This standard specifies a test method for evaluating the combustion performance of insulating liquids by means of the Oxygen Index (OI). The principle involves placing a glass cup containing the liquid sample in a specific combustion tube and introducing a controlled mixture of oxygen and nitrogen. By igniting the sample and precisely adjusting the oxygen concentration, the minimum oxygen concentration required to sustain steady combustion for 60 seconds is determined, which is defined as the Oxygen Index. The core equipment for performing this test is the Oxygen Index Tester, which mainly consists of a heat-resistant glass combustion tube, a sample cup and holder, a precise flow measurement and control system, an igniter, and a timer. This method is primarily applicable to various insulating liquids, such as transformer oil and capacitor oil, with a viscosity not exceeding 50 mm²/s at 40°C, providing critical data for their flame-retardant safety assessment in electrical equipment.