CA1193322A - Ground conductor monitoring system - Google Patents
Ground conductor monitoring systemInfo
- Publication number
- CA1193322A CA1193322A CA000403539A CA403539A CA1193322A CA 1193322 A CA1193322 A CA 1193322A CA 000403539 A CA000403539 A CA 000403539A CA 403539 A CA403539 A CA 403539A CA 1193322 A CA1193322 A CA 1193322A
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- CA
- Canada
- Prior art keywords
- ground conductor
- test signal
- impedance
- ground
- wire
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Landscapes
- Testing Of Short-Circuits, Discontinuities, Leakage, Or Incorrect Line Connections (AREA)
Abstract
ABSTRACT
A monitoring system for continuously monitoring the ground conductor of an electrical cable having power conductors and a pilot wire in addition to the ground conductor. An a.c. test signal is connected onto a loop comprised of the ground conductor and pilot wire and a terminating impedance. A current transformer samples the test signal on the ground conductor and checks its magnitude. By making the transformer impedance considerably lower than the mutual inductance between the ground conductor and pilot wire, large, expensive blocking impedances are not required to prevent the test signal from following sneak paths from the test loop to earth ground.
A monitoring system for continuously monitoring the ground conductor of an electrical cable having power conductors and a pilot wire in addition to the ground conductor. An a.c. test signal is connected onto a loop comprised of the ground conductor and pilot wire and a terminating impedance. A current transformer samples the test signal on the ground conductor and checks its magnitude. By making the transformer impedance considerably lower than the mutual inductance between the ground conductor and pilot wire, large, expensive blocking impedances are not required to prevent the test signal from following sneak paths from the test loop to earth ground.
Description
f Title: Impro~ed Ground Condl~ctor .~onitorin~
System Inventor: John R~ Sherwood Background of the Invention This i~vention relates to a ground monitoring system and more particularly to a system fox monitoring the integrity of the ground wire in a multi wire pow r cable o~ a type used in surface mining operations, i.e., strip ~ines. In surface mining operations~ extensive use is made of large electrically powered machinery such as power shovels, p~mps, drills, etc. A~ an example, 600 volt to 25 kilovolt, three phase electrical service may be provided at a remote sur~ace mining site. Electrical utility company power lines are brou~ht to a sub-station at or close to the mining site. Because much of the mining machinery must be movable, long insulated power ca~les that can be dragged along the ground connect the machinery to mobile switch houses that in turn are connect2d to the substation by additional long insulated cables. There are a number of different types of power cables used in surface mining. One such type of cable include the three power conductors or phase wires, each with a metallic shield around it, two bare ground conductors, and an insulated ground cheek wire that commonly is ` 7 J
called a pilot wire. All are enclosed in an insuXating elas~omeric jacke~. Typically, the cahles are in 1,000 or 1~500 foo~ leng~hsO Two or more cables, and sometimes up to ten cables are series connected.
If an insulation breakdown occurs in one of the large electrically powered pi2Ges of machinery the ~ntire machine may be at a dangerous high voltage unless it is protected by a suitable metallic ground circuit. In many instances the machine will be in contac~ with the earth, but because ~he machinery is mo~ed around rom place to place, a dependable earth ground resulting only from contact with the eaxth cannot be relied upon~ Consequently, a metallic ground circuit in the power cable is essential. The Federal Coal Mine Health and S~fety Act of 1969 requires that the electrical ground circuit in the power cable be continuously monitored to detect short circuits, open circuits, and abnormally high resis-tanoe conditions in the ground circuit. Upon detection of a fault condition the ground monitor must open one or more circuit breakers to disconnect the electrical source from the ~aulty portion of the system.
A ground monitor intended for use at a surface mining location must be able to detect the desired condition that it is the metallic ground 9~
conductor of the power cable that is p~oviding ground continuit~ in the system and not an earth ground resulting from the machine being in contact with the earth or water.
A system for monitoring the integrity, ox continuity, of the ground conductor of an elec-trical power cable is disclosed in my patent 4,228,475, issued October 14, 1980. In that systemr corresponding ends of the pilot wire and ground conductor o a power cable are connected together through a terminating resistor Rt. The other ends of the ground conductor and pilot wire are connected to the primary ground oE the mining site. A 1 kHæ
sensing signal is coupled onto the loop formed by the pilot and ground conductors. The ground sensing circuit of that patent continually monitors the loop and produces a fault indication when the resistance "seen" by the sensing circuit increases by more than two oh~s from the value of the te.rminating resistance Rt~ An objecti~e in that system was to assure that the sensing signal in the loop was not diverted through any sneak paths to earth ground at a switch hou5e ~ s~lice skidsl and machinery frames, for exam~le. To isolate the pilot wire and ground conductor loop from potential sneak paths, a large blocking impedance in the form of a large iron core 3~;~
inductor was connected in series with the ground conductor just beyond the junction with the terminating resistor Rt.
