State and explain properties of transformer oil.  
in Testing and maintenance of electrical equipments by

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Following are the properties of transformer oil:- 

1. Dielectric strength :- It should be have a high dielectric strength 

2. Specific resistance:- It should be have a high Specific resistance. 

3. Dielectric dissipation factor (DDF) (tanδ):- It should be as low as possible. 

5. Relative permittivity (Dielectric constant):- It should be 2.2 

4. Flash Point :- Oil should have very high flash point. (1600c minimum, or greater than 1400c) 

5. Fire point - It should have high fire temperature (not less than 200°C) it should be 25% greater than flash point. 

6. Pour Point: - (Indicates the lowest temperature at which the insulating oil will flow.) It should be low (Pour point of transformer oil is an important property mainly at the places where climate is extremely cold.)(- 60c to – 400c) 

7. Viscosity:- Oil should have low viscosity at 1000c 34 say bolt 

8. Density:- Oil should have low density. Density of oil at 200C should be 0.89 gm/cm3. 

9. Moisture content:- Oil should be free from moisture (moisture content should be less than 10 ppm (Parts per million) Water content in oil is allowed up to 50 ppm 

10. Dissolved gas:- Oil should be free from dissolved gas. 

11. Acidity content:- Oil should be chemically stable. Acidity content should be very low. (0.03mg KOH/mg Maximum) 

12. The oil should be clear & plane in colour, transparent & free from suspended matter. 

13. For mineral oil, the power factor of new oil should not exceed 0.05 percent at 250c. 

14. It should be not contain impurities such as sulpher & its compounds to avoid rusting & sludge formation. 

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