For an insulating material dielectric strength and dielectric loss should be respectively

dielectric constant

For an insulating material dielectric strength and dielectric loss should be respectively high or low

why need dielectric test for transformer

The dielectric,usually the insulator between the plates of a capacitor, can be overstressed by the application of too high voltages applied to the capacitor plates. The dielectric breaks down and a current flows between the plates until,either they are discharged, or an equilibrium is reached,below the working voltage of the capacitor. If the dielectric is damaged in this process he capacitor must be replaced. Some dielectric material self heal and can recover from an over voltage.