A Doble test measures the power factor or dissipation factor of insulation by applying an AC voltage, commonly 10 kV, and comparing the in-phase loss current with the capacitive charging current. A rising percentage on a bushing or winding compared with nameplate or earlier results points to moisture, contamination or ageing. The brand name stands for the whole family of tests, including capacitance, excitation current and tip-up checks.
Doble test on the C phase bushing came back at 0.9 percent against 0.3 on the nameplate, that bushing is coming out.
Also heard as
- power factor test
- tan delta test
- dissipation factor test
Do not mix it up with
- Power factor: Insulation power factor describes losses in a dielectric, not the power factor of a load.
Related terms
Insulation resistance test
DC test that applies a high voltage between conductors and earth and reads leakage resistance in megohms.
Partial discharge
Tiny discharge in a void, on a surface or at a sharp edge that never spans the whole insulation, an early sign of decay.
Power factor
Ratio of active power to apparent power, showing how much of the current drawn does useful work.