A device for determining the location of a fault at a fault position (F) on a power transmission line between two stations (P, Q), and in which the power transmission line has an unknown conduction impedance (Z.sub.PQ), wherein, at one of the stations (P), capacitive voltage measuring transformers measure the phase voltage (U.sub.PM) and the phase currents (I.sub.P) or changes in the phase currents (.DELTA.I.sub.P) are measured on the occurrence of a fault. The phase voltage, phase currents or changes in the phase currents are low pass filtered; and the filtered phase voltage, phase currents or the change in the phase currents are converted from an analog to instantaneous digitized phase voltage, phase currents or changes in phase current values, respectively.The fault distance .alpha. and a value of an apparent fault resistance R.sub.F at the fault location are calculated, using linear regression, on the basis of the relationship:U.sub.PM =.alpha.Z.sub.PQ I.sub.P +R.sub.F I.sub.P +.DELTA.U.sub.CVT ;where .alpha. represents a measure of the fault position, U.sub.F represents a voltage across the fault position and .DELTA.U.sub.CVT represents a fault voltage introduced by said capacitive voltage measuring means.The fault distance is compared with an upper (.alpha..sub.max) and a lower (.alpha..sub.min) limit value and the apparent fault resistance is compared with an upper (R.sub.Fmax) and a lower (R.sub.Fmin) limit value. A tripping signal is generated when the fault position and the apparent fault resistance, respectively, are within the aforesaid limit values.