tan delta & capacitance test

the tan delta test is a measure of the quality of insulation and is applied to cables and i've seen it applied to a generator stator too. the insulation resembles a capacitor in that it is a conductor separated by insulation from the neutral and in a perfect capacitor the current leads the voltage by 90 degrees. any deterioration in insulation will result in an increase in resistance and therefore the angle between current and voltage will be less than 90 degrees (because of resistive current component in phase with voltage). the resistive current and capacitative current are measured and Ir/Ic gives the tangent of angle delta. delta being the angle between the capacitative current component and the resultant current. the angle delta is zero for a perfect capacitor and any increase indicates insulation contamination.