Fault wave arrival determination

The present invention relates to a method and apparatus for determining the time of arrival of a fault wave at a measurement point of a power transmission system. The apparatus (20) comprises a measurement unit (32) measuring a power quantity (PQ) of the system at the measurement point (Pl) for obtaining a measurement quantity being a potential fault wave, a time keeping unit (40), a storage unit (38) storing the measurement quantity, a comparing unit (36) comparing the measurement quantity with a threshold (T1) for detecting the presence of a fault wave (W1) and an analyzing unit (42). The analyzing unit analyzes measurements made before the fault wave presence was detected, determines the starting point (SP) of the fault wave based on the analysis and sets the time of the starting point to be the fault wave arrival time (T1).