Should insulating oil flash point test be open or closed?

The flash point refers to the lowest temperature at which a flash fire occurs when the oil is heated to a certain proportion of its steam and air mixed with air, and a flash fire occurs when it is close to the specified flame, accompanied by a short popping sound. It reflects the ease with which the oil catches fire and the amount of light fractions in the oil. The more light fractions in the oil, the lower the flash point. There are open cup method and closed cup method for flash point test. The insulating oil used in the power system is mostly used in a closed container, so the test results of the closed cup method shall prevail. For new oils, a low flash point indicates an oil with too much light distillate or an oil mixed with light distillates. For operating oils, the reduction in flash point is often due to the breakdown of the insulating oil to produce volatile and flammable low-molecular-weight hydrocarbons due to internal faults in the equipment.