What does the letter 'O' signify in the cooling class of transformers?

The letter 'O' in the cooling class of transformers stands for 'Oil' and specifically indicates that the transformer uses oil as a coolant. More precisely, it refers to the use of an oil with a flash point that is less than or equal to 300°C. This classification is important in determining the thermal properties and safety considerations of the transformer.

Using this type of oil means the transformer is designed to operate safely under the thermal conditions established for oils with this flash point. The classification ensures that users understand the cooling mechanism and the associated risks of operating the transformer, particularly in relation to fire hazards and thermal management.

In contrast, oils with a flash point greater than 300°C would suggest a higher level of fire resistance. The reference to no measurable flash point would indicate a different type of cooling method or fluid, typically associated with non-combustible fluids. A cooling class that indicates no requirement for cooling is typically indicative of self-cooled or entirely passive systems, which does not apply to those using oil as a cooling medium.