Abstract A method of determining a characteristic of a power transformer is provided. The power transformer has an input current, an input voltage, an output current and an output voltage. The method comprises measuring the input current and a voltage 5 difference between the input voltage and the output voltage of the power transformer while the power transformer is in use within a power distribution network. Information that is indicative of at least one property of a test locus plot from the measured input current and voltage difference of the power transformer is derived and compared to information that is indicative of at least one property of a reference locus plot io associated with a reference power transformer. From the comparison, it is determined whether a characteristic of the power transformer differs from that of the reference power transformer. 9822063_1 (GHMatters) P90871.AU.1
