Abstract: A security system for controlling access to an enclosed conduit along which runs fibre optic cable, the conduit having a cover for access to the cable, the system having a communications receiver and a modulator responsive to an output of the receiver for coupling non-intrusively to the fibre optic cable adjacent the cover, the communications receiver being responsive to the presence or absence of an access authorisation signal from an operator to cause the modulator to introduce a signal to the fibre optic cable indicative of the authorisation status.