CP TD1 User Manual
38 OMICRON
4.3 "UST" and "GST" Measurements Using the Guard
Technology
The CP TD1 has three external measuring inputs, IN A, IN B and ground. Those three inputs can be
individually switched to guard or to the measuring unit. If an input is switched to the measuring path, the
connected capacitance is part of the measurement. If it is switched to guard, the current bypasses the
measurement path and is not included in the measurement. The advantage of using a switch matrix to
configure the measurement is that multiple measurements can be made without changing the
measurement setup or wiring.
The terms “UST” for “ungrounded specimen test” and “GST” for “grounded specimen test” have
historically evolved. “UST” describes a measurement setup, where ground is not connected to the
measurement path, whereas “GST” describes a measurement setup, where ground is part of the
measurement path. In the “UST” configuration, 3 configurations are possible. Depending on
configuration, the currents via IN A and IN B are measured or not (see Table 4-1 below). The
measurement result is the sum of all measured channels.
In the “GST” configuration, 4 configurations are possible (see Table 4-2). The naming is a bit different
compared to the “UST” modes as it indicates the channels which are guarded, not the channels which
are measured. The measurement result is also the sum of all measured channels.
In general, the UST measurement is less influenced by external noise or stray capacitances than the
GST measurement and should be preferred if both modes are possible.
The different configurations allow multiple measurements with only a few re-connections at the device
under test. On the following pages examples are shown for the common cases of a two-winding and a
three-winding transformer.
Table 4-1: UST measurement modes
Mode IN A IN B Ground
UST-A Measured Guarded Guarded
UST-B Guarded Measured Guarded
UST-(A+B) Measured Measured Guarded
Table 4-2: GST measurement modes
Mode IN A IN B Ground
GST Measured Measured Measured
GSTg-A Guarded Measured Measured
GSTg-B Measured Guarded Measured
GSTg-(A+B) Guarded Guarded Measured
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