Abstract:
The resistance of a cable conductor between a measuring point and a conductive fault is measured using an electronically regulated current source and a very high resistance voltmeter in such a way as to eliminate contact and fault resistance as a source of error. Distance to the fault is determined by calculation from the known resistance per unit length.

Description:
This is a continuation of co-pending application Ser. No. 07/138,939 filed Dec. 29, 1987, now aband. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the location of conductive faults in telephone and similar shielded cables. 
     BACKGROUND OF THE INVENTION 
     Location of conductive faults in telephone and similar shielded cables is most often accomplished by measuring the resistance of one of the faulted conductors between a reference point and the fault. Actual distance is determined by calculation using the known resistance per unit length and the measured resistance. 
     Current practice is to make the resistance measurement with an electrical bridge which is made up of the faulted conductors and one or more non-faulted conductors which may or may not be in the same cable. The arrangement is such that the fault resistance ends up in one of the power supply leads to the bridge and not within the bridge loop itself. The conductor between the reference point and the fault; however, becomes one of the bridge arms and its resistance is determinable by ratiometric means to the resistance of the non-faulted conductor. The resistance of the non-faulted conductor is known or is readily measureable, since its end point is known and accessible. Conventional forms of the bridge arrangement are known as Varley bridges and Murray loops and are well covered in contemporary engineering literature. 
     These bridges have shortcomings which make them difficult to use in some situations, notably when any of the parameters or external interfering voltages are varying. Often the fault is due to water in the cable. This causes conduction between conductors or from a conductor to the shield which, when it becomes great enough, constitutes a fault. Other conductors, which have voltage on them may have faults to the faulted conductors and thus inject interfering currents into the fault. Electrical current through any of the water based paths will both electrolyze the water and tend to dry out the fault. Furthermore, the non-faulted reference conductor and its connection to the faulted conductor is a temporary arrangement which may have unexpectedly large and unstable series resistance. In this condition bridge parameters will be varying with time and it is more difficult to balance the bridge. Furthermore, the mere task of balancing a bridge, even when the parameters are constant, is a slow and tedious process. 
     It is the objective of this invention to provide an improved method for locating conductive faults in telephone and similar shielded cables. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     FIG. 1 is a schematic diagram showing apparatus and circuitry to be used to practice the method of the invention in accordance with a preferred embodiment. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     In our invention a novel approach to measurement of the resistance between a reference point and the fault is employed. Use of our method is illustrated in FIG. 1. Here an electronically regulated constant current source, 1, forces a known and constant current through a faulted conductor, 2, an unknown fault resistance, 4, and a faulted conductor, 3. One of the faulted conductors, say 3, is connected, at a point &#34;F&#34;, beyond the fault, to another non-faulted conductor, 6, at a point &#34;G&#34; with a strap connection. This non-faulted conductor extends to the location of the constant current source. A high resistance voltmeter is connected to the ends (points &#34;D&#34;, &#34;H&#34;) of the conductors 3 and 6 opposite the strap. The resistance of this voltmeter must be high enough that current flow through it is negligible compared to the regulated current under all conditions of fault and conductor resistance within the measuring range desired, and its resistance must be high compared to that of any of the conductors. Suitable current regulated sources and high resistance voltmeters are commonly available and their design is not part of our invention. 
     In practice the current flows from the regulated current source, 1, through point &#34;A&#34; to point &#34;B&#34; of conductor 2, thence through fault resistance, 4, to point &#34;E&#34;, back through a portion of conductor 4 to point &#34;D&#34; and thence to the regulated current source. This current is known and constant, regardless of the resistance of the conductors or the fault, within the range of the setup. Because conductor 3 has an unknown resistance between points &#34;D&#34; and &#34;E&#34; there will be a voltage drop across this section of conductor 3 which is directly proportional to its resistance. Because the current from point &#34;E&#34; to point &#34;H&#34;, through conductors 3 and 6 and the strap is negligible the voltmeter will accurately read this voltage drop regardless of the resistance of the conductors 3 and 6. Knowing the voltage drop and the current causing it allows calculation of the resistance and hence the distance from point &#34;D&#34; to point &#34;E&#34; along conductor 3. It is understood, of course, that points &#34;A&#34;, &#34;D&#34; and &#34;H&#34; are at the location of the regulated current source 1 and high resistance voltmeter 7; and that the points &#34;B&#34;, &#34;E&#34; are at the fault location. 
     A number of enhancements to this basic invention are anticipated. The voltmeter may be calibrated to read directly in distance and the sensitivity may be adjusted with calibrated variable resistors to compensate this distance reading for wire gage and temperature. Alternately the value of the regulated current can be adjusted to compensate for wire gage and temperature. A zero adjustment can be added to the meter to zero out any voltage contributed by leakage to other pairs with voltage on them. All compensation adjustments, and conversions can be accomplished automatically by adding a microprocessor to the equipment used to practice our method. All of these enhancements are also used with the Varley and Murray methods and are not claimed as part of our invention. 
     An advantage of our method is elimination of the necessity for adjusting the apparatus for a null which both saves time and allows operation by less skilled persons. Another advantage is that steady and accurate measurements can be made even when the fault resistance or strap and non-faulted conductor resistance is changing.