In U.S~ Patent No. 4,321,643, R. Vernier, issued on March 23, 1982, the hlocking impedance is located in the line that connects to the local ground sta~e rather than in series with the cable ground conductor or conductors.
The ~round conductor monitoring system disclosed in my above mentioned patent performs well to achieve its desired purpose~ but does have several disadvantages a The blocking impedance is quite expensive, is difficult to install on equipment in the field, and is not fail-safe because it appears as a high impedance to the sensing signal whether it is functioning as intended or is faulty because it has an open circuit therein.
Another shortcoming of the system disclosed in my above mentioned patent results from the fact that the fault sensing circuitry is `'looking for`' a resistance change of two ohms in the loop formed by the pilot wire and ground conductor. In order for a two ohm change in resistance to appear to be significant and thus easily detected, the total resistance in the loop must be a low value. This means that the impedance in the ground sensing circuitry that is in series with the loop has to be ~33~
as small as possible. ThiS consideration severly restricts the use of noise suppressing and transient suppressing circuit means in the fault sensing circuitry. Because of the restriction on the use of noise suppressing means, the fault sensing circuitry of my previous system was to some ex~ent susceptible to damage and destruction of components due to noise and transients.
Summary of the Invention Quite contrary to my previous thinking, I have discovered that it is not necessary to use blocking impedances to isolate the monitored pilot wire and ground conductor from potential sneak, or parallel, paths to earth ground. Theoretical analysis shows, and actual use confirms, that a monitoring system for a cable can function without the blocking impedances and can in fact operate with improved results as compared with my prior system.
Additionally, the ground sensing monitor of my present invention is not "looking for" a two ohm resistance change in the monitored loop comprised of the pilot wire and ground monitor. The present system is looking, in one instance, only far a change i~ the magnitude of the test signal current from some nominal or predetermined value~ This is i an important consideration because it means that the above-mentioned restriction on adding impedance to the ~onitored loop no longer is present and I now am able to add additional noise and transient suppression means to the fault sensing circuitry.
According to a broad aspect of the present invention there is provided an apparatus for monitoring the condition of a ground conductor that extends through-out the length of an electrical cable that has a plurality of power carrying wires and at least one other wire, said apparatus comprising means at one end of said ground conductor for earth grounding said ground conductor, said ground conductor having no other earth grounding means connected thereto in the absence of a fault condition, an AC test signal source coupled between said one ènd of the ground conductor and a corresponding end of said other wire, impedance means connected between the opposite ends of the ground conductor and said other wire to form a ground monitoring loop, said impedance means having a reactanc~ value at the frequency of said test signal thatlis opposite to, and greater in magnitude than, the reactance value of said ground conductor and other wire wîthout the impedance means connected thereto, means at said one end of the cable for sampling the voltage and current of the test signal that is coupled onto said monitoring loop, means for comparing the phases of said 3~
- 6a -sampled voltage and current and for producing a fault signal when the comparison indicates that the reactance of the monitoring loop changes from the reactance value of a properly operating monitori.ng loop with the termi-nating impedance connected therein, means at said one of the ground conductor for sampling the test signal current flowing thereon, said last named means for sampling the test signal current flowing on said ground conductor presenting an impedance to said test signal that is small compared to the impedance represented by the mutual inductance between said ground conductor and other wire at the ~requency of said ~est signal, and means for produc-ing a fault signal when the test signal current sampled on the ground conductor is different from a predetermined magnitude.
According to a further broad aspect of the present invention there is provided the method of monitoring the condition of the ground conductor of an electrical power cable that also includes a plurality of power or phase ~0 wires and at leas~t one other wire, all of which extend from one end to the opposite end of the cable, said method comprising the steps of earth grounding said ground conductor only at said one end, coupling an AC test signal between said one end of the ground conductor and the corresponding end of said other wire, terminating the ground conductor and the other wire at said opposite end ,I~L
- ~b -of the cable in an impedance that presents at least a predetermined capacitive reactance at the test signal frequency when monitored at said one end of the cable, monitoring the reactance of said terminating impedance and producing a fault signal when the reactance of the cable as monitored at said one end changes from said predetermined capacitive reactance, sampling the test signal current flowing in said ground conductor at said one end of the cable, and producing a fault signal when the sampled test signal current is below a predetermined magnitude.
