Transformer oil in good condition must withstand _____kV applied across two flat electrodes submerged in a sample of the oil without arcing.

• A. 10
• B. 20
• C. 30
• D. 40

Answer: (C)

Dielectric strength is the voltage that the insulating oil can withstand across a gap without forming an arc. In a standard test, two flat electrodes are submerged in a sample of oil and the voltage is raised until breakdown occurs. For oil that’s in good condition, this breakdown voltage is typically around 30 kV in that setup, which is why 30 kV is given as the benchmark. If the oil only withstands 10 or 20 kV, contaminants or moisture are likely reducing its insulating quality. A value as high as 40 kV could occur with exceptionally clean oil, but the common minimum specification used to judge “good condition” in exams is about 30 kV.