Abstract:
A fault indicator for three phase sheathed cables includes a flexible collar unit which fits over the cable. Magnetic pole pieces within the collar unit having associated magnetic windings are aligned with the three conductors within the cable to produce signals indicative of the current level in each conductor. An alarm unit receives the signals and sounds an alarm in the event the current in any conductor exceeds a predetermined threshold level to indicate the occurrence of a fault. The voltage level of the current indicating signals may be utilized to align the magnetic core assemblies with the conductors.

Description:
BACKGROUND OF THE INVENTION 
     In electrical power distribution systems it is frequently necessary to sense the occurrence of fault currents in three phase underground cables, which typically consist of three 120° radially spaced conductors encased within an electrically conductive lead or lead sheath and an overlaying PVC jacket. The cables are typically run from vault to vault, the conductors being buried in the ground or routed through conduits between the vaults and being accessible through the uncovered vault or manhole. 
     Conventional current transformers cannot be used for sensing fault current in three phase sheathed cables because the sum of the balanced 3 phaser currents is zero, so that the net current sensed by the transformer is zero. Consequently, prior devices for sensing fault currents in such cables have been installed at cable splices or terminations, where individual conductors are accessible and the phase currents can be separately sensed. 
     The present invention is directed to a fault indicator which senses faults in a three phase cable without physical access to the individual conductors within the cables, thereby allowing the indicator to be positioned at any location along the cable, such as where the cable passes unterminated through a cable vault. For optimum performance in this environment, the fault indicator is moisture-proof and includes an aural alarm which, in the event of a fault, can be heard from outside the cable vault, thereby obviating the need for physical inspection of individual fault indicators along the cable. 
     Accordingly, it is a general object of the present invention to provide a new and improved fault indicator for sensing the occurrence of a current in three phase power distribution cables. 
     It is a more specific object of the present invention to provide a fault indicator which senses fault currents in three phase sheathed cables without the need for physical access to individual conductors within the cable. 
     It is another specific object of the present invention to provide a fault indicator which can be installed at any location along the length of a three phase sheathed cable. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a fault indicator for sensing a fault current in a three phase three conductor sheathed cable of the type having first, second and third electrical conductors. The fault indicator comprises first, second and third magnetic pole pieces arranged primarily in the magnetic fields surrounding the first, second and third conductors, respectively, first, second and third magnetic windings in magnetic communication with the first, second and third pole pieces, respectively, for developing first, second and third signals indicative of the current level in the first, second and third conductors, respectively, indicator circuit means responsive to the first, second and third signals for indicating the occurrence of a fault current in the conductor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with the further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which: 
     FIG. 1 is a perspective view partially broken away of a fault indicator constructed in accordance with the invention shown installed on a three phase sheathed cable of the type used in underground power distribution systems. 
     FIG. 1A is a cross-sectional view of the alarm sound reflector of the fault indicator taken along line  1   a — 1   a  of FIG.  1 . 
     FIG. 2 is an enlarged cross-sectional view of the fault indicator taken along line  2 — 2  of FIG.  1 . 
     FIG. 3 is an enlarged cross-sectional view of the fault indicator taken along line  3 — 3  of FIG.  1 . 
     FIG. 4 is a schematic diagram of the fault indicator of FIGS.  1 - 3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the Figures, and particularly to FIG. 1, a fault indicator  10  constructed in accordance with the invention includes a fault sensing collar unit  11  and a remote alarm unit  12 . The two units, which may be some distance apart, are connected by a cable  13 . 
     The collar unit  11  is shown installed on a three phase sheathed conductor  14  of the type used in underground power distribution systems. In accordance with conventional practice, conductor  14  may include three standard copper electrical conductors  15 ,  16  and  17 , corresponding to respective phases of a three phase circuit. Conductors  15 ,  16  and  17  are provided with electrically insulating jackets  20 ,  21  and  22 , respectively. The three jacketed conductors are contained with a metallic sheath  23 , which may be formed of a corrugated aluminum or, in older cable constructions, a layer of lead. The metallic sheath is covered with a jacket  24  formed by a layer of PVC or similar material. One example of a sheathed cable of this construction is the C-L-X™ type MV-105 cable marketed by The Okomite Company. 
