The present document contains requirements and measurements methods for the physical interface "A" that is situated between the power supply system(s) and the power consuming ICT equipment. The nominal voltage at power interface "A" of ICT equipment defined in the present document is DC voltage -48 V. The DC power can be supplied by a DC output power system (e.g. based on AC rectifiers on grid or DC/DC converters on solar system, fuel cell, DC engine or fuel cell generator) and also directly supplied by a battery backup in this DC power system. The purpose of the present document is to use a power supply system with the same characteristics for all ICT equipment defined in the area of application: - to facilitate inter working of different types of load units; - to facilitate the standardization of ICT equipment; - to facilitate the installation, operation and maintenance in the same network of ICT equipment and systems from different origins. The present document aims at providing electrical compatibility between the power supply equipment and the power consuming ICT equipment, between different system blocks and loads connected to the same power supply feeding the interface "A" (e.g. control/monitoring, cooling system, etc.). The requirements are defined for: - the power supply input of any type of ICT equipment installed at telecommunication centres that are connected to interface "A" powered by DC; - any type of ICT equipment, installed in access networks and customers' premises, the DC interface "A" of which is also used by equipment requiring a DC supply source; - any type of ICT equipment powered by DC, used in the fixed and mobile networks installed in different locations such as buildings, shelters, street cabinets. Disturbances on the power supply interface "A" relating to the continuous wave phenomena below 20 kHz are covered within the present document. The present document does not cover safety requirements, they are covered by relevant safety standards. The present document does not cover EMC requirements, they are covered by relevant EMC standards. NOTE: Annex B gives guidance on -60 VDC supply systems.