Bxief Descri~tion of the Drawin~s The invention will be described in connection with the accompanying drawings wherein:
Fig. 1 is a cross-sectional illustration of a type of cable that is used in practicing the present invention, Fig. 2 is a simplified illustration of an elec-trical power distribution system employing a cable of the type illustrated in Fig. 1 and incorporating the improved ground cable monitoring features of this invention;
Figs. 3 - 7 are schematic illustrations used in explaining the circuit theory that forms a basis for the improvement of this invention, and Fig. 8 is a simplified diagram that illustrates the features of the ground conductor monitor of this invention.
, .,~
Detailed Description of the Invention Before explaining the principle o~ this in~ention, the type of electrical power cable that may be employed in the powex distribukion system at a surface mining operation first will be described~ The type of cable chosen for represen-tation is identi~ied as SHD-GC-8KV to 25 KV.
Other types are available, but the above-iden~ified type is represented in Fig. 1 and used as the example throughout this description. In Fig. 1, the individual phase wires 11, 12~ and 13 are stranded metallic conductors and each is surrounded by respecti~e insulating material lla, 12a, and 13a, and by a respective stranded conductive shield llb, 12b, and 13b. The cable also includes two bare stranded ground wires 16, and an insula~ed stranded ground check, or pilot wire 1~. A jacket or sheath 20 of elastomeric insulation materi~l surrounds al~ the conductors and wires. Although the cable actually include~ two ~round conductors 16, they and the shields llb, 12b, and 13b function as a system ground conductor. Accordingly, in the following discu~sion and in the accompanying schematic drawingsg just one ground conductor will be illu-strated~ it being understood that in practice it may be a number of conductors and shields.
~o.J~Iq_j~fi~J
~ simplified illust:ration of anelectxical power di~tribution system that might be used at a surface mining site is illustrated in ~ig. 2. ~igh voltage from ~he Plectric utility company three phase system is coupled sver open wires 24, 25, 26 to the delta connected primary windings 30 of transf~rmer Ts at the substation 40~ The substation structure is grounded by a local ground stake, as indicated. The ~econdary winding 31 of transformer Ts is wye connected and its neutral point i5 connected through resistor Rp to the primary ground of the mining site power distribution system. Resistor ~p is a current limiting resistor that is intended to limit the magnitude of any ground fault current to no more than 50 amps~
Substation 40 includes large circuit breakers 33 in each of the phase wires 11, 12 and ~3 of cable 10 of the mine power distribution system. The circuit breakers operate to open the phase wires upon detection of a fault in the grounding system by the fault sensor circuit 35.
The circuit breakers also operate to open the phase wires upon detection of an over-current, in the conventional manner. The cable ground conductor ~3~
g 16 and ground monitor are grounded at poin~ 36 to the primary ground of the ~ubstation. As illustrated, the ground monitor or fault sensor circui~ 35 is ~oupled betwe2n pilot l.ine lB and ground point 36.
Cable 10 extends to a distantly located machine 43 such as a drillO shovel, etc.
At machine 43 ground conductor 16 and pilo~ wire 18 are terminated in an impedance means ~t. It is seen that a ground cable test loop is formed by grou~d conductor, terminating impedance and ground monitor 35.
In a cable of the type illustrat~d in Fi~ . 1 , i.e., the cable that extends between sub-station 40 and machine 43 in Fig. 2, there i~
mutual coupling between the wires of the cable.
The mutual inducta~ce M between t~e wires may be expressed as ~ollows.
M = 0.00508 ~(2.203 log10 2~ , where D is the distance between the wires, and ~is the length of the wires, the lPngth X ~eing much greater than the distance D. M is in microHenries. In a cable of the type under consideration the pilot and ground conductors are close together so tha~ ~
is small. On the other hand, the distance D between the pilot wire and an earth ground return path will be greater than the distançe hetw~en the pilot and ground c:onductors of the cable. Conse~uerltly, the mutual. induc~ance M between the pilQ~ and ground conductor of the c:able is greater than that between the pilot wire or ground conductor and the earth ground return path.
