•Methods for voltage sag source detection are discussed that use only line currents.•Instantaneous current vector is introduced using Clarke’s components.•The proposed method is compared to the already known phasor-based method.•Both methods are evaluated by numerical simulations, laboratory and field testing.•The proposed method shows considerably higher effectiveness for transient events. The presented research reflects those methods for detecting voltage sag sources that only use measurements of line currents. It is shown that the phasor-based method known from the literature is unsatisfactory, especially in the cases of transient voltage sag events. Therefore, a generalised method is proposed that is based on the instantaneous current vector written in Clarke’s components. The phasor-based method and the proposed method for voltage sag source detection were evaluated by applying extensive simulations, laboratory tests, and field testing. The obtained results show considerably higher effectiveness for the proposed method. The correctness of the instantaneous vector-based approach is verified in this way. Moreover, the proposed method that uses only measurements of line currents is just as effective as some other instantaneous vector-based methods that use measurements of line currents and line voltages.