Abstract: Method and apparatus for detecting defects in an optical cable composed of a series of cable units. Light pulses are sent through one cable conductor and returned to the starting end via a second conductor and an electrical signal representative of the light intensity exiting the second conductor is derived. The electrical signal is applied to a sensitivity control circuit which is switched through a succession of sensitivity levels starting from a minimum level and proceeding to a maximum level. Each sensitivity level corresponds to the attenuation produced by a cable composed of a given number of cable units and is associated with a particular ratio between the output signal provided by the sensitivity control circuit and the electrical signal applied thereto.

Abstract: A cooling system for communications equipment includes a closed container for housing the equipment and protecting the equipment against environmental influences. The container has an outer wall made of heat conductive material and is constructed for at least partial burial in the earth and for a heat transfer relationship therewith. A fluid filled conduit system includes conduits disposed outside the container and in heat transfer relationship with the outer wall thereof. The conduits outside the container are arranged to be buried in the earth for a heat transfer relationship with the earth. Heat conducting means are in thermal contact with the equipment and the container wall for transmitting heat generated by the equipment to the wall and to the conduit system. Thus, heat generated by the equipment is transferred to the fluid and heat in the fluid is transferred to the earth, and the fluid is circulated in the conduit system solely due to temperature and density variations of the fluid.