What changed in IEC 62305:2024
The 2024 third edition revised the lightning risk method in ways that can change a pass into a fail. This guide explains each change in plain English, why it can move a verdict, and why being on the current edition matters to auditors, insurers and authorities.
The IEC 62305:2024 third edition revised the lightning risk method, not just the wording around it. It moved the calculation of how often dangerous events occur onto a ground strike-point density in place of the older flash density, combined loss of human life and loss from fire into a single risk view, added a measure for the frequency of damage to internal systems, and formally recognised thunderstorm warning systems as a way to reduce risk. Because these touch the actual inputs to the calculation, the same building can compute a different answer under the 2024 method than it did under the 2010 second edition.
That is the headline most people miss: a new edition of a standard usually reads like housekeeping, but here the changes sit inside the formula that decides whether a structure needs protection. This guide takes each change in turn, in plain English, and explains why it can flip a pass into a fail. It then sets out why an assessment built on the older method can land on the wrong side of the line today, and why the edition you compute on is part of the verdict, not a footnote. New to the standard? Start with what is IEC 62305, or read the method itself in the IEC 62305-2 risk assessment.
The method is a chain, and the edition changed links in it
To see why a revision of the standard can change an answer, it helps to hold the shape of the method in mind. In IEC 62305-2 each risk is built from components, and every component is the same three-step chain: how often a dangerous event occurs, multiplied by the probability that the event causes the damage, multiplied by the loss that results. Written plainly, that is rate times probability times loss. The risk for a type of loss is the sum of its components, and the verdict is that risk compared against a tolerable level the standard sets.
A revision matters when it touches one of those links. If the edition changes how the rate is computed, every component scales with it. If it changes which probabilities a measure is allowed to reduce, the design conversation changes. If it changes how losses are grouped into a single risk, the sum lands somewhere new. The 2024 edition does all three, which is why it is more than an editorial pass. The sections below map each change onto the link it moves.
The tolerable line did not move, but several of the inputs feeding the risk did. A computed risk that sat just under the line on the 2010 method can sit just over it on the 2024 method, and the structure that read as safe now reads as needing protection. That is the whole reason the edition is load-bearing.
From flash density to ground strike-point density
The most consequential change is in the rate, the first link of every component. Earlier editions drove the rate of dangerous events from a lightning flash density, written NG, the number of cloud-to-ground flashes per square kilometre per year. The 2024 edition replaces it with a ground strike-point density, written NSG, the number of points struck on the ground per square kilometre per year.
The distinction is physical, not bookkeeping. A single lightning flash does not always reach the ground at one tidy point. One flash can have several ground strike points, each capable of doing harm, so counting flashes undercounts the places a strike actually touches down. By counting strike points, NSG describes the hazard a structure is exposed to more faithfully. A collection area pulls strikes from the ground around a structure, and what matters for risk is how many of those ground contacts it gathers, not how many parent flashes they came from.
Because the rate of dangerous events scales directly with this input, swapping the density term changes every component at once. Where the local ground strike-point density works out higher than the flash density figure that was used before, the computed rate rises across the whole assessment, and a risk that was comfortably under its tolerable value can be pushed over it. The change is not guaranteed to move a given building in one direction, but it is guaranteed to be the term the whole result scales with, which is why it is the change most likely to flip a verdict.
Loss of human life and loss from fire, in one combined risk
The risk of loss of human life gathers several ways a strike can harm people: injury from touch and step voltages, and the consequences of physical damage such as a fire that a strike sets off in an occupied building. The 2024 edition brings loss of human life and loss from fire into a single combined risk view, so a structure that carries both an occupancy hazard and a fire hazard is assessed as one risk rather than as two partial pictures looked at in isolation.
This is a change to how the components are summed into the verdict, the last link of the chain. For a building that holds both people and a fire load, a warehouse with staff and flammable stock, a venue packed with a crowd, a workshop with a combustible process, the combined treatment can push the computed risk of loss of human life higher than an approach that weighed the two hazards separately. A structure whose fire risk and life risk each looked tolerable on their own can exceed the tolerable line once they are assessed together, which again can turn a pass into a fail.
