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Important properties


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This guide describes Specifications for electrical insulating oils of petroleum origin intended for use in electrical cables, transformers and other electrical apparatus where the oils are used as insulating, or heat transfer media, or both.


1- Aniline Point: The aniline point of an insulating oil indicates the solvency of the oil for some materials that are in contact.

2- Color: A low color number is an essential requirement for inspection of assembled apparatus in a tank. An increase in the color number during service is an indicator of deterioration or contamination of the insulating oil

3- Flash Point: The flash point test gives an indication of the Flammability of an oil.

4- Interfacial Tension: 

Interfacial tension measurements on electrical insulating oil provide a sensitive means of detecting small amounts of soluble polar contaminants and products of oxidation. The test is frequently applied to service – aged oils as an indication of the degree of deterioration.

5- Pour Point : The pour of a transformer oil gives an indication of the temperature below which it may not be possible to pour or remove the oil from its container.

6- Relative Density (Specific Gravity ) : The relative density (Specific gravity ) of a mineral insulating oil influences the heat transfer rates and may be pertinent in determining suitability for use in specific application. In certain cold climates, ice my form in de-energized transformers exposed to temperatures below 0°c, and the maximum specific gravity of the oil used in such equipment should be at a value that will ensure that ice will not float in the oil at any temperature the oil might attain.

7- Viscosity : Viscosity of electrical insulating oils influences their heat transfer properties, and consequently the temperature rise of apparatus containing the liquid. At low temperature, the resulting higher viscosity influences the speed of moving parts.

Viscosity controls insulating oil processing conditions, such as dehydration, degasification and filtration, and oil impregnation rates. High viscosity may adversely affect the starting up of apparatus in cold climates.

8- Visual examination: the observation of the color and condition of the oil in a field inspection permits a determination of whether the sample should be sent to a central laboratory for full evaluation.


1- Dielectric Breakdown Voltage: The ability of a fluid to resist breakdown under the test condition is an indication of the ability of the fluid to perform its insulating function in electrical apparatus.


1-1- Breakdown Voltage – The square – edged disk electrodes: This method is recommended for acceptance tests on unprocessed insulating liquids received from vendors in tank cars, tank trucks, and drums. It may also be used for the routine testing of liquids from selected power systems apparatus.

1-2-Breakdown Voltage – VDE electrodes : This test method is more sensitive to the deleterious effects of moisture in solution. This method is recommended for testing filtered, degassed, and dehydrated oil prior to, during and after the filling of power systems apparatus.

2- Dissipation Factor: This property is a measure of the dielectric losses in an oil. A low value of dissipation factor indicates low dielectric losses and a low level of soluble polar ionic or colloidal contaminants.


1- Oxidation Inhibitor Content: The quantitative determination of 2,6- ditertiary – butyl para – cresol or 2,6 – ditertiary – butyl phenol measures the amount of this material that has been added  to new electrical insulating oil as protection against oxidation or the amount remaining in a used oil.

2- Sulfur, Corrosive: In most of their uses, insulating oils are continually in contact with metals that are subject to corrosion, such as copper or silver. Since the presence of detrimental corrosive sulfur compounds will result in deterioration of these metals to an extent dependent upon the quantity and type of corrosive agent.

3- Water Content: A low water content of insulating oil is necessary to  achieve adequate electrical strength and low dielectric loss characteristics, to maximize the insulation system life, and to minimize metal corrosion.

4- Neutralization Number: A low total acid content of an insulation oil is necessary to minimize electrical and metal corrosion and to maximize the life insulation system.

5- Oxidation Stability: The development of oil sludge and acidity resulting from oxidation during storage, processing and long service life should be held to a minimum. This minimize electrical conduction and metal corrosion, maximizes insulation system life and electrical breakdown strength, and ensures satisfactory heat transfer.

6- Gas Content: Some types of electrical equipment require use of electrical insulating liquids of low gas content. In filling electrical apparatus, a low gas content reduces foaming and also reduces available oxygen in sealed equipment, increasing the service life of the insulating oil.

7- Polychlorinated Biphenyl (PCB) Content: Electrical apparatus and electrical insulating fluids containing PBC be handled and disposed of through the use of specific procedures as determined by the PBC content of the fluid. The result of this test method (D-4059) can be useful in selecting appropriate handling and disposal procedures.