Method, device and system for determining a line property in the form of an arc on a power transmission line, wherein at least one test signal is coupled into the power transmission and an interference signal at an interference point, which is formed by the arc, is generated along the power transmission line with the aid of the at least one test signal, which interference signal is acquired as at least one measuring signal, and the line property is determined from the at least one measuring signal, where the line property is determined by detecting at least one intermodulation product from the at least one test signal and the mains signal of the power transmission line in the at least one measuring signal.