Nearly every high-voltage equipment prior to delivery to the customer will undergo a test related to C and tan-d for a final quality control. Such tests are in generally conducted at the frequency for which the equipment is designed. The reasons for the tests are firstly that excessively high losses (= high tan-d) may cause a thermal breakdown or, secondly, the manufacturer of a specific equipment knows the typical magnitudes which can be tolerated and which provide information about the quality of newly manufactured equipment; and these measurements can also serve for a comparison with later preventive measurements.

Such tests are in general made in dependence of the test voltage applied; both magnitudes, capacitance and dissipation factor shall be essentially constant with increasing voltage, as insulation systems are linear systems and any "tip-up" of the tan- d with voltage level, called "ionisation knee", is a preliminary indication of partial discharges in the system which can lead to a failure of the insulation system.