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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