IEC 61202-1: 2016 applies to isolators used in the field of fibre optics, all exhibiting the following features: - they are non-reciprocal optical devices, in which each port is either an optical fibre or fibre optic connector; - they are passive devices containing no opto-electronic or other transducing elements; - they have two optical ports for directionally transmitting optical power. This fourth edition cancels and replaces the third edition published in 2009. It constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: - the terms and definitions were reconsidered; - quality assessment level was deleted from classification; - the clause numbers of Annexes A and B have been rearranged.