The mutually coupled pilot wire 18 and ground conductor 16 of a cable of the type illustrated in Figs. 1 and 2 are schematically illustrated in Fig. 30 R1 7 L1, and R2, L2 are, respectively, the resistance and inductance of the pilot wir~ and ground conductor, and M is the mutual inductance between the two. Il is the current in the pilot wire and I2 is the current in the ground conductor a As is well known in circui~ theory, mutually coupled wires may be considered as an air core transformer. Accordingly, the representation in Fig. 3 of the mutually coupled wires also may be represented as an air core transformer, as shown in Fig. 4. It also is well known in circuit the~ry that an equivalent circuit ~or the air core transformer is a T network of the type illustrated in Fig. 5t (see Electrical Engineering Science, page 523, by Clement and Johnson, Mc~ra~J Hill Book Co. ~ . Because the currents in the pilot wire and ground conductor are in opposite directions, the 1,3~
mutual induc~ance is negative ~
The circuit of Fig~ 5 may be redrawn to the circuit of Fig~ 6 to make it look more like the pilot and sround eonductc)rs of the cable lOo Carryiny the representation o~ Fi~. 6 into the y~ system of the surface mine e~ectrical distribution system of Fig. 2, and illustrating only the ground conductor monitoring loop, results in the simplified illustration of Fig. 7~ It is seen that the impedance component due t~ the mutual inductance M is subtracted ~actually 2M
is substxacted) fr~m the impedance of the ground sensing loop comprised of pilot wire 18 and ground conductor 16, i.e., points 1, 4, 5, 2, 7, Fig. 7, but is included in the earth ground return path, i.e., points 1, 4, 5, 3, 6.
Representative values of circuit parameters for a cable of the type mentioned above are as f~llows~
Ll = 393 microHenries L2 ~ 282 microHenries M = 268 ~ 5 micro~Ienries
System Inventor: John R~ Sherwood Background of the Invention This i~vention relates to a ground monitoring system and more particularly to a system fox monitoring the integrity of the ground wire in a multi wire pow r cable o~ a type used in surface mining operations, i.e., strip ~ines. In surface mining operations~ extensive use is made of large electrically powered machinery such as power shovels, p~mps, drills, etc. A~ an example, 600 volt to 25 kilovolt, three phase electrical service may be provided at a remote sur~ace mining site. Electrical utility company power lines are brou~ht to a sub-station at or close to the mining site. Because much of the mining machinery must be movable, long insulated power ca~les that can be dragged along the ground connect the machinery to mobile switch houses that in turn are connect2d to the substation by additional long insulated cables. There are a number of different types of power cables used in surface mining. One such type of cable include the three power conductors or phase wires, each with a metallic shield around it, two bare ground conductors, and an insulated ground cheek wire that commonly is ` 7 J
called a pilot wire. All are enclosed in an insuXating elas~omeric jacke~. Typically, the cahles are in 1,000 or 1~500 foo~ leng~hsO Two or more cables, and sometimes up to ten cables are series connected.
If an insulation breakdown occurs in one of the large electrically powered pi2Ges of machinery the ~ntire machine may be at a dangerous high voltage unless it is protected by a suitable metallic ground circuit. In many instances the machine will be in contac~ with the earth, but because ~he machinery is mo~ed around rom place to place, a dependable earth ground resulting only from contact with the eaxth cannot be relied upon~ Consequently, a metallic ground circuit in the power cable is essential. The Federal Coal Mine Health and S~fety Act of 1969 requires that the electrical ground circuit in the power cable be continuously monitored to detect short circuits, open circuits, and abnormally high resis-tanoe conditions in the ground circuit. Upon detection of a fault condition the ground monitor must open one or more circuit breakers to disconnect the electrical source from the ~aulty portion of the system.
A ground monitor intended for use at a surface mining location must be able to detect the desired condition that it is the metallic ground 9~
conductor of the power cable that is p~oviding ground continuit~ in the system and not an earth ground resulting from the machine being in contact with the earth or water.
A system for monitoring the integrity, ox continuity, of the ground conductor of an elec-trical power cable is disclosed in my patent 4,228,475, issued October 14, 1980. In that systemr corresponding ends of the pilot wire and ground conductor o a power cable are connected together through a terminating resistor Rt. The other ends of the ground conductor and pilot wire are connected to the primary ground oE the mining site. A 1 kHæ
sensing signal is coupled onto the loop formed by the pilot and ground conductors. The ground sensing circuit of that patent continually monitors the loop and produces a fault indication when the resistance "seen" by the sensing circuit increases by more than two oh~s from the value of the te.rminating resistance Rt~ An objecti~e in that system was to assure that the sensing signal in the loop was not diverted through any sneak paths to earth ground at a switch hou5e ~ s~lice skidsl and machinery frames, for exam~le. To isolate the pilot wire and ground conductor loop from potential sneak paths, a large blocking impedance in the form of a large iron core 3~;~
inductor was connected in series with the ground conductor just beyond the junction with the terminating resistor Rt.
In U.S~ Patent No. 4,321,643, R. Vernier, issued on March 23, 1982, the hlocking impedance is located in the line that connects to the local ground sta~e rather than in series with the cable ground conductor or conductors.
