How an IEC 62305 risk assessment is calculated
Follow the full IEC 62305-2 calculation on one simple example building, from the inputs you collect, through the collection areas, the dangerous-event rate, the probabilities and losses, the risk components, and the sum into the risk of loss of human life R, to the moment you compare it against the tolerable risk and read the verdict.
An IEC 62305 risk assessment is calculated by turning a building into a few annual risk figures and checking each against a level the standard accepts. You collect the inputs, work out a dangerous-event rate from the area that collects strikes and the local lightning activity, multiply that rate by a probability of damage and by a loss to get each risk component, sum the components into the risk of loss of human life R and the frequency of damage F, and compare each against its tolerable value. A value at or below the tolerable level passes; one above it means the building needs protection.
That is the shape of every assessment, but it is easier to follow on a real building than in the abstract. This page runs the whole calculation once, step by step, on a single simple example: a small standalone clinic. We invent the building, its dimensions and its numbers so you can see how each value feeds the next, from the first input to the final verdict. For the concepts behind the method, the risk of loss of human life and the frequency of damage, and the components, read the IEC 62305-2 risk assessment method; new to the standard as a whole, start with what is IEC 62305.
Every number on this page is illustrative. The figures here are values we invented for the example clinic, chosen only to show the shape of the calculation and how the steps connect. They are not coefficients from IEC 62305. The real strike-point densities, location factors, probability values, loss factors and tolerable-risk levels all come from the tables in the standard itself, and a genuine assessment must take them from there. Read the numbers below as a worked illustration, never as a source of standard values.
What does the example building look like?
Picture a small single-storey clinic standing on its own at the edge of a town. It is a plain rectangular block, and for the calculation we describe it with a handful of facts. These are the raw inputs every IEC 62305 assessment starts from, and the rest of the method is built out of them.
Size and shape
40 metres long, 20 metres wide, 6 metres high. A low, simple block, illustrative dimensions for the example.
Lightning activity
An illustrative ground strike-point density of 3 strike points per square kilometre per year for the location.
Surroundings
Standing alone with no taller neighbours, on open ground. Nothing nearby shields it from strikes.
People and contents
Patients and staff inside through the day, some of them hard to evacuate quickly. No flammable or explosive stock.
Services that enter
One overhead power line running to the building, feeding lighting and medical equipment inside.
Existing protection
None yet. We start the clinic with no lightning protection system and no surge protective devices, then add them if it fails.
That is enough to run the method. From here, every later number is derived from these inputs, so it is worth being clear about them before the arithmetic starts.
How much ground collects strikes for the clinic?
A strike does not have to hit the clinic to count against it. The building gathers flashes from a zone of ground around it, the collection area, and the taller and larger it is, the wider that zone. The method works out four of these: the area for a direct flash to the structure (Ad), the area for a flash to the ground near it (Am), the area for a flash to the power line (Al), and the area for a flash near that line (Ai).
For the structure itself, picture the building footprint with a margin added all the way round, and that margin grows with height because a taller building reaches out to catch strikes that would otherwise have hit the nearby ground. Our clinic is low and wide, so its direct-strike collection area Ad works out, for the example, at about 0.0035 square kilometres. The near-strike area Am is a broader band around the building and comes out larger. The line areas Al and Ai depend on the length of the overhead power line and the strip of ground along it; for the example we take a single line giving a line collection area of about 0.004 square kilometres. These are illustrative figures to show the shape, not values you can read off the standard.
The point of this step is simple: each thing exposed to lightning, the structure and every connected line, pulls strikes from its own patch of ground, and the size of that patch is the first number in the rate. A taller clinic, or a longer overhead line, would collect from a wider area and carry more risk before anything else is even considered.
How often does a dangerous event happen per year?
With a collection area in hand, the rate of dangerous events is the local lightning activity multiplied by that area, then adjusted by a location factor for shielding. The lightning activity is the ground strike-point density, the number of ground strike points per square kilometre per year. Our example clinic sits in a location we gave a density of 3 strike points per square kilometre per year.
