“A Stitch in Time...”

Values of C at 20°C
60-Hertz
25-Hertz
Tanked oil-filled type 1.5 1.0
Untanked oil-filled type 30.0 20.0
Dry or compound-filled type 30.0 20.0
This formula is intended for single-phase transformers. If the transformer
under test is of the three-phase type, and the three individual windings are
being tested as one, then:
E = voltage rating of one of the single-phase windings
(phase to phase for delta connected units and phase to
neutral for star connected units)
kVA = rated capacity of the completed three-phase winding under test
Tests Using Multi-Voltage Megger Insulation Testers
Maintenance practice trends indicate the value of testing insulation with
DC voltages at levels somewhat higher than the peak value of the rated AC
voltage of the equipment being tested. Such DC tests have in some cases
been shown to reveal non-destructively incipient weaknesses in insulation
which could not otherwise be found, except possibly by partial discharge
detection at non-destructive AC test voltage levels.
The technique involves the application of two or more DC voltages, and
critically observing any reduction of insulation resistance at the higher
voltage. Any marked or unusual reduction in insulation resistance for a
prescribed increase in applied voltage is an indication of incipient weakness.
It is important to mention that the merits of this technique arise from more
recent investigations which indicate that rather high DC voltage can be used
to detect weaknesses without damaging the insulation. The maximum value
of voltage which should be used will depend largely on the cleanliness and
dryness of the insulation to be tested.
In making tests on insulation at such DC voltages, the ohmmeter method
has at least two advantages. First, prescribed fixed voltages are switched
into use, and one instrument measurement made with the direct reading
ohmmeter. This is a simple and reproducible method compared to one in
which many choices of voltage are available. Another important advantage
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A STITCH IN TIME