PF/DF and capacitance measurements are performed to verify the insulation condition of the transformer and feedthroughs. Standard PF/DF measurements at 50 Hz or 60 Hz can only detect the effects of moisture and advanced ageing. Combining the TESTRANO 600 with the accessory CP TD1 allows PF/DF measurements across a wide frequency range from 15 Hz to 400 Hz and detects faults at an earlier stage.