Multiply that density by the direct-strike collection area Ad of 0.0035 square kilometres and you get a base rate of 3 times 0.0035, which is 0.0105 events per year, roughly one direct strike every ninety-five years. Because the clinic stands alone with nothing to shield it, its location factor stays at its full value rather than reducing the rate, so the adjusted rate for direct strikes stays near 0.0105 per year. A building hemmed in by taller neighbours would carry a location factor that cut this figure down, and that is exactly how the method credits shielding. The same multiplication, density times area times location factor, gives the rate for the near-strike source and for each line source, using their own collection areas.
So the first term in every component, the rate, is now a concrete frequency: how many dangerous events of each kind the clinic faces per year. For our worked example we will carry the direct-strike rate of about 0.0105 per year forward, and treat the line and near-strike rates the same way when their components come up. Remember the density, the area and the location factor here are all illustrative; the real strike-point density for a site and the real location factors come from the standard and the site data.
Given a strike, how likely is it to cause harm?
The rate tells you how often a dangerous event happens. The probability tells you how often that event actually causes the damage in question, and it is a number between zero and one. This is the term protection changes, so it is where the design conversation lives.
Take the danger to people from touch and step voltages near the clinic after a direct strike. With no protection in place, we set an illustrative probability of 1.0 for that harm: nothing is stopping the strike current reaching a person, so given the event the harm is taken as certain for the example. Now add a lightning protection system. An LPS captures the strike and leads it safely to earth, and a higher class captures a wider range of strike currents, so it cuts that probability hard. The class of system follows the lightning protection level; for the example, fitting a class II system might pull the probability of physical damage down to an illustrative 0.1, a tenfold reduction in that one term. Coordinated surge protective devices do the same job for a different component: they might lower the probability that an induced surge from the power line knocks out the clinic's equipment from an illustrative 1.0 down to 0.03.
Each measure is credited only where it physically helps. The LPS lowers the probability of physical damage from a direct strike but does nothing for the surge arriving along the power line; the SPDs lower the surge-failure probability but do not stop a roof catching fire. Keeping the probabilities separate per component is what lets the calculation pick the right measures rather than over-fitting one. The reductions shown are illustrative; the real probability values and the credit each measure earns come from the standard's tables.
How much is actually at stake when damage happens?
The third term, the loss, scales the consequence. It is the fraction of life, service, heritage or value lost when the damage actually occurs, and it captures the difference between a strike on an empty shed and one on a building full of people. For the clinic, the loss that matters most is the loss of human life, because patients and staff are inside through the day and some cannot evacuate quickly.
Loss rises with the people exposed and with special hazards, and falls with measures that limit how far harm spreads. For the example we set an illustrative loss factor for injury from a strike of 0.01, reflecting an occupied but not crowded building with no explosive or flammable stock. A packed venue, or one with a hard-to-evacuate occupancy and flammable contents, would carry a higher loss factor and so a larger risk from the same strike. Going the other way, fire detection and suppression, compartmentation that stops a fire crossing the building, and bonding that limits the reach of touch and step voltages all reduce the loss. Because the loss multiplies the rate and the probability, cutting it is a real route to bringing a risk down even where the strike itself cannot be made less likely. As before, this 0.01 is an illustrative value for the worked example; the real loss factors come from the standard.
Putting rate, probability and loss together
Now the three terms combine. A risk component is the rate multiplied by the probability multiplied by the loss. That single line is the engine of the whole assessment, and we can run it for the clinic with the numbers gathered so far.
Take the component for injury to people from a direct strike, before any protection. The rate was about 0.0105 events per year, the probability of harm was 1.0, and the loss was 0.01. Multiply them: 0.0105 times 1.0 times 0.01 gives an illustrative component value of about 0.000105 per year, written 1.05 times ten to the minus four. That one number is the annual risk this single way-of-being-harmed contributes. Now apply the class II LPS, which we said pulls the probability from 1.0 down to 0.1. The same component becomes 0.0105 times 0.1 times 0.01, which is about 0.0000105 per year, ten times smaller, purely because one term in the product moved.
This is the lever the assessment turns on. Halve a probability and you halve that component; cut the loss with fire measures and you shrink the consequence without touching how often strikes land. A real building has many components, one for each source-and-damage pairing, but every one is this same three-term product. The values above are illustrative, chosen so the arithmetic is easy to follow.
