Most physical test objects can be accurately represented as a two or three-terminal network. An example of a two-terminal capacitor is an apparatus bushing without any test tap. The centre conductor is one terminal and the mounting flange (ground) is the second terminal. An example ofa three-terminal capacitor is an apparatus bushing which has a power factor or capacitance tap. The centre conductor is one terminal, the tap is the second terminal, and the mounting flange (ground) is the third terminal.

It is possible to have a complex insulation system that has four or more terminals. A direct measurement of any capacitance component in a complex system can be made with this test set since it has the capability for measuring both ungrounded and grounded specimens.

Figure 1 shows a simplified measuring circuit diagram ofthe DELTA 4000 test set measuring a two-winding transformer in UST test mode. The test voltage is connected to the HV terminal and the current is measured at the LV terminal.Voltage and current are accurately measured in amplitude and phase and CHL capacitance, dissipation factor, power loss etc are calculated and displayed.