Lightning risk assessment
A lightning risk assessment is a check on a building that calculates the risk of a surge caused by lightning and the likely damage caused by that surge to people, or equipment and the increased likelihood of fire. The outcomes can be manipulated by changing factors affecting the electrical infrastructure, the building and people within or near the building. IEC 60364-4-44 always requires the installation of a surge protective device (SPD) at the power line entrance in the structure when the consequence caused by over-voltages affects: – care of human life, e.g. safety services, medical care facilities, – public services and cultural heritage, e.g. loss of public services, IT centres, museums, – commercial or industrial activity, e.g. hotels, banks, industries, commercial markets, farms.
The PQRS lightning risk assessment calculator guides you through the process to amongst other things; confirm whether Type 2 SPD’s are sufficient or whether it might have to be stepped up to include more robust protection. Please take a look at the short-form calculator in the link below.
Open the short-form calculator 2011 vs 2024
We simplified the wording and terminology to make the standard’s requirements and variable fields easy to understand and applicable to the trade.
How the assessment works
Lightning can strike the building, the ground nearby, or travel in on a power line or a data line by striking the line directly or near the line. The assessment looks at all four options of how a surge can enter a building.
The form then covers three topics with a series of questions:
- Is there a chance of harm to people?
- Any risk of fire and damage to the building?
- Would the electrical and electronic systems inside the building sustain any damage?
The calculator uses edition IEC 62305-2:2024 and/or SANS 62305-2:2025.
The output from the assessment tells you whether you still need an outside lightning protection system (a LPS), surge protection devices (SPDs), or both. Building the actual LPS is guided by IEC 62305-3 and how to actually fit surge protection is guided by IEC 62305-4. This tool does not yet do that design, but rather guides the user in understanding what still needs to be added — a LPS, surge protection devices, or both — in order to reduce the risks and possible damage.
A simple site, as an example
Let’s use a house built with bricks, on a plot, powered by an overhead power line. There is no LPS on the building, and no surge protection in the DB board.
The short-form calculator asks how often lightning occurs in that area, and how big the house is, as well as the length of the power line. When that length is unknown, the standard uses 1 000 m, and the short form uses that value in the lightning risk assessment.
It then calculates how many strikes you would expect on the building and on the power-line supplying the building, and whether the three factors (damage to people, fire and equipment) are within tolerable risk levels (which you select). If they fail, the report points you back to the step where the changes can be made to reduce the risk such as setting bonding or surge protection at the DB Board, or including fire prevention measures and then running the assessment again.
The short-form Lightning Risk Assessment is simplified so users can get to the required answers faster and familiarise themselves with the terminology as the standard can become quite complex.
What makes the standard challenging
The calculations are buried in tables that share factors, letters and numbers. An incorrect selection can change several results at once as the same factor might be applied in multiple calculations or affect the outcome of multiple calculations.
Risk to People and damage as a result of fire can pass the assessment through the calculations, while the damage to electrical equipment and systems still fails the assessment.
When you need an assessment that is more detailed
A high-voltage section in front of the low-voltage supply, a neighbouring building might be close enough to make a difference, a second incoming supply such as PV, or a cable with a screen that is actually bonded, all change the outcome.
Those fields are accessible on the detailed calculator and have not been included as extra fields on the short form.
The Calculator is broken down into 6 Sections
The calculator is the same maths as the 2024 Part 2 check.
- Section 1: How often lightning occurs in this area.
- Section 2: The building, the surroundings, and the power-lines.
- Section 3: The four ways lightning can reach the site. Nothing to type in this section, but this is so step 4 makes sense.
- Section 4: Looks at what is already there: the structure, any LPS, bonding, surge protection, and how the wiring is run.
- Section 5: Covers people and fire: who is in the zone, and how serious a fire would be.
- Section 6: Does the actual assessment calculation and produces the report to see if the risk is within certain boundaries. If it is not, the page shows where the user can make adjustments to reduce the overall outcome.
Two assessments for two types of users
As the terminology may be new for most users, the LRA calculator is available in both a simplified and advanced version. In the advanced version, zones can be added or taken away, there are more factors to set or adjust and the input fields are significantly more advanced.
| Page | Who it is for | What you get |
|---|---|---|
| Short form | Easy for Electricians + A first look or rough draft for a client | A quick run to confirm whether you need SPDs or a LPS. |
| Detailed calculator | Bigger or more complex sites. People who know the standard. | Real line lengths, high-voltage sections, neighbouring buildings, cable screens, and extra site factors. |
Start on the short-form assessment calculator. If you would like to have access to the advanced LRA calculator, leave your details here and we will mail you when the detailed calculator is up.
Open the short-form calculator Create a PQRS account
What this assessment does not do:
- It is not a certificate or an approval.
- It is not a Lightning Protection or surge protection design with down conductors, or earth spikes.
- It is not a surge-protection schedule.
- It does not replace Parts 3 and 4 of IEC 62305 but can guide users on what to add and why.
- It is not a guarantee that if the steps are followed, no damage will be sustained in the event of lightning. It is a tool that calculates the mathematical probability of risk.
If you still use the 2011 standard, the mechanism and calculation process itself changed in 2024/2025 version of the 62305 standard. Read our comparison between 2011 and 2024 next, or run the calculator on the 2024 standard methodology set in Part 2 of the standard.
Public pages on this site cite IEC 62305. In South Africa the 2024 IEC text was adopted as SANS 62305 in 2025. PQRS uses IEC as a reference to assist international users outside South Africa as they can still use the assessment calculator.