Lightning Risk Assessment (IEC 62305) 

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IEC 62305 'Protection against lightning' requires a risk assessment be carried out to determine the characteristics of any lightning protection system to be installed. There is a lot of hype about about this being a complicated process, however in reality it is very simple.

Risk Assessment

The procedure requires assigning a tolerable level of risk. A lightning protection level is then chosen to ensure that the calculated risk level is lower than the assigned tolerable level.  This is carried out for each of the following risk types:

  • Loss of Human Life
  • Loss of Public Services
  • Loss of Cultural Heritage
  • Economic Loss

Each country defines it's own tolerable risk values (for example the UK assigns the tolerable risk for loss of cultural heritage as 10-4).  General representative values listed in the standard are:

Type of Loss

Tolerable Risk

Loss of human life or permanent injuries


Loss of service to the public


Loss of cultural heritage


Economic loss



Note: surge protective devices (SPD) when installed, can lower the calculated risks obtained during any assessment exercise.

Lightning Protection Level (LPL)

Four protection levels (I, II, III and IV) are defined by considering the peak current, charge, specific energy, duration of various lightning strokes and the probability of these being exceeded.  Lighting Protection Level I is the most severe, with level IV being the least.

Once a lightning protection level has been selected, this fixes many of the parameters for the design and installation of the system.  For example, the size of the rolling sphere for calculating protection zones is 20 m for a protection level of I, and increases to 60 m for level IV.


After reading the standard, calculating the risk level may seem like a complicated process. Help is at hand however. In purchasing the standard the 'IEC Risk Assessment Calculator' software is provided free of charge. This is a very straightforward piece of software to use requiring the structure dimensions to be entered and various parameters selected from drop down lists. The entire process can be completed in only a few minutes and assessment reports generated.

The attached images show the input screen for the software and an example of the assessment report generated. Click on the images to view a bigger picture. this being a complicated process, however in reality it is very simple.

Related Links

The standard can be purchased at:

IEC Risk Assessment Calculator


Lightning Assessment Report 1.2


Lightning Assessment Report 2.2


Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

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