08 Gezondheids- en werkplekaspecten
 >  What to do in the event of a thunderstorm
Discussion > Discuss this with your colleagues!
  • As soon as a thunderstorm approaches, I seek shelter. I don’t wait until it has reached my workplace.
  • On this project, I know where I can safely shelter when a thunderstorm approaches.

The danger of a thunderstorm should not be underestimated. On average, in the Netherlands there are 25 days with thunderstorms each year. It is important to be aware of the dangers of lightning.

During our activities it is important to be alert to a thunderstorm, both during indoor and outdoor activities. The place and the time of a lightning strike can never be determined precisely.

In view of climate change and the increasing number of thunderstorms, identifying safe sheltering sites is not a superfluous action. This can be included as an item in setting up the company emergency response (CER) organization at the construction site itself. In this respect, it is also important to take the potential damage of a lightning strike and how to act into account.

The ‘10-second’ rule is generally used to provide an indication of the distance between you and the lightning strike. Sound travels more slowly than light and the time difference between seeing a lightning flash and hearing the thunder can therefore be used to estimate how far the thunderstorm is away. The number of seconds between seeing the lightning flash and hearing the thunder divided by three provides a rough estimate of the distance in kilometres.

If you hear the thunder within ten seconds, activities must be halted.

Risks > What can happen?
  • Being hit by lightning.
  • Electric devices can be disrupted or seriously damaged by lightning.
  • Under (noisy) work conditions, lightning often is not heard or seen on time.
  • Hearing protection shields low frequency sound. The rumble of thunder is within the same low frequency region and therefore is blocked.
Measures > What you must do

What you must do:

  • Immediately stop working when a thunderstorm approaches and look for safe shelter.
  • Leave workplaces in open areas
  • Go to a safe location
  • Shelter in the site hut/mobile canteen or in a building
  • Stay in your car (keep doors and windows closed)
  • A metal cabin is a safe shelter (keep doors and windows closed)
  • Leave elevated workplaces (scaffolding, decks and wall formwork)
  • Do not stay together as a group, but spread out
  • Do not stand in front of an open window
  • Make arrangements with colleagues working under noisy conditions to have them alerted by other colleagues
  • Make arrangements about when to react in case of a thunderstorm: when do you act

Safe locations include:

  • Car or cabin
  • Buildings
  • Mobile canteen/site hut


  • Halt crane and pile driving operations
  • If possible, lower booms and masts of cranes and pile driving rigs
  • Be sure you are not in the proximity of fences, construction lifts, scaffolding, light masts and other metal objects
  • Halt work on metal conductive parts (gas, electricity, water and central heating pipes, air conditioning ducts, etc); also stay away from these objects


  • A machine connected to a structure that is also earthed (sheet pile wall, earthing pin) does not have to be disconnected when there is an approaching thunderstorm. By contrast, a machine that is connected to a return line via a surge arrester must be disconnected when there is an approaching thunderstorm, however only if this can be done safely. If the thunderstorm is already close by, simply leave things as they are. In other words, personal safety comes first. When it is disconnected, the machine must be outside the falling range, however.

What you must not do:

If you are not able to reach a safe spot on time, you should act as follows:

  • Do not stand under a free-standing tree
  • Do not shelter under a metal mast
  • Never stand near large metal material
  • Do not lie down
  • Make yourself as small as possible by crouching
  • Put your arms around your knees
  • Keep your head down
  • By keeping your feet together, the chance that a currently will flow through your body is small
  • Avoid using your mobile phone
Tips > For more information

The distance of a thunderstorm can be estimated using the following rule of thumb:

Thunderstorms travel fast; distances can be estimated using the following rule of thumb:

Three (3) seconds between the lightning flash and the thunderclap is 1 kilometre!