How does a short in the windings cause gas to form in transformer oil?

• A. The impurities in the oil break down creating gas
• B. Oil expands with heat
• C. Air leaks into the oil
• D. The winding generates gas directly

Answer: (A)

When a short occurs in the windings, the fault concentrates energy in a small region of the insulating oil, causing intense electrical stress and heating. This energy breaks down the oil molecules, and impurities in the oil—such as moisture or contaminants—make this breakdown easier by lowering dielectric strength and providing reactive sites. The decomposition of the oil under these conditions produces gaseous byproducts, which evolve as gas in the oil. So the formation of gas isn’t from the winding itself but from the oil breaking down due to electrical fault energy, with impurities accelerating that process. Gas presence is a diagnostic sign of insulation breakdown under fault conditions.