as LEMP (D3). One strike commonly causes more than one type at once, which is why a single source can feed several components.

Pairing the sources and the types gives the set of components that make up each risk. A direct flash to the structure can hurt people (D1), set it alight (D2) or fry its electronics (D3); a flash near a line mainly drives a surge that causes an internal failure (D3). The method adds the components that apply to the risk in question, which is why a structure with no connected lines, or no people inside, simply drops the components that cannot occur.

Each component follows the same three-step chain, and it is worth holding this in mind because it is what a protection measure acts on. A component equals how often a dangerous event occurs, multiplied by the probability that the event causes the damage in question, multiplied by the loss that results. Written plainly: rate times probability times loss.

The first term, the rate, is a frequency: how many dangerous events of that kind happen per year, driven by the lightning activity of the location and the area that collects strikes. The second, the probability, is between zero and one: given the event, how likely it is to actually cause that damage, which is exactly where protection bites. The third, the loss, scales the consequence: the fraction of value, people or service lost when the damage does occur. Multiply the three, sum the components, and you have the risk for that loss type.

This is the lever the whole assessment turns on. A measure that halves a probability halves that component. A measure that cuts the loss, such as fire compartmentation, shrinks the consequence without touching how often strikes land. The method makes both visible, so the report shows not just the final number but which term each measure moved.

A strike does not have to hit the building to count. A structure and each line connected to it gather flashes from a zone of ground around them, called the collection area. The taller and larger the exposed object, the wider that zone: a tall mast on a hill draws flashes from a much bigger area than a low building on a plain, because its height lets it intercept strikes that would otherwise have hit the ground nearby. The method works out a collection area for direct flashes to the structure (Ad), for flashes near the structure (Am), for flashes to each line (Al) and for flashes near each line (Ai). The idea is the same in every case: estimate the patch of ground from which that object pulls strikes.

The rate of dangerous events is then the local lightning activity multiplied by the collection area, adjusted by a location factor that accounts for shielding from nearby structures, trees and terrain. A building hemmed in by taller neighbours collects far fewer direct strikes than the same building standing alone, and the location factor is how the method credits that. For the line sources, the rate also reflects the length and type of the line and whether it runs overhead or buried, since a long overhead power line is exposed to far more flashes than a short buried cable.

The local lightning activity is the input that ties the whole assessment to a place. In the 2024 third edition this is the ground strike-point density, written N_SG, the number of points struck per square kilometre per year. This replaces the older flash density, N_G, used by earlier editions. The change is not cosmetic: a flash can have several ground strike points, so the new measure describes the hazard a structure actually faces more accurately, and it can shift the computed rate enough to change a verdict. Wherever you take the figure from, it should reflect the real location, since the rate scales directly with it.

The probability term is the heart of the design conversation, because it is mostly what protection changes. Given that a dangerous event has occurred, the probability says how likely it is to actually cause the damage in question. Without protection these probabilities sit high. The job of a protection measure is to pull them down, and the method gives explicit credit for doing so.

Physical damage from a direct strike. Fitting a lightning protection system lowers the probability that a direct flash causes physical damage, and the higher the class of system the more it lowers it. An LPS of class I, the most demanding, intercepts the widest range of strike currents and substantially reduces that probability; a class IV system reduces it less. The class follows the lightning protection level (LPL I to IV) the assessment computes, so the level chosen feeds straight into the risk through this probability.

Failure of internal systems. Coordinated surge protective devices lower the probability that an induced surge knocks out the electronics inside. Coordination matters: SPDs at the service entrance and at the equipment work together so that what gets past the first is clamped by the next, and the better the coordinated set, the lower the probability of an internal failure. Shielding, careful routing and bonding reduce it further by cutting the surge the wiring picks up in the first place.

Injury to people. The probability of injury from touch and step voltages is reduced by physical measures around the structure, such as insulation, equipotential bonding, restrictions and warnings near down conductors, and the layout of the earth termination. These act on the D1 components without changing how often strikes arrive.

The point throughout is that protection is credited where it physically helps and nowhere else. An LPS does not lower the probability of an internal surge failure; SPDs do not stop a direct strike setting a roof alight. Keeping the probabilities separate per component is what lets the method recommend the right combination rather than over-specifying one measure to cover risks it cannot touch.