Adding the components into the risk
One component is not the whole risk. The risk of loss of human life, R, is the sum of every component that can take a life at the clinic: injury from touch and step voltages, physical damage such as a fire that follows a strike, and the failure of any life-safety systems, from both direct strikes and strikes on the power line. You compute each relevant component as rate times probability times loss, then add them.
For the example, say the unprotected clinic has three components feeding the risk R: the injury component of about 0.000105 we worked out, a physical-damage component of about 0.00006, and an internal-failure-leading-to-harm component from the power line of about 0.00003. Add them and the illustrative risk R is roughly 0.000195 per year, written about 2 times ten to the minus four. The same summing builds the frequency of damage F from the components that bear on the availability of the internal systems, which would matter more for a data centre or a control room than for this clinic. Loss of service to the public, loss of cultural heritage and economic loss are further loss categories the method can weigh where they apply. Our clinic is dominated by the risk to human life, so that is the figure that decides its verdict, and the numbers shown are illustrative example values, not figures from the standard.
Comparing the risk R against the tolerable risk
With the risk R summed, the test is a single comparison. The risk of loss of human life R has a tolerable risk RT the standard sets, and the frequency of damage F its own tolerable frequency, and the rule is plain: a value at or below its tolerable value passes, and a value above it means the building needs protection. Economic loss is handled differently, with no tolerable line; instead the annual cost of the loss it prevents is weighed against the annual cost of protection, and protection is justified when it saves more than it costs.
For the worked example we use an illustrative tolerable risk for loss of life of 0.00001 per year, written ten to the minus five, the kind of small number a life-safety limit takes. Compare it with the unprotected clinic's risk R of about 0.000195 per year. The computed risk is well above the tolerable value, by roughly twenty times, so the unprotected clinic fails: it needs protection. The comparison is the verdict. A building can clear the risk to human life yet still fall down on the frequency of damage to its internal systems, or on economic grounds, which is why each figure is checked against its own tolerable value. This tolerable value, like every other number here, is illustrative; the real tolerable-risk levels are fixed in the standard.
That single comparison is the answer the whole calculation exists to produce: does this building need lightning protection, yes or no, for each kind of loss it is exposed to. Everything before it, the inputs, the areas, the rate, the probabilities, the losses and the components, exists to make this verdict defensible and traceable back to the clause behind each number.
Adding measures and recomputing
A failed risk is not the end of the assessment, it is the start of the design. You add the measures that act on the components carrying most of the risk, recompute those components, and re-sum the risk, repeating until it sits below the tolerable value. Because the method shows which components dominate, the choice of measure is targeted rather than a blanket specification.
For the clinic, the risk R was dominated by physical damage and life safety from direct strikes, so we fit the class II LPS from Step 4. That pulled the injury component from about 0.000105 down to about 0.0000105 and the physical-damage component down in step. Adding coordinated SPDs on the incoming power line cut the surge-failure component too. Re-summing the reduced components, the illustrative protected risk R falls to roughly 0.000008 per year, now below the tolerable 0.00001. The clinic passes, and the assessment records exactly which measures earned the pass. Had it still failed, you would add the next measure, fire detection and compartmentation to cut the loss, or a higher LPS class to cut the probability further, and recompute again. These figures stay illustrative throughout; a real recomputation uses the standard's coefficients for every measure.
The calculation, end to end
Nine steps take the clinic from a description to a defensible verdict. Read down the list and the structure of any IEC 62305 calculation is visible at a glance.
Every value used along the way was illustrative, invented for the example clinic to show how the steps connect. A real assessment runs the identical chain but draws its strike-point density, location factors, probabilities, loss factors and tolerable-risk levels from the IEC 62305 standard and the site data, not from numbers like these.
From the worked example to a filed report
The chain above 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, our IEC 62305 platform, runs the full method on the building you describe with the real coefficients from the standard, 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. See how an IEC 62305 assessment works or explore the platform. To go deeper on the concepts, read the IEC 62305-2 risk assessment method, or start from what is IEC 62305.