The present document contains requirements for: - the output performance of the direct current (dc) power equipment at the interface "A"; - the input of the telecommunications equipment connected to interface "A" powered by dc. The dc voltage at interface "A" may be derived from the AC primary supply. The dc supply may incorporate a backup battery. If any other equipment e.g. datacom equipment, are connected to interface "A" in parallel to telecommunications equipment, all requirements addressed in clause 4 of the present document must be fulfilled. The present document aims at providing compatibility between the power supply equipment and the power consuming telecommunications equipment, and also between different system blocks connected to the same power supply. NOTE 1: The present document is applicable only to -48 Vdc power supply interfaces. However, during a transitional period, other dc voltages may be used in existing installations. Annex A gives guidance on working in conjunction with existing -60 Vdc supply systems. The power supply interface, interface "A", is a physical point to which all the requirements are related (see figure 1). This point is situated between the power supply system(s) and the power consuming telecommunications equipment. NOTE 2: Interface "A" is located at the power terminals of the telecommunications equipment. Subject to the installation preconditions this point may be located at any other point between the power supply system and the telecommunications equipment by mutual agreement of the relevant parties. The requirements at interface "A" apply to: - the output of the power supply equipment or power supply installation of telecommunications centres; - the power supply input of telecommunications equipment installed at telecommunication centres; - telecommunications equipment, installed in customers' premises, whose dc interface "A" is also used by equipment requiring a supply to this specification. NOTE 3: Normally there is more than one load unit connected to interface "A". In these cases, interface "A" will undergo further restrictions with respect to noise limits or other disturbances specified in clause 4.8. NOTE 4: Disturbances on the power supply interface "A" relating to the continuous wave phenomena below 20 kHz are covered within the present document. NOTE 5: An example of a configuration in which interface "A" is identified is given in annex D.