A method and apparatus for characterizing a three phase transformer (3) using a single phase power supply (1). Pairs of input terminals (H0-H3) of the transformer are sequentially energized for each energization and the voltage between pairs of output terminals (x0-x3) of the transformer are measured. Measured voltages are processed in order to characterize the winding configuration of the transformer. Either simultaneously or subsequently the presence of neutrals on the primary and/or secondary side of the transformer are identified to enable the winding configuration to be further characterized. Subsequently any phase displacement of the transformer is determined. The apparatus comprises means (2) for selectively applying a single phase power supply (1) to pairs of terminals (H0-H3) on the transformer (3) and for measuring voltages between pairs of terminals (x0-x3) of the transformer and for measuring phase difference between the primary and secondary sides of the transformer all under automatic control of a control means (13) which includes a processing means to process measured voltages and phase differences in order to characterize the transformer.