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Examoogle • User • info@examoogle.com • EE-2024-8821
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What is standard dielectric strength of oil?
35KV
50KV 40KV
40KV 50KV
30KV
30KV
Quick Summary: The standard dielectric strength of transformer oil, also known as the breakdown voltage, is officially specified as 30 kV (rms) for a 2.5 mm gap using standard spherical electrodes according to IS 335 standards. This value serves as the minimum threshold to ensure the oil can effectively act as an insulator and coolant within power transformers.
The standard dielectric strength of transformer oil, also known as the breakdown voltage, is officially specified as 30 kV (rms) for a 2.5 mm gap using standard spherical electrodes according to IS 335 standards. This value serves as the minimum threshold to ensure the oil can effectively act as an insulator and coolant within power transformers.
E=dV — where E is dielectric strength, V is breakdown voltage, and d is electrode gap distance.
The dielectric strength of oil is primarily governed by its purity, specifically the presence of moisture, dissolved gases, and suspended particles. Under an applied electric field, these impurities align to form conductive bridges across the gap, leading to electrical breakdown. Maintaining the dielectric strength above 30 kV ensures that the insulation system remains robust against transient overvoltages and steady-state operating stress.
The standard test uses a gap of 2.5 mm between two 12.5-13 mm diameter spherical electrodes.
Dielectric strength is measured using an oil testing set following IS 335 specifications.
Presence of water even in parts per million (ppm) significantly reduces the dielectric strength.
Oil is used for both internal insulation and heat dissipation in transformers.
High insulation resistance
Effective cooling medium
Oxidation stability
Flammable nature of mineral oil
Sensitive to moisture contamination
Requires periodic testing and maintenance
Power transformers
Distribution transformers
Circuit breakers (OCB)
Tap changers
The 30 kV value is the minimum limit; healthy oil typically shows values between 40 kV to 60 kV depending on treatment.
Oil with a dielectric strength below 30 kV indicates significant contamination, necessitating filtration or replacement.
D is correct — The standard dielectric strength for fresh transformer oil is 30 kV when measured across a 2.5 mm gap.
Always remember that dielectric strength is inversely proportional to the level of moisture and impurities; a drop below 30 kV is a primary trigger for oil filtration in maintenance scheduling.