Choose the tan delta test mode from the insulation section you need to include or exclude: UST measures an ungrounded specimen isolated from ground, GST measures a grounded specimen, and GSTg uses the guard terminal to exclude part of the insulation from a grounded-specimen measurement. The correct mode is the one whose circuit matches the test object and the question you are answering.
Why the Measurement Mode Changes What the Instrument Sees
A tan delta test set measures the dielectric loss and capacitance of the insulation between the energised terminal and the measuring circuit. The mode determines which parts of the insulation are energised, which are grounded and which are excluded by the guard. Two modes on the same transformer can produce different readings because they measure different physical circuits, not because the insulation changed.
The practical consequence is that a reading is only comparable with other readings taken in the same mode on the same circuit. If a previous test used UST for the bushing-to-tank section and the next test uses GST, the difference in results is expected and must not be interpreted as a change in condition.
The guard principle is simple in concept: the guard terminal is connected to the insulation that should carry no measuring current, so the instrument measures only the section between the energised and measuring terminals. Because the guard current bypasses the measuring circuit, the reported tan delta and capacitance reflect the intended section alone. Getting this circuit right is the single most important connection task in tan delta testing.
In practice, the mode label on the instrument and the physical circuit must be verified together each time: confirm which terminal is energised, which is grounded and which is guarded, and check that the leads are connected to the correct bushings and taps. A mode selected in software without the matching physical connection produces a reading that is internally consistent but meaningless for comparison.
GST for Grounded Specimen Measurements
In GST (grounded specimen test), one side of the test object is connected to ground and the energised terminal is measured against that ground path. This mode suits insulation-to-ground measurements, such as the insulation from a transformer winding or bushing conductor to the grounded tank or mounting flange, and it is the natural choice when the test object is already connected to ground through its normal configuration.
The GST reading includes all insulation between the energised terminal and ground that lies within the measuring circuit, which can be more than the single section you intended to test. When that extra insulation would confuse the result, the guard mode is the next option.
In a transformer, GST is used for measurements such as the insulation from a de-energised winding to the grounded tank, with the other windings grounded according to the test procedure. The reading then represents the combined ground-side insulation of the energised circuit, which is useful for overall assessment but not for isolating one component.
GSTg for Guarded Grounded-Specimen Measurements
GSTg (grounded specimen test with guard) adds the guard terminal to a grounded-specimen circuit. The guard is connected to the insulation sections that should be excluded from the measurement, so the instrument measures only the section between the energised terminal and ground while the guarded section carries no measuring current.
This is the mode used to isolate one bushing from a transformer winding, or to measure one insulation section while excluding the others that share a common terminal. The connection of the guard is critical: a floating or wrongly connected guard changes the circuit and produces a reading that cannot be compared with the baseline.
For example, when a winding has several bushings, GSTg connects the guard to the other bushings’ terminals so that the measurement includes only the target bushing’s insulation to ground. The same logic applies to excluding a section of winding insulation from a bushing measurement. The mode converts a broad ground measurement into a component-level measurement.
UST for Isolating an Ungrounded Insulation Section
UST (ungrounded specimen test) measures insulation between two terminals when neither side is grounded, or when the section to be measured is isolated from ground on both sides. In a transformer, this mode measures insulation between windings, such as the high-voltage to low-voltage insulation, with both windings ungrounded during the measurement.
For bushings, UST is used for the principal capacitance C1 between the centre conductor and the tap or flange layer, with the bushing isolated from its normal ground connection. The mode is also used for measuring capacitance between two separate windings or between phases where the configuration allows. Confirm that the test object is safely isolated before applying the test voltage.
Because UST requires both sides of the test object to be isolated from ground, the safety preparation is part of the mode decision. Verify the isolation state, apply the approved grounding and discharge procedures, and confirm that no other test activity shares the circuit. The mode choice and the safety preparation belong together in the work instruction.
Typical Applications for Bushings and Transformer Windings
On a power transformer, the three modes work together to separate the insulation sections: GST measures the winding-to-ground insulation, GSTg excludes a selected bushing or section from that ground measurement, and UST measures winding-to-winding or bushing C1 insulation. On a bushing, UST measures the principal capacitance C1, GSTg measures the tap-to-flange capacitance C2, and the correct mode depends on the bushing’s internal construction and the manufacturer’s test guidance.
The reference documents also define context: IEEE C57.12.90 describes factory test methods for liquid-immersed transformers, IEEE C57.152 addresses diagnostic field testing of the same class, and IEC 60137 covers insulated bushings for alternating voltages above 1000 V. Use the document and section that matches your test object and stage, and record the reference in the report so the mode and voltage choice can be audited.
For each measurement, record the capacitance and tan delta with their units, the temperature, the mode and the test voltage, and keep the C1 and C2 results for bushings in separate sections of the record. This structure makes the trend comparison straightforward when the next test date arrives and prevents a C1 value from being compared with a C2 value by mistake.
| Test object | Mode | What is measured | What is excluded |
|---|---|---|---|
| Transformer HV to LV insulation | UST | Insulation between the two windings | Ground paths and external bushings |
| Transformer winding to ground | GST | Insulation from winding to grounded tank | Nothing; full ground circuit |
| One bushing on a winding | GSTg | Bushing insulation to ground | Other bushings and winding sections |
| Bushing C1 | UST | Centre conductor to tap capacitance | Flange and ground path |
| Bushing C2 | GSTg | Tap to flange capacitance | Centre conductor circuit |
Connection Checks That Prevent Misleading Results
Before each measurement, verify the mode label on the instrument matches the physical connection: energised terminal, ground connection and guard connection all have to be in the right place. Check that the guard lead is continuous and connected to the intended terminal, that no floating metal is left in the circuit, and that the transformer is isolated and grounded according to the approved procedure.
Record the mode and connection diagram in the test report. A reading without its mode is not comparable with history, and a mode change between test dates is one of the most common causes of false condition alarms in tan delta trending. The connection record protects both the current interpretation and every future comparison.
Before energising, run a connection verification step: confirm the instrument’s displayed mode matches the selected circuit, check continuity of the guard lead, and scan the work area for floating conductors or tools. A two-minute verification saves a repeat measurement later and keeps the trend data clean.
Use a connection checklist for each mode so the same circuit is built every time, and keep the completed checklist with the test record as evidence of the setup.
Frequently Asked Questions
Which mode is used for bushing testing?
For bushings, UST measures the principal capacitance C1 between the centre conductor and the tap, while GSTg measures the tap-to-flange capacitance C2. The exact mode and test voltage follow the bushing type and the manufacturer’s or applicable standard’s guidance, so confirm before testing.
How do the modes apply to transformer windings?
UST measures insulation between windings with both sides ungrounded, GST measures winding-to-ground insulation, and GSTg excludes a selected section such as a bushing from that ground measurement. The three modes together separate the insulation sections for a complete assessment.
Why does the guard circuit matter so much?
Because the guard defines which insulation carries measuring current. A floating, broken or wrongly connected guard changes the measured circuit, producing a reading that cannot be compared with the baseline and can falsely suggest a condition change.
For the overall tan delta decision framework, see the tan delta testing guide. To compare test sets with full UST, GST and GSTg capability, review tan delta test set options and request a technical proposal.