The ~round conductor monitoring system disclosed in my above mentioned patent performs well to achieve its desired purpose~ but does have several disadvantages a The blocking impedance is quite expensive, is difficult to install on equipment in the field, and is not fail-safe because it appears as a high impedance to the sensing signal whether it is functioning as intended or is faulty because it has an open circuit therein.
Another shortcoming of the system disclosed in my above mentioned patent results from the fact that the fault sensing circuitry is `'looking for`' a resistance change of two ohms in the loop formed by the pilot wire and ground conductor. In order for a two ohm change in resistance to appear to be significant and thus easily detected, the total resistance in the loop must be a low value. This means that the impedance in the ground sensing circuitry that is in series with the loop has to be ~33~
as small as possible. ThiS consideration severly restricts the use of noise suppressing and transient suppressing circuit means in the fault sensing circuitry. Because of the restriction on the use of noise suppressing means, the fault sensing circuitry of my previous system was to some ex~ent susceptible to damage and destruction of components due to noise and transients.
Summary of the Invention Quite contrary to my previous thinking, I have discovered that it is not necessary to use blocking impedances to isolate the monitored pilot wire and ground conductor from potential sneak, or parallel, paths to earth ground. Theoretical analysis shows, and actual use confirms, that a monitoring system for a cable can function without the blocking impedances and can in fact operate with improved results as compared with my prior system.
Additionally, the ground sensing monitor of my present invention is not "looking for" a two ohm resistance change in the monitored loop comprised of the pilot wire and ground monitor. The present system is looking, in one instance, only far a change i~ the magnitude of the test signal current from some nominal or predetermined value~ This is i an important consideration because it means that the above-mentioned restriction on adding impedance to the ~onitored loop no longer is present and I now am able to add additional noise and transient suppression means to the fault sensing circuitry.
According to a broad aspect of the present invention there is provided an apparatus for monitoring the condition of a ground conductor that extends through-out the length of an electrical cable that has a plurality of power carrying wires and at least one other wire, said apparatus comprising means at one end of said ground conductor for earth grounding said ground conductor, said ground conductor having no other earth grounding means connected thereto in the absence of a fault condition, an AC test signal source coupled between said one ènd of the ground conductor and a corresponding end of said other wire, impedance means connected between the opposite ends of the ground conductor and said other wire to form a ground monitoring loop, said impedance means having a reactanc~ value at the frequency of said test signal thatlis opposite to, and greater in magnitude than, the reactance value of said ground conductor and other wire wîthout the impedance means connected thereto, means at said one end of the cable for sampling the voltage and current of the test signal that is coupled onto said monitoring loop, means for comparing the phases of said 3~
- 6a -sampled voltage and current and for producing a fault signal when the comparison indicates that the reactance of the monitoring loop changes from the reactance value of a properly operating monitori.ng loop with the termi-nating impedance connected therein, means at said one of the ground conductor for sampling the test signal current flowing thereon, said last named means for sampling the test signal current flowing on said ground conductor presenting an impedance to said test signal that is small compared to the impedance represented by the mutual inductance between said ground conductor and other wire at the ~requency of said ~est signal, and means for produc-ing a fault signal when the test signal current sampled on the ground conductor is different from a predetermined magnitude.
According to a further broad aspect of the present invention there is provided the method of monitoring the condition of the ground conductor of an electrical power cable that also includes a plurality of power or phase ~0 wires and at leas~t one other wire, all of which extend from one end to the opposite end of the cable, said method comprising the steps of earth grounding said ground conductor only at said one end, coupling an AC test signal between said one end of the ground conductor and the corresponding end of said other wire, terminating the ground conductor and the other wire at said opposite end ,I~L
- ~b -of the cable in an impedance that presents at least a predetermined capacitive reactance at the test signal frequency when monitored at said one end of the cable, monitoring the reactance of said terminating impedance and producing a fault signal when the reactance of the cable as monitored at said one end changes from said predetermined capacitive reactance, sampling the test signal current flowing in said ground conductor at said one end of the cable, and producing a fault signal when the sampled test signal current is below a predetermined magnitude.
Bxief Descri~tion of the Drawin~s The invention will be described in connection with the accompanying drawings wherein:
Fig. 1 is a cross-sectional illustration of a type of cable that is used in practicing the present invention, Fig. 2 is a simplified illustration of an elec-trical power distribution system employing a cable of the type illustrated in Fig. 1 and incorporating the improved ground cable monitoring features of this invention;
Figs. 3 - 7 are schematic illustrations used in explaining the circuit theory that forms a basis for the improvement of this invention, and Fig. 8 is a simplified diagram that illustrates the features of the ground conductor monitor of this invention.
