NTT performs tests according to standards approved by the American
Society for Testing and Materials (ASTM). These tests are designed to
evaluate the ability of the oil to perform its job. Tests include moisture
in oil, interfacial tension, acid number, color number, visual, dielectric
strength, viscosity, specific gravity, power factor at 25oC & 100oC,
oxidation inhibitor, refractive index, pour point, and flash point.
Water in Insulating Fluids
ASTM D 1533B
The presence of water can adversely affect the
dielectric strengths of an insulating fluid. Water content is reported in
parts per million.
ASTM D 974
Oxidation of insulating fluids and/or additives in the
fluid results in the production of acidic compounds. The periodic
measurement of acidity provides a means of monitoring the progress of
oxidation. The build-up of acidic compounds precedes the formation of
sludge in the transformer which is the end-product of oxidation.
ASTM D 971
Determining the presence of polar contaminants in insulating oil is
accomplished by measuring the tension of oil against water. The
interfacial tension of an oil is sensitive to the presence of the products
of oxidation of the oil and can be used, together with acidity
measurements, as an indicator to monitor sludge development. Foreign
substances such as dissolved varnishes andother organic coating materials
can also affect IFT. The presence of polar contaminants generally lowers
the interfacial tension value.
Visual Examination and Color
ASTM D 1524
Monitoring the color and visual appearance of an
insulating oil provides a rapid assessment of oilquality. Insulating oils
tend to darken due to oxidation and/or presence of contamination. Materials
suspended in the oil or sediment are assessments of oil quality and/or
Dielectric Breakdown Voltage
ASTM D 877
ASTM D 1816
The dielectric breakdown voltage is the
voltage at which an insulating fluid begins to conduct. This voltage
denoted the electrical stress that an insulating fluid can withstand
ASTM D 1298
The specific gravity of an oil is the ratio of the
weights of equal volumes of oil and water at the same temperature.
Specific gravity is applicable in determining suitability for use in
specificsituations. In cold climates, specific gravity can be used to
determine whether ice resulting from freezing water in the oil will float
on the oil.
ASTM D 445
ASTM D 2161
Viscosity is the measured resistance of
the insulating fluid to flow under specific conditions. Viscosity
influences heat transfer characteristics of the insulating fluid.
ASTM D 924
Power factor is a measure of the dielectric losses in an
insulating fluid due to heat dissipation when the fluid is placed in an
electric AC field. Power factor is usually performed in conjunction
with other oil quality tests to determine the state of the insulation
This is the measurement of the amount of inhibitor remaining in the
fluid after oxidation has reduced its concentration. Inhibitors can be
added to oil in prescribed amounts to increase the service life of the
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