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Lightning protection of modern wind turbines presents a number of new challenges due to the geometrical, electrical and mechanical particularities of the turbines. The risk assessment requires the estimation of the number of expected strikes. In the case of modern turbines, most of the expected lightning flashes will be upward. In addition, due to the rotation of the blades, modern wind turbines may trigger their own lightning. Moreover, since wind turbines are becoming tall structures, lightning currents that are injected by return strokes into the turbines may be affected by the structure itself. The presence of carbon reinforced plastics in the blades introduces a new set of problems. The validation of lightning protection systems for wind-turbine blades is therefore an important issue. This paper presents some open questions related to lightning protection of large wind turbines. The paper also presents two full-scale blade impulse current tests that allow the evaluatation of current distributions and potential differences for the first and second return stroke currents defined in the corresponding IEC standards.