The same strike on the same building can cost very different amounts depending on what is inside and who is present, and the loss term is how the method captures that. The loss scales the consequence of each component: the fraction of what is at stake, life, the structure and its contents, or the function of the internal systems, that is lost when the damage occurs. It is what separates a strike on an empty store from one on a packed venue with flammable stock.

Loss rises with the people exposed and with special hazards. A structure holding a crowd, or one with a risk of explosion or a hard-to-evacuate occupancy, carries a much larger loss for physical damage than a sparsely occupied building, and the method increases the loss factor accordingly. Conversely, measures that limit the spread of harm reduce the loss: fire detection and suppression, compartmentation that stops a fire crossing the building, and bonding and protective measures that reduce the reach of touch and step voltages all cut the fraction lost when damage happens. Because the loss multiplies the rate and probability, reducing it is a legitimate route to bringing a risk down even where the strike itself cannot be made less likely, which is exactly the case for many existing buildings.

With every component computed and summed, each figure is compared against its tolerable value. The risk of injury to people R is judged against a tolerable risk R_T, and the test is simple: a risk at or below its tolerable value passes, and a risk above it means the structure needs protection. The frequency of damage F is checked the same way against its tolerable frequency F_T. Those two comparisons are the whole verdict.

The value that decides is not a fixed one. Clause 7.3 NOTE 1 gives R_T = 1×10^-5 per year as a representative value of tolerable risk and adds that another value may be set once the case has been investigated in detail. Printed p.12 then lets national or local regulations fix R_T, the tolerable frequency of damage F_T, and the Annex A, B, C and E calculation rules and parameter values. Which value applies is a property of the jurisdiction the structure sits in, so a Lumex assessment states it explicitly. The 2010 edition additionally weighed a purely economic loss on cost and benefit, with no tolerable line; the 2024 edition removed that loss type, and its Annex D now covers the evaluation of P_SPD instead.

Where a risk fails, the assessment is iterative. You add the measures that act on the dominant components, recompute, and check again, repeating until each risk sits below its tolerable value. Because the method shows which components carry most of the risk, the choice of measure is targeted rather than a blanket specification: if the risk is dominated by internal-system failure, coordinated SPDs do the work; if it is dominated by physical damage and life safety, an LPS of the right class and fire measures are what move the number. The output is a clear, defensible verdict for each risk, with the reasoning traceable back to the clause and the components behind it. This traceability is what an auditor, an insurer or an authority looks for, and it is what a manual spreadsheet most often fails to show.

The third edition, published in 2024 and adopted nationally across most of the world, is the version an auditor or authority cites today, and it changes the method in ways that can flip a verdict. Building on a 2010-era spreadsheet is not just out of date, it can compute a different answer.

The most consequential change is the move to a ground strike-point density, N_SG, in the dangerous-event rate, in place of the older flash density. Because a single flash can strike the ground at more than one point, this describes the real hazard more faithfully and can raise or lower the computed rate. The edition also brings injury to people and loss from fire into a single combined risk view, so the two are no longer assessed in isolation from each other. It adds a frequency-of-damage measure aimed at the availability of internal systems, recognising that for some sites keeping the electronics running is itself the thing being protected. And it formally recognises thunderstorm warning systems, in line with IEC 62793, as a measure that reduces risk by enabling temporary precautions ahead of a storm. Each of these can change which measures a structure needs, which is why the edition an assessment is built on is part of the result, not a footnote.

The method is exact but unforgiving by hand: dozens of components, each with its own collection area, rate, probability and loss, all recomputed every time a measure is added or a dimension changes. Lumex runs the full IEC 62305-2 method on the building you describe, computes every component, compares each risk against its tolerable value, and shows which measures bring a failing risk into line, with the reasoning traceable clause by clause. To see the chain end to end with an illustrative worked example, follow the numbers on one building, read how an IEC 62305 assessment works, or explore the platform.

New to the standard? Start with what is IEC 62305, or see which structures need an assessment and the types of loss it weighs. Once an LPS is installed, Part 3 governs its periodic inspection and testing, the maintenance cycle that keeps an assessment valid.