The invention relates to a method for determining states and locating faults in installed isolated down conductors in the external lightning protection means, wherein the isolated down conductors have a coaxial structure with a metal inner conductor, dielectric medium and voltage-controlled outer casing made of weak-conducting or semiconductor material, as well as having an earth connection. According to the invention, in order to locate faults, after removal of the earth connection, an impulse voltage is applied between the inner conductor and the outer casing of the respective down conductor, causing a discharge in the event of down conductor faults at the fault location, wherein the discharge is subjected to an evaluation to determine the state of the respective down conductor.