Which letter of the transformer cooling class represents the external cooling medium?

The correct answer indicates that the third letter of the cooling class designation in transformers represents the external cooling medium. Transformer cooling classes are expressed in a standard format, where each letter denotes specific characteristics of the cooling system employed.

In this system:

• The first letter signifies the type of cooling method used within the transformer itself, such as oil or air.

• The second letter specifies the cooling method for the transformer, indicating whether it is naturally cooled or has forced cooling.

• The third letter indeed reflects the external cooling medium, which is crucial for determining how the transformer dissipates heat to the surrounding environment. This could indicate the use of air or water as a cooling medium, and effectively influences the transformer's thermal performance and cooling capacity.

Understanding this classification helps in assessing the cooling efficiency and operational capabilities of different transformers, critical for ensuring their reliability and longevity in various applications.