, .,~
Detailed Description of the Invention Before explaining the principle o~ this in~ention, the type of electrical power cable that may be employed in the powex distribukion system at a surface mining operation first will be described~ The type of cable chosen for represen-tation is identi~ied as SHD-GC-8KV to 25 KV.
Other types are available, but the above-iden~ified type is represented in Fig. 1 and used as the example throughout this description. In Fig. 1, the individual phase wires 11, 12~ and 13 are stranded metallic conductors and each is surrounded by respecti~e insulating material lla, 12a, and 13a, and by a respective stranded conductive shield llb, 12b, and 13b. The cable also includes two bare stranded ground wires 16, and an insula~ed stranded ground check, or pilot wire 1~. A jacket or sheath 20 of elastomeric insulation materi~l surrounds al~ the conductors and wires. Although the cable actually include~ two ~round conductors 16, they and the shields llb, 12b, and 13b function as a system ground conductor. Accordingly, in the following discu~sion and in the accompanying schematic drawingsg just one ground conductor will be illu-strated~ it being understood that in practice it may be a number of conductors and shields.
~o.J~Iq_j~fi~J
~ simplified illust:ration of anelectxical power di~tribution system that might be used at a surface mining site is illustrated in ~ig. 2. ~igh voltage from ~he Plectric utility company three phase system is coupled sver open wires 24, 25, 26 to the delta connected primary windings 30 of transf~rmer Ts at the substation 40~ The substation structure is grounded by a local ground stake, as indicated. The ~econdary winding 31 of transformer Ts is wye connected and its neutral point i5 connected through resistor Rp to the primary ground of the mining site power distribution system. Resistor ~p is a current limiting resistor that is intended to limit the magnitude of any ground fault current to no more than 50 amps~
Substation 40 includes large circuit breakers 33 in each of the phase wires 11, 12 and ~3 of cable 10 of the mine power distribution system. The circuit breakers operate to open the phase wires upon detection of a fault in the grounding system by the fault sensor circuit 35.
The circuit breakers also operate to open the phase wires upon detection of an over-current, in the conventional manner. The cable ground conductor ~3~
g 16 and ground monitor are grounded at poin~ 36 to the primary ground of the ~ubstation. As illustrated, the ground monitor or fault sensor circui~ 35 is ~oupled betwe2n pilot l.ine lB and ground point 36.
Cable 10 extends to a distantly located machine 43 such as a drillO shovel, etc.
At machine 43 ground conductor 16 and pilo~ wire 18 are terminated in an impedance means ~t. It is seen that a ground cable test loop is formed by grou~d conductor, terminating impedance and ground monitor 35.
In a cable of the type illustrat~d in Fi~ . 1 , i.e., the cable that extends between sub-station 40 and machine 43 in Fig. 2, there i~
mutual coupling between the wires of the cable.
The mutual inducta~ce M between t~e wires may be expressed as ~ollows.
M = 0.00508 ~(2.203 log10 2~ , where D is the distance between the wires, and ~is the length of the wires, the lPngth X ~eing much greater than the distance D. M is in microHenries. In a cable of the type under consideration the pilot and ground conductors are close together so tha~ ~
is small. On the other hand, the distance D between the pilot wire and an earth ground return path will be greater than the distançe hetw~en the pilot and ground c:onductors of the cable. Conse~uerltly, the mutual. induc~ance M between the pilQ~ and ground conductor of the c:able is greater than that between the pilot wire or ground conductor and the earth ground return path.
The mutually coupled pilot wire 18 and ground conductor 16 of a cable of the type illustrated in Figs. 1 and 2 are schematically illustrated in Fig. 30 R1 7 L1, and R2, L2 are, respectively, the resistance and inductance of the pilot wir~ and ground conductor, and M is the mutual inductance between the two. Il is the current in the pilot wire and I2 is the current in the ground conductor a As is well known in circui~ theory, mutually coupled wires may be considered as an air core transformer. Accordingly, the representation in Fig. 3 of the mutually coupled wires also may be represented as an air core transformer, as shown in Fig. 4. It also is well known in circuit the~ry that an equivalent circuit ~or the air core transformer is a T network of the type illustrated in Fig. 5t (see Electrical Engineering Science, page 523, by Clement and Johnson, Mc~ra~J Hill Book Co. ~ . Because the currents in the pilot wire and ground conductor are in opposite directions, the 1,3~
mutual induc~ance is negative ~
The circuit of Fig~ 5 may be redrawn to the circuit of Fig~ 6 to make it look more like the pilot and sround eonductc)rs of the cable lOo Carryiny the representation o~ Fi~. 6 into the y~ system of the surface mine e~ectrical distribution system of Fig. 2, and illustrating only the ground conductor monitoring loop, results in the simplified illustration of Fig. 7~ It is seen that the impedance component due t~ the mutual inductance M is subtracted ~actually 2M
is substxacted) fr~m the impedance of the ground sensing loop comprised of pilot wire 18 and ground conductor 16, i.e., points 1, 4, 5, 2, 7, Fig. 7, but is included in the earth ground return path, i.e., points 1, 4, 5, 3, 6.