A frequency-of-damage measure for the availability of internal systems
Not every site is dominated by the threat to life or the threat of a fire. For a data centre, a control room or any operation where the cost of a strike is measured in downtime, the thing being protected is the availability of the internal electrical and electronic systems. The 2024 edition adds a measure aimed at the frequency of damage to those systems, giving the assessment a way to reason about how often a strike would disrupt the electronics rather than only whether it would injure someone or burn the building.
This recognises that an internal-system failure can be the dominant concern in its own right. A strike does not have to start a fire or hurt anyone to take a facility offline: the electromagnetic pulse a strike radiates, known as LEMP, can induce a surge that knocks out controls, servers or instrumentation. For sites where that is the real exposure, the new measure makes the assessment speak to what they actually care about, and it can change which measures are specified, since coordinated surge protective devices, shielding and bonding become the components that move the number rather than fire measures or an air-termination layout.
Thunderstorm warning systems, recognised in line with IEC 62793
The 2024 edition formally recognises thunderstorm warning systems, in line with IEC 62793, as a measure that reduces risk. This acts on the probability link of the chain. By detecting an approaching storm and triggering temporary precautions ahead of it, moving people to safety, pausing an exposed activity, clearing an open area, a warning system lowers the probability of harm to people during the window of greatest exposure.
The 2010 edition gave no credit for this, so a warning system did nothing for the computed risk under the old method even where it genuinely reduced the danger to people. Under the 2024 edition it becomes a recognised way to bring the risk of loss of human life down, particularly for sites where people are exposed in the open and can be moved in time, such as sports grounds, open-cast sites and outdoor venues. A structure that relies on such a system can compute a lower risk under the current method than it could under the old one, which is the reverse case: a change that can turn a fail into a pass without any physical change to the building.
Corrigendum 1 to Part 2
The third edition of Part 2, the risk management part, also carries Corrigendum 1. A corrigendum is a published correction: it fixes errors and inconsistencies in the text rather than changing the intent of the method. It is easy to overlook, but it is part of what the current edition means. Working to the 2024 edition means working to Part 2 as corrected by Corrigendum 1, so an assessment that quietly ignores it is not fully on the current edition, even if it has adopted the headline changes above.
None of these five changes is cosmetic. Three of them, the density term, the combined risk and the warning systems, move links in the calculation that decides a verdict. The frequency-of-damage measure adds a concern the method could not previously express. The corrigendum makes the text it all rests on correct. Taken together they are why a 2024 assessment and a 2010 assessment of the same building are not interchangeable.
What moved, and which link of the chain it moves
Each change in the third edition acts on a specific part of the risk method. Reading them this way is what shows why a verdict can land somewhere new.
An assessment on the 2010 method can compute a different answer
Put the changes together and the conclusion is direct: an assessment built on the 2010 method can compute a different answer for the same building today. The density term feeding every component is different, loss of life and loss from fire are now summed into one risk rather than weighed apart, and a warning system that earned no credit before can now reduce the risk to people. Nothing about the building has to change for the verdict to move. The change is in the method, and the method is what the tolerable line is applied to.
That has practical weight beyond being current for its own sake. An auditor, an insurer or an authority having jurisdiction checks an assessment against the edition in force, which is the 2024 third edition. An assessment computed by the older method can be rejected as out of date, and because the method genuinely changed, it can also leave a structure under-protected or over-protected relative to the current standard. A building that reads as safe on a 2010-era calculation may need protection under the 2024 method, and signing off on the old number is a real exposure for whoever put their name to it.
The safe position is to compute on the current edition, with the reasoning traceable clause by clause so a reviewer can follow it. Where an assessment is being relied on for approval, insurance or a client specification, or where the structure or its surroundings have changed, recomputing it under the 2024 edition is how you confirm the verdict still holds. The edition is not metadata on the report. It is part of the answer.
Compute on the 2024 method, not the 2010 one
Lumex's Voltrace engine computes the IEC 62305-2 risk method on the current 2024 third edition: it drives the dangerous-event rate from a ground strike-point density, applies the combined view of loss of human life and loss from fire, accounts for the frequency of damage to internal systems, and credits thunderstorm warning systems in line with IEC 62793, with Part 2 taken as corrected by Corrigendum 1. Every risk is compared against its tolerable value and the reasoning is traceable clause by clause, so the result holds up when an auditor, insurer or authority reviews it. See how an IEC 62305 assessment works end to end, or explore the platform.
New to the standard? Start with what is IEC 62305, then read the method in full in the IEC 62305-2 risk assessment.