     As best shown in FIG. 3, in order to sense current flow in conductors  15 - 17  collar unit  11  includes three magnetic sensing assemblies  25 ,  26  and  27  associated with conductors  15 ,  16  and  17 , respectively. The sensing assemblies includes individual generally U-shaped magnetic pole pieces  30 ,  31  and  32  each disposed in a plane perpendicular to the conductors and each having projecting pole portions arranged to bracket a respective one of the conductors. Individual magnetic windings  33 ,  34  and  35  are provided on the center portions of respective ones of the pole pieces to sense the magnitude of the magnetic flux surrounding the conductors, and hence the magnitude of the current in the conductor. Since the U-shaped pole pieces are aligned to provide a magnetic circuit for only the field in the bracketed adjacent conductors, and typically span only approximately 60° of the circumference of the cable, their associated windings are sensitive only to the associated conductor. 
     For optimum sensing it is necessary that the three magnetic pick-up assemblies  25 ,  26  and  27  be accurately aligned with the three conductors  15 ,  16  and  17 . To this end, the collar assembly, which is formed of a flexible elastomeric material, is provided with a slot  36  which allows it to be opened up and slipped over cable  14 , and thereafter to be rotated around the cable until the magnetic sensing assemblies are aligned with the conductors. An index mark  37  may be provided on the collar to enable alignment with a mark  38  made on the cable jacket  24  when the collar is removed and subsequently reinstalled. 
     Encapsulated full-wave bridge rectifier circuits  40 ,  41  and  42  may be connected to windings  33 ,  34  and  35  such that each of the magnetic sensing assemblies  25 ,  26  and  27  (FIG. 3) each develop a DC output signal indicative of the current in their respective conductor. In the illustrated embodiment of the invention, the outputs of the three bridge rectifier circuits are connected in parallel and applied to a winding  44  in alarm unit  12  such that a fault current in any one of conductors  15 ,  16  and  17  will result in a current flow in the winding sufficient to close a reed switch  45  disposed adjacent to and in magnetic communication with the winding. Closure of the reed switch  45  causes a trigger capacitor  46  to be charged by an internal battery  47 . This in turn is connected to the gate electrode of a field effect transistor (FET)  48 , conditioning the FET to a conductive state and allowing battery  47  to supply operating current to a conventional DC powered alarm transducer  50 . After the fault current has subsided, reed contacts  45  open and capacitor  46  slowly discharges through resistor  51 . Eventually capacitor  46  discharges to the extent that FET  48  opens and the alarm ends. Also, the alarm can be manually reset by means of a push button switch  52 , which rapidly discharges capacitor  46  through a current-limiting resistor  53 . 
     To assist in aligning collar unit  11  on cable  14  an optional voltmeter  54  may be connected across the outputs of rectifier circuits  40 ,  41  and  42  and the collar rotated for maximum voltage reading. To this end, terminals  55  and  56  may be provided on the housing of alarm unit  12 . 
     To protect the alarm transducer  50  from rising water levels, such as may occur in a cable vault, the output port of the transducer is connected to a vertical tube  57  which extends downwardly from the alarm unit housing  58  such that trapped air in the tube prevents rising water from coming into contact with the transducer port. An adjustable sleeve extension  60  attached to sleeve  57  by a machine screw  61  may be provided to assist in directing the audio output of the alarm transducer. 
     While the output signals generated by the three magnetic sensing assemblies  25 ,  26  and  27  have been connected to actuate an alarm, it is also possible to actuate an electromechanical fault indicator, such as shown, for example, in U.S. Pat. No. 4,414,543, or to actuate an LCD fault indicator, such as shown in, for example, U.S. Pat. No. 5,406,195, both issued to the present inventor. Also, instead of using a single alarm or indicator, it will be appreciated that the outputs could be individually connected to individual alarm units or indicators, thereby indicating, provided the phases were first identified, which of the three phases had experienced the fault current. Moreover, the alarm circuit could provide for contact closure, thereby enabling a remote alarm to be sounded by means of a conventional SCADA circuit or by carrier current modulation on the monitored conductors. 
     Also, it is possible to combine the collar unit  11  and the alarm unit  12  to obtain a single collar assembly wherein the fault indication, aural and/or visual, is contained on the collar. Moreover, instead of the optional voltmeter  54  and contacts  55  and  56 , it is possible to use a separate magnetic sensing assembly which is rotated around the collar to locate and mark the correct alignment for the cable, the collar unit then being installed according to the marking. 
     While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made therein without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.