The breakdown strength of transformer oil, also known as its dielectric strength, refers to the maximum voltage that the oil can withstand without undergoing electrical breakdown (i.e., without forming an electrical arc). Typically, new transformer oil should have a breakdown strength of at least 30 kV (kilovolts) for a 2.5 mm gap between the electrodes in a standard BDV test. Higher breakdown strength indicates better insulating properties and is critical for the reliable performance of transformers.