A lightning protection system of a wind turbine (10) comprises electrically grounded receptors (51, 52) on turbine blades. A corresponding electrical ground connection must be tested regularly. For such a test, an unmanned aerial vehicle or UAV (4) places an electrode (41) near a receptor (51, 52). Using a high voltage generator (42), a high voltage is created between the electrode (41) and the receptor (51, 52), causing an electrical arc (45) between the two. A corresponding current through the lightning protection system (5) indicates whether the ground connection is in order. Thanks to the high voltage used, the electrical arc (45) can bridge a gap between the electrode (41) and receptor (51, 52) of at least several millimetres. It is not necessary to establish a direct physical contact between the electrode (41) and the receptor (51, 52). This simplifies controlling the UAV and speeds up the testing procedure.