FIELD OF THE INVENTION
The present invention relates generally to the repair of systems using a cable locating current for determining the position of underground cables from above ground. More particularly, the present invention is an apparatus and method for finding a fault in a cable sheath that is causing the cable locating current to leak to ground.
BACKGROUND OF THE INVENTION
Many utilities bury pipes and cables (“utility conveyances” or “conveyances”) underground for reasons of both safety and aesthetics. Underground burial often provides protection to 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 rely on electromagnetic signaling techniques to precisely locate such conveyances.
U.S. Pat. No. 5,644,237, issued Jul. 1, 1997 and incorporated herein by reference in its entirety, describes a principle for electromagnetic signaling for locating a buried utility conveyance. To locate a buried conveyance, a locating tone or signal is applied to a metallic component of the conveyance. In one currently used embodiment, the signal is an AC signal between 80 and 120 volts and having a frequency of from 220 to 440 Hz. As shown in FIG. 1, in the case of a cable 100 containing fiber optic bundles 110, a cable locating conductor may be included in the cable as a metallic sheath 120 or a copper tracer wire (not shown) within an insulating sheath 130 surrounding the cable. Using a signal detector of a type well known in the art, a technician locates the conveyance by operating the detector above ground to detect a signal generated by current passing through the cable locating conductor.
In addition to the AC locating tone, a DC cable locating signal is also sometimes used in conjunction with a vector bar detector in order to confirm the information from the AC signal. The DC signal in a currently used system is approximately 80 to 100 volts.
The cable sheath or insulating sheath 130 surrounds the cable locating conductor in the buried cable and insulates the conductor from ground. The insulating sheath is applied during the cable manufacturing process, and is formed from a durable, flexible insulating material such as polypropylene. In one commonly used fiber optic cable, the sheath 130 has a thickness 138 of approximately ⅛ inch.
The insulating sheath of an underground cable has been known to contain cable sheath faults such as fault 135 caused by cracking, puncturing or preexisting manufacturing defects. Those faults may permit water or other conducting media to penetrate the cable, providing a ground path from the cable locating conductor within the sheath. Sheath faults are most common in locations where a cable is bent to a relatively small radius, such as within manholes or splice boxes, where extra cable length is commonly wrapped or coiled for later use in repairs or maintenance.
A cable sheath fault, once located, can often be repaired by resealing the sheath in the area immediately surrounding the fault. One commonly used method for repairing a sheath fault is by wrapping a sealing tape around the area that includes the fault, thereby sealing the leak. The tape may be a poly material and may include an adhesive designed to withstand moisture and to adhere securely to the sheath.
Cable sheath faults that are leaking current to ground may be roughly located using outside plant fault detection equipment as is known in the art. For example, a drop in the cable locating signal received above ground usually indicates the presence of a ground fault in that general area. Those techniques are effective in isolating a sheath fault to a certain manhole or splice box. The exact location of a cable sheath fault along a cable is, however, very difficult to pinpoint even after it has been isolated to a specific manhole or splice box. Manholes often fill with rainwater, submerging the cable. When a cable is pulled from the water, current flow to ground will often cease, making the fault difficult to pinpoint. In many cases, the fault is not visible on the sheath because the defect is extremely small or because a crack in the sheath partially closes when the cable is unwound from a splice box.
One technique frequently used by an outside plant technician is to feel along the cable for any imperfection in the sheath. Often, before the fault is detectable in that manner, the fault must deteriorate significantly. That deterioration also makes it impossible to use the cable locating current to locate the cable underground in the event of a third party dig alert, cable maintenance, cable repairs, etc. If a technician is not successful in pinpointing the fault either tactilely or visually, it is not possible to repair the sheath at the fault location. In those cases, entire sections of cable, typically hundreds of feet between splices, must be replaced.
There is therefore presently a need for a method and apparatus for locating a sheath fault on a cable such as a fiber cable, with sufficient precision to repair the fault without replacing the entire section. To the inventors' knowledge, there is currently no such apparatus or method currently employed to satisfactorily accomplish that task.
SUMMARY OF THE INVENTION
The present invention addresses the needs described above by providing a an apparatus and a method for locating on an optical fiber cable a fault where a cable locating current is leaking to ground. One embodiment of the invention is an apparatus for locating such cable faults. The apparatus includes a body adapted to be positioned adjacent the cable, at least one voltage probe mounted in the body and positioned in the body to probe the leaking cable locating current, a reference voltage input for receiving a reference voltage, and a voltage comparator electrically connected to the at least one voltage probe and to the reference voltage input, the comparator configured for measuring a test voltage between the reference voltage and the at least one voltage probe.
The body may further be adapted to at least partially surround a transverse section of the cable. In that case the at least one voltage probe may be a plurality of voltage probes angularly spaced around the transverse section of the cable. The at least one voltage probe preferably presents a conductive surface facing the cable.
The reference voltage may be ground, or may be a DC voltage applied to the cable.
In another embodiment of the invention, a method is provided for locating on a cable a fault where a cable locating current is leaking to ground. The method includes the steps of positioning a voltage probe adjacent the cable, applying a conductive medium between the cable and the voltage probe, displacing the voltage probe along the cable, measuring a voltage between the voltage probe and a reference voltage, and, based on the voltage, detecting the fault at a position of the voltage probe along the cable.
The conductive medium may be water, a water-based paste or a gel. The voltage probe may include a plurality of conductive surfaces facing the cable. In that case, the step of positioning a voltage probe adjacent the cable may include at least partially surrounding the cable with the voltage probe.
The step of displacing the voltage probe along the cable preferably includes maintaining the probe in a position at least partially surrounding the cable. The step of measuring a voltage between the voltage probe and a reference voltage includes measuring a voltage between the voltage probe and ground.
The method may further include the step of applying a reference DC voltage to the cable. In that case, the step of measuring a voltage between the voltage probe and a reference voltage includes measuring a voltage between the voltage probe and the reference DC voltage.
The method may include the step of sounding an alarm when the fault is detected. An initial step may be included to determine an approximate position of the fault by determining a position along the cable where an above-ground detectability of the cable locating current degrades.
The step of detecting the fault may include detecting a drop in the measured voltage, or it may include detecting an increase in the measured voltage.