IEC 61753-052-3:2016 contains the minimum initial test and measurement requirements and severities which a fibre optic attenuator satisfies in order to be categorised as meeting the requirements of single-mode fibre non-connectorized fixed attenuator devices used in uncontrolled environments. IEC 60869-1 contains the generic specification of the optical attenuator. Optical performances specified in this document relate only to non-connectorized configurations optical attenuators. This second edition cancels and replaces the first edition published in 2001. This edition constitutes a technical revision. This edition includes the following significant technical change with respect to the previous edition: reconsideration of performance requirements. Keywords: non-connectorized fixed attenuator devices, Category U in uncontrolled environments