This part of IEC 61753 contains the minimum test and measurement requirements and severities which a fibre optic circulator as specified by IEC 62077 should satisfy in order to be categorized as meeting the requirements of circulators used in controlled environments as specified in IEC 61753-1:2018, COR1:2019 and AMD1:2020. The requirements cover non-connectorized single-mode fibre 3-port incompletely circulated type optical circulators for category C used in controlled environments.