Abstract: A sensor for use on the front end of a flexible, resilient probe for use in an underground duct of the type which contains one or more current-carrying cables. The sensor is provided with a signal generating device for detecting a particular operating condition of a cable in the duct when the sensor moves through the duct adjacent to the cable. In one embodiment of the sensor, acoustical signals are sensed by a crystal when such acoustical signals originate from a leak in the pressurized sheath surrounding the cable. In another embodiment of the sensor, a coil senses the magnetic field surrounding the cable so that electrical shorts occurring in the cable can be monitored. In a third embodiment of the sensor, a heat detector is provided so that the sensor can detect hot spots in a duct.

Abstract: A duct probe having a coaxial cable is provided to advance a microphone through an underground or other duct containing a cable in a pressurized sheath for sensing fluid leaks in the sheath. The coaxial cable is embedded in a resilient fiberglass rod of one-piece construction. The rod is sufficiently flexible so that it can be wound onto and unwound from a reel having a cage-like configuration. A stationary base is provided to rotatably mount the reel, and the base is provided with a length measuring device responsive to the movement of the duct probe as it is unwound from the reel so that the distance by which the duct probe penetrates a duct can be determined. At the outer end of the rod, a microphone is mounted for sensing the sounds caused by a fluid leak in the pressurized sheath in the duct.