Electricity: electrical systems and devices – Safety and protection of systems and devices – High voltage dissipation
Reexamination Certificate
1997-09-04
2001-09-18
Leja, Ronald W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
High voltage dissipation
C361S111000
Reexamination Certificate
active
06292344
ABSTRACT:
TECHNICAL FIELD
The present invention relates to communications cable locating systems and, in particular, to a floating ground isolator for connection to a communications cable sheath to facilitate cable locating activities.
BACKGROUND OF THE INVENTION
It is imperative that communications cables be grounded to discharge damaging and/or annoying induced energy resulting from transient surges (caused by lightning, transient voltages and power crosses), periodic signals (such as sixty cycle interference from power lines) and other known causes. Conventional cable sheath grounding methods require a fixed electrical connection at selected locations (typically at cable splice points) between the cable sheath and earth ground. Grounding of the cable sheath at each splice point provides a plurality of signal paths for bleeding off induced energy present on the sheath and protecting the cable and associated communications equipment from damage and interference.
With buried communications cables, however, facilitating long range cable locating using conventional signal radiating techniques is equally imperative to protecting the cable from induced energy. In the conventional locating method, a locating signal is transmitted along the length of the cable sheath. The cable sheath functions as an antenna radiating the transmitted locating signal through the earth for subsequent surface detection. The strength of the received radiated signal gives an indication of the approximate location of the buried communications cable.
Fixed grounding of the communications cable sheath as described above, however, precludes long range cable locating using conventional signal radiating techniques because the locating signal transmitted on the cable sheath will be shunted to ground at the first cable splice point encountered along the length of the cable. Thus, to enable long range cable locating, the ground connection at each splice point must be disconnected (lifted) by cable maintenance personnel to allow the locating signal to propagate along the entire length of the communications cable. Over the course of a thirty to sixty mile length of communications cable, it becomes prohibitively inconvenient, inefficient and expensive to have the ground connection lifted at each one of dozens of splice points.
Of further concern with respect to long range cable locating is the difficulty experienced with locating communications cables on side legs (or spurs) emanating from a main communications cable. Due to the potential differences in impedance of the sheath caused by differences in side leg lengths, it is likely that the cable locating signal will propagate at different, sometimes insufficient strengths, down each side leg. If the locating signal has insufficient signal strength, radiation from the sheath and surface detection may become difficult or impossible, thus adversely affecting the accuracy of the cable locating activity.
SUMMARY OF THE INVENTION
The present invention comprises a floating ground isolator for protecting a communications cable sheath from induced energy while simultaneously facilitating cable locating using conventional signal radiating techniques. The floating ground isolator includes circuit means for providing a connection to ground for shunting selected damaging and/or annoying induced energy signals from the cable sheath. The use of the term “floating” refers to the fact that the circuit means of the floating ground isolator also simultaneously presents an open circuit (lifting or floating the hardwired ground connection) with respect to certain selected signals on the cable sheath, for example, a transmitted cable locating signal. Thus, with the floating ground isolator of the present invention, cable protection and cable locating services are efficiently provided in a simultaneous manner, thus obviating the need for lifting of sheath ground connections at each splice point prior to engaging in cable locating activities.
In one embodiment of the present invention, the floating ground isolator comprises a surge protection circuit that functions to shunt transient surges on the sheath exceeding a threshold voltage to ground. With respect to any other signal present on the sheath, including the cable locating signal and surges below the threshold, the ground connection through the surge protection circuit “floats” to effectively present an open circuit. The surge protection circuit preferably comprises a metal oxide varistor (“MOV”) and gas tube connected in parallel between the cable sheath and ground.
In another embodiment of the present invention, the floating ground isolator comprises a bandpass filter circuit that functions to shunt periodic signals on the sheath that are within a predetermined bandpass frequency response to ground. With respect to any other signal present on the sheath, including the cable locating signal, transient surges and those periodic signals outside the bandpass frequency, the ground connection through the filter circuit “floats” to effectively present an open circuit. The filter circuit preferably comprises an inductor connected in series with a capacitor between the cable sheath and ground.
Because the filter circuit cannot function to protect the communications cable from damage due to transient surges, in yet another embodiment of the present invention, the floating ground isolator comprises the filter circuit connected in parallel with the surge protection circuit. Thus, periodic signals within the bandpass response and transient surges exceeding the acceptable voltage threshold on the cable sheath will be shunted to ground. With respect to any other signal present on the sheath, including the cable locating signal, the ground connection “floats” to effectively present an open circuit.
In yet another embodiment of present invention, the floating ground isolator further includes variable impedance means connected between the cable sheath and ground for use at cable termination points on side legs. The variable impedance compensates for the differences in impedance encountered with side lengths of varying lengths. By adjusting the variable impedance, the strength of the cable locating signal propagating down each side leg is adjusted to enable surface detection and locating of the cable using conventional techniques.
REFERENCES:
patent: 3668640 (1972-06-01), Driscoll
patent: 4021759 (1977-05-01), Campi
patent: 4023071 (1977-05-01), Fussell
patent: 4262317 (1981-04-01), Baumbach
patent: 5122921 (1992-06-01), Koss
patent: 5136455 (1992-08-01), Billingsley
patent: 5224878 (1993-07-01), Lurie et al.
patent: 5721662 (1998-02-01), Glaser et al.
Glaser James A.
Glaser Ronald W.
ACT Communications, Inc.
Carr & Storm, L.L.P.
Leja Ronald W.
LandOfFree
Floating ground isolator for a communications cable locating... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Floating ground isolator for a communications cable locating..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Floating ground isolator for a communications cable locating... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2479546