Abstract: The apparatus comprises a plurality of connectors (9, 10, 11, 12 . . .) for connecting to the component to be tested (e.g. a transformer), a signal generator (19), a measuring system (23) and control means (22) which is arranged to control which of the connectors (9, 10, 11, 12 . . .) are connected to the signal generator (19) and the measuring system {23) and arranged to control the function of the signal generator (19) and the measuring system (23) so that the same signal generator (19) and measuring system (23) can be used to carry out a variety of different tests on the component, the signal generator (19) also being arranged to provide a synchronising signal (28) to the measuring system (23) so that the phase relationship between measurements at different times can be calculated by comparing the phase relationships of those measurements with the synchronising signal (28).