Wind turbines are normally erected far away from houses, industry, etc., as the wind conditions are not favourable in the vicinity of large obstacles. Furthermore, with regard to acoustic noise emission and shadow flicker certain distances are required by national regulations, when wind farms are planned in the neighbourhood of residential areas.
Thus, wind turbines should not cause risks as far as ice throw is concerned. However, the turbines are erected close to roads or agricultural infrastructure in order to avoid long and expensive access roads for erection and maintenance. This induces a risk for persons passing by the wind turbines, cars passing the streets if ice fragments fall down from a turbine.
Especially in the mountainous sites or in the northern areas icing may occur frequently and any exposed structure - also wind turbines - will be covered by ice under special meteorological conditions. This is also true if today's Multi Megawatt turbines with heights from ground to the top rotor blade tip of more than 150 m can easily reach lower clouds with supercooled rain in the cold season, causing icing if it hits the leading edge.
Figure 1 Nice view, but the rime ice accretion on the grass and the fence signalizes danger of ice throw in the neighbourhood of the wind turbines.
If a wind turbine operates in icing conditions which are described in , two types of risks may occur if the rotor blades collect ice. The fragments from the rotor are thrown off from the operating turbine due to aerodynamic and centrifugal forces or they fall down from the turbine when it is shut down or idling without power production. It depends upon the weather and especially the wind conditions, on the instrumentation of the wind turbine's control system, and on the strategy of the control system itself.