IEC TR 62713-2013 pdf Safety procedures for reduction of risk outside a structure

6 How to act in the presence of a thunderstorm 6.1 How to detect a lightning risk Advance information on the probability and corresponding approach of lightning is available from local weather forecasts in many regions and internet sites are available to provide near real-time lightning location information. Lightning warning systems can also be used, both for industrial sites and private activities. Networks of detectors exist in some countries and provide, by internet or by other means (such as fax, phone, e-mail, dedicated communication lines), a warning of the occurrence of a lightning event. A dangerous industrial activity can be stopped, people can be transferred to safe shelters and exposed activity (for example, on roof or tanks) can be avoided. Local detectors also exist. Some sensors can be portable but are generally less reliable than others. Being light and quite inexpensive, they can be used as a last resort for people involved in outdoor activity. Fixed sensors are more bulky but also more reliable, especially in their capability of giving an early warning. They can be used for industrial sites but are also efficient for golf courses and camping sites, for example. The warning time is generally less than 30 min, so a safe shelter on the site is strongly recommended. You can evaluate the risk for your activity, by watching the approach of the thunderclouds with the accompanying far flashes and by listening to the thunder. The actual distance of a thunderstorm can be roughly estimated: the number of seconds between the flash and the thunder divided by 3 gives you the distance in kilometres. From 5 km, an outdoor sports event should be interrupted and you should avoid dangerous places, like isolated trees. Half an hour after the last flash you see or the thunder you hear, you can assume that the lightning risk is over.