This paper describes the application of EN 15085 in the manufacture of an power transformer. The standard refers to the welding of the steel structure and its alloys. The same standard EN 15085 is aimed at welding railway vehicles and railway components. Power transformers which is transported by railways to the desired location are manufactured according to EN 15085. The purpose and role of the transformer in energetics are described in the paper, described are "step up" and "step down" transformers, and their role in the energy chain. The basic components of an power transformer (steel construction) are described and described, those that are subject to the EN 15085 standards. In order to clarify the purpose and meaning of EN 15085 in a short term, all parts of the standard (five parts of the standard) are described. The main requirements of the standard are outlined and explained. The qualification of the welding process is described in detail in the example illustrated in the paper. Qualification of the procedure is described from welding the test sample until the qualification is issued by the accreditation body. In the same section non-destructive tests are described, while destructive tests are listed according to norms. In order to clearly understand in which part of the manufacturing process of a transformer tank (steel construciton) and its components is represented the EN 15085 standard, a review and description of the production activities is given. An overview and description of production activities is given from the start of production, including technological processing, cutting positions, etc. to the surface protection and shipment of the finished product in order to gain a better insight into the entire production process. Since EN 15085 is about welding and related processes, a larger and more detailed part of this paper is devoted to project welding documentation. In this section are listed activities to be carried out before the start of welding the project and it is shown a review of the required documents for traceability in the welding process of the whole project. In the experimental part of this paper, a production weld tests was made. The production weld tests is described step by step and supported by the images, diagrams and test results.