This invention relates to a technique for selectively locating a short haul cable or pipe (a utility conveyance) that runs off a main conveyance.
Many utilities, such as ATandT, bury their pipes and cables (xe2x80x9cutility conveyancesxe2x80x9d) underground both for reasons of safety and esthetics. Underground burial often provides protection to such utility conveyances against weather and other sources of potential damage. Utilities that undertake burial of their conveyances usually make extensive efforts to plot the location of each buried conveyance on a map to facilitate its location in case of repair or replacement. While a map will indicate the general location of a buried conveyance, more precise location information often becomes necessary, particularly in urban environments. For that reason, most utilities that bury their conveyances underground typically rely on electromagnetic signaling techniques to precisely locate such conveyances.
U.S. Pat. No. 5,644,237, issued in the names of Hossein Eslambolchi and John Huffman, on Jul. 1, 1997, and assigned to ATandT, (incorporated by reference herein) discloses an electromagnetic signaling technique for locating a buried utility conveyance. A signal generator applies a locating signal and a confirmation signal to a metallic part of the conveyance. In the case of an optical fiber cable, the metallic part comprises either a metallic sheath or a copper trace wire within the cable. Using a signal detector, a technician detects both the locating signal and the confirmation signal radiated above ground to precisely locate the buried conveyance.
Conveyance location via electromagnetic signaling works well for long haul conveyances. However, some conveyance routes, and in particular, certain fiber cable routes in urban areas, include one or more short haul cables (xe2x80x9cside-legxe2x80x9d) cables that run between a long haul (backbone) cable and a telecommunications facility. Each side leg cable connects the metallic sheath of the backbone cable to ground, thus allowing the locating signal on the backbone cable to pass into the side leg cable. All other considerations being equal, a side leg cable having a smaller impedance (i.e., a shorter signal path to the cable locating signal generator) will radiate a higher strength locating signal. Conversely, the side leg cable having a larger impedance (i.e., a longer signal path to the cable locating signal generator) will radiate a lower strength locating signal, making locating of such a side leg cable more difficult. In the past, overcoming the problem of a low-strength locating signal on a side leg cable required the addition of a separate locating signal generator for that side leg. Such locating signal generators are expensive, and adding a separate locating signal generator for each side leg will greatly increase costs.
Thus, there is a need for a technique that enables accurate location of each of a plurality of side leg cables by means of electromagnetic signaling without the need for a separate locating signal generator for each side leg.
Briefly, in accordance with a preferred embodiment, there is provided a method for selectively applying a conveyance-locating signal to each of a plurality of short haul (side leg) underground utility conveyances, each coupled to a backbone conveyance. In accordance with the method, a conveyance-locating signal generator applies a plurality of different frequency conveyance-locating signals on the backbone conveyance. A notch filter, comprising part of a filter arrestor assembly, connects each short haul conveyance to ground for selectively passing a particular frequency locating signal only. In this way, each short haul conveyance carries a locating signal of a particular frequency whose strength remain unaffected by locating signals on the other short haul conveyances.