EN 50216-9 deals with oil-to-water heat exchangers that means a heat exchanger for the cooling of the transformer oil using a forced oil circuit and a forced water circuit. The oil-side of the oil-to-water heat exchangers is not included in the scope of the Pressure Equipment Directive 97/23/EC according to Article 1, § 3.12. The water-side falls into Article 3, § 3 of the Pressure Equipment Directive, therefore the rating plate must not contain a CE sign according to Article 15 of the Pressure Equipment Directive. This standard establishes essential dimensions and the requirements to ensure interchangeability and adequate mounting of the oil-to-water heat exchangers. This standard applies for dimensions and requirements of oil-to-water heat exchangers. The increase of versions of oil-to-water coolers with higher ratings and constructional changes as well are considered in the revised standard.