What does the second letter of the four-letter cooling code on liquid-immersed transformers identify?

The correct answer isB. Forliquid-immersed transformers, the four-letter cooling code is used to describe both thecooling mediaand themethod of circulationfor the internal and external cooling systems. In this code structure, thefirst letteridentifies theinternal cooling mediumthat is in contact with the windings, while thesecond letteridentifieshow that internal cooling medium circulates. This may be by natural circulation or by forced circulation, depending on the transformer design. Thethird letterthen identifies theexternal cooling medium, and thefourth letterindicates thecirculation method of that external medium.

This coding is important in source inspection because the cooling class affects transformer rating, heat dissipation capability, accessories, and verification of nameplate data. During inspection and surveillance, the source inspector checks that the specified cooling class on the nameplate, drawings, and test documentation matches the purchase requirements and transformer design. Misidentifying the cooling code can lead to misunderstanding of the transformer’s thermal performance and service suitability. Therefore, thesecond letterspecifically represents thecirculation mechanism for the internal cooling medium.