There is provided mechanisms for detecting a fault of a transmission line (20) in a power system (10) comprising at least one of an extreme weak system (10a) and an extreme strong system (10b). A method comprises obtaining travelling wave polarities from two terminals (21a, 21b) of the transmission line during occurrence of the fault, the travelling wave polarities being defined by two current polarities and two voltage polarities. The method comprises determining the obtained travelling wave polarities to be detectable and the obtained travelling wave polarities to be non-detectable. The method comprises detecting the fault to be internal based on the detectable travelling wave polarities and the non-detectable travelling wave polarities. There is also provided an arrangement configured to perform such a method.