This Technical Specification defines the classification of the electric onboard auxiliary power converter system and defines its basic characteristics and interfaces. The onboard auxiliary power converter system consists of the auxiliary converter and the battery charger function. This Technical Specification applies to locomotive hauled passenger trains and electric multiple units with distributed power as well as trains with concentrated power heads. Relevant train configuration and concerned energy supply subsystems are defined in CLC/TS 50534. This Technical Specification provides a technical base for implementation of onboard auxiliary power systems on different trains. The objective of this specification is to define the required interfaces and characteristics of the onboard auxiliary power converter system in order to enable further standardisation: - interface between onboard auxiliary power converter system and onboard traction power system; - interface of the onboard auxiliary power supply system to the low voltage grid and to a shore supply (stationary workshop supply or external supply); - interfaces of the auxiliary converter and the battery charger; - characteristics of the onboard auxiliary power converter system. The electrical operational behaviour is defined by requirements. Requirements for the type tests as well as the routine test are referred.