It is known to provide wind turbine rotor blades with lightning protection systems to protect them from strokes of lightning. As an example WO 96/07825 discloses a lightning protection arrangement, in which the blade tip is provided with a so-called lightning receptor of an electrically conducting material. This lightning receptor may capture a stroke of lightning and conduct the current through a lightning downconductor extending in the longitudinal direction of the blade and being earthed via the wind turbine hub. This system has proved to provide a particularly effective protection.
It is furthermore known to drill a drain bore in the tip of the wind turbine blade to drain off water, which has accumulated in the interior of the wind turbine blade in particular due to condensation.
Experience has shown that lightning may strike at such a drain bore, as this contains water. The lightning current heats the water and thus creates “steam explosions” resulting in such severe increases of the pressure that the blade is damaged. Lightning may strike in these drain bores even if the blade is provided with a lightning receptor. Due to the electric conductivity of the water in the drain bore, a spark may also fly between the lightning receptor and the drain bore. An effective drainage of the water accumulated in the interior of the blade is, however, necessary, the water otherwise accumulating in the cracks and pores of the blade material and drawing lighting thereto. Finally it should be noted that the accumulation of water in the interior of the blade is undesirable in view of corrosion and as a large amount of water will act as a displaceable ballast and interfere with the rotation of the blades.