Representative values of circuit parameters for a cable of the type mentioned above are as f~llows~
Ll = 393 microHenries L2 ~ 282 microHenries M = 268 ~ 5 micro~Ienries
2 6 ohms . 10 ohms In practice, at l kilohertz, the effective impedance value of the mutual inductance was measured to be
3~
~ 12 -approximate~y 1.8 ohms for a 1, oao feet length of cable. The minimum value of contact resistance that one might expect between a switch house and earth is of the order of two ohms, and .in ~ractice probably will be ~v~n larger. With the parameter values given above it is seen that the impedance between term;~ls 1 and 2 is much small~r than the impedances between terminals 1 and 3~ or between terminals 2 and 3. This means that even if the earth ground ha~ zero impedance the ground conductor of the cable still will be a lower imPedance current path than the earth ground return path that has ~he mutual inductance M in series with it. In one actual example, the measured value of impedance between term;n~l.s 3 and 5 of Fig. 7 was 18 times greater than the measured value of impedance between te~minals 2 and 5. It will be remembered that the yround conductor 16 of Fig. 7 actually is two braided ground wires and the braided shields of the phase wires. Conse~uently, the resistance of the path between points ~ and 5 will be ~ess than the resistancP between points 1 and 4 and between 5 and 6. Assuming negligible impedance in the primary of transformer Tc, less than 5%% of the current on the ground monitor loop would flow through 1~933Z~
the earth ground return path e~en if it had zero impedance.
Ba~ed on my above cmalysis I made the discovery that with the design of my present ~ystem I do not need the large and expensi~e blocking impedances ~b that are included in the ground monitoring syst~m of my above mentioned patent since the leakage current that was being blocked actually was small enough to ignore.
A ground conductor monitoring ~ystem according to my present invention i5 illustra~ed in simplified form in Fig. 8. Ground monitor 3.S
include~ an oscillator and power amplifier 50 that produces a l kiloHer~z test ~ignal~ The signal passes through a l KHx series xesonant filter compri.sed of ~he pximary winding of transforme~ T2 and capacitor Cl The signal is further filtered in an ~MI and RFI filter 52 which provide~
transient and voltage spike suppressionO The thoroughly filtered tes~ signal then is directly connect~d to pilot wire 18 of the mine power cablP lO.
Pilot wire 18 and ground conductor 16 are connected together by a terminating impedance ~t at the far end of the cable where a machine i~
located. Terminating impedance ~ is comprised of the parallel connected capacitor Ct and the i~ductor.
'g l p ~ 14 ~
Lt. A test loop therefore i~ formed by pilot wire 18, ground conductor 16, ~erminati~g impedanee ~t~ and monitor 35.
The 1 kHz curren~ that flows ~o pilot wire 18 is sampled by sampling transformer ~2~
The current from the secondary of transformer T2 is coupled as the I~ input to phase detector 56. The other input to phase detector 56 is a voltage sample E~ of the 1 k~z ~est signal that is connected to pilot wire 18. The voltage sample E~ is filter~d by a l kHz parallel resonant filter 54. Because the ~urrent sample signal I~ is coupled through transformer T2, the input signals I~ and E~ to phase detector 56 are in phase auadratureD
The 1 k~z parallel resonant filter 54 filters ~0 Her$z ~ign~ls and transients. Filter 54 introduces a minimal phasa shift o not more than 10 degrees over the operating voltage and temperature ranges.
The operating principle underlying the ~bove mentioned phase comparison is as fol~ows.
Under normal conditions the combined reactance of the ground conductor 16 and pilot wire 18 of the type of cable under con~ideration is inductive~
ThP reactance at 1 kHz of terminating impedance ~t is capacitive, being about 10 microfarads in one embodiment of the invention~ The i~ductance elemen_ Lt pr~vides a low impedan~e, hi~h current carrying path at 60 Hz. ~t SO Hæ, terminating impedance ~t appear~ as a 2.5 milliHenry ~nduc~or. This prevents high voltag~ build-up on the pilot wire in the event of a phase wire to pilst wire short, and a~so assures that the phase wire curr~nt will no$ be li~ited, thereby assuring that the circuit breaker will trip on an overcurrent.
Even with as many as ten 1,000 foot ca~les connected in seri~s, the reactance of ter~;nating impedance ~t at lkH~ will ~ppear to be capacitive to ground monitor 35 if the ground con~
ductor moni~oring loop is in accepta~le condition.
