OMICRON 61
High-voltage bushings
6.5 Grounded Specimen Test (GST)
This test measures the quality of the insulation between the current carrying or center conductor and the
mounting flange of a bushing. This test is conducted on bushings that have been removed from
equipment, bushings connected to de-energized equipment, spare bushings, or bushings that have
been isolated from connected windings and interrupters. The test is performed by energizing the bushing
conductor and grounding the flange.
Figure 6-4: GST bushing test (C
2
)
6.6 Hot collar test
This test measures the condition of a specific small section of bushing insulation between an area of the
upper porcelain rain shed and the current carrying or center conductor. The test is performed by
energizing one or more electrodes placed around the bushing porcelain with the bushing center
conductor grounded. This test is used to supplement the three previous tests. It is also used to test
bushings in apparatus when the three tests are either inapplicable or impractical, such as, with SF6
bushings. Perform a hot-collar test at every third skirt on SF6 bushings.
Hot-collar tests are effective in locating cracks in porcelain, deterioration, or contamination of insulation
in the upper section of a bushing, low compound or liquid level, or voids in compound often before such
defects are noticeable with the previous tests.
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