Why does pressure (in the form of gas) sometimes build up in a transformer?

• A. A short in the winding
• B. Overheating
• C. External gas
• D. Mechanical fault

Answer: (A)

Gas pressure inside a transformer mainly comes from the insulating oil breaking down when the transformer overheats. The oil used for cooling and insulation can decompose at high temperatures and release gases such as hydrogen and methane. As these gases accumulate, pressure builds up inside the transformer and may trigger protective relays or vents.

A short in a winding can cause overheating or arcing, which can accelerate gas formation, but the direct cause of the gas buildup is the overheating of the oil, not the short itself. External gas can’t accumulate inside the transformer, and while a mechanical fault might worsen cooling and contribute to overheating, it isn’t the primary reason for gas generation.

So overheating is the best answer because it directly drives the chemical processes that produce gas and raise internal pressure.