If a short circuit exists between ground conductor 16 and pilot wire 18, as illustrated at 61 in Fig. 8, ground monitor 35 no longer will "see' the capacitive reactance of terminating impedance ~t but will "see~ the inductive reactance of the conductors 16 ana 180 Phase detector 56 senses this change from a negative ~o a positive reactance and produces a fault signal in response thereto.
The current of the 1 kHz test signal flowiny in ground conductor 16 of cable lO i5 sampled ~y current transformer Tc that is located at the ground monitor end of cable 10, i.e., remote from the machinery. The primary of this transformer is a 2~
straisht conductor that is series connected 1:o the c~ble ground conductor 16. The secondary winding is comprised of multiple ~"indings on a ferromagnetic toroid that encircles the straight primary wire. Because the primary winding of trans:Eormer Tc is a straight conductor, its impedance will be negligible compared with the actual value of the mutua~ inductance M. This is consistent with the theory and explanation given above in connection with Figs. 3 ~ 7r The sampled ground conductor current Igc is coupled to EP~I and RFI ilter 58 which functions to re~ect 60 and 120 Hertz signals and provide~ .
transient and voltage spike protection. This f iltered ground conduc:tor current sample Igc then ,:
i~ coupled to amplitude detector 60 where it is compared with a reference voltage Vlref . If the ground conductor monitoring loop is in acceptable operating condition the voltage Vgc corresponding to current Ig~ is greater in magnitude than th~
reference voltage Vlr~f and the output of comparatvr 60 will be high. If an o~en circuit or high resis-tance fault condition exists in the ground conductor monitoring loop, the current in the loop will fall and voltage Vgc will be smaller in magnitude than reference voltage VlreEO W.it~ this condition, comparator 60 will produce a fault signal~ which in one embodiment is a low signal.
The u~e of the current sensing transformer Tc for monitoring the current of the test signal on ground conductor 16 provides a number of advantages.
First, it is failsafe in that if the test current is bypas~ed around ~he transformer or if the transformer should become open circui~ed in i~s primaxy or secondary, the out~ut of the transformer will all and a fAult signal will be produced by ~mplitude detector 60. Additionally, because the primary of transformer is a straight conductor with substantial current carrying capacity, ~h~ use of transformPr Tc does not appre~iably increase the impedance to 60 ~z current. The system of this invention also may be used in a power distribution sy~tem that is carrying d.c. power.
As further illustrated in Fig. 8, the outputs of phase detector 56 and amplitude detector 6Q are coupled to logic circuitry 64 which responds to a fault signal on either of its inputs to produce 2 corre~ponding output signal that actuates a respective one of the circuit breaker~ CBl ox CB2.
In r~sponse to an open circuit fault signal or a short circuit fault signal logic cixcuitry 64 also produces a secondaxy trip signal on output line 66.
3~
~ ~ 8 --Secondary trip line h6 incllldes the two conta~ts CBl-l and CB2~1 of circuit breakexs CB1 and CB2. These contactC; are closed, as illustrated, when the circuit breakers are set and a xespective one of the contacts opens when a fault signal is coupled from logic circuitry 64 to one of the ciruit breakers CBl or CB2. Secondary trip line 66 is energized in the absence o~ a fault condition on the ground conductor monitoring loop and the coils of relays Kl and K2 normally are energized. The energization of the,se relay coils-causes contacts Kl~1, K1~2 and K2~1, K2-2 to be closed. The closed contacts energize a holding coil, or coils, in the cable circuit breakers to connect the phase wires of cable 10 to the substation.
Upon occurrence of a fault signal from logic circuitry 64, circuit breaker CBl or CB2 is tripped to open a corr~sponding set of contacts CBl-l or CB2-1 to open secondary trip line 66. Addition~lly, secondary trip 66 is deenergized upon occurrence of a fault signal to logic circuitry 640 Consequently, relay coils Kl and K~ are deen~rgized and their corresponding contacts Kl-l, Kl-2 and K~-l and K2-2 opPn to deenergize the holding coil of the cable circuit breakers~ thereby causing them to open and disconnect cable 10 from the substation~
~9 ~,3Z~
The particular circuitry that is included in logic circuitry 64 may be conventional and may be similar to that illus~ra~ed in Figs. 4 and 5 of my above identi~ied patent, and as in Figs. 4 and 5 of the above-mentioned Vernier application. In the accompanying Fig. 8, phase detector 56 would include the signal pakh comprised o squaring amplifiers 111, 112 exclusive or circuit 113, and integrator R18 C18 of my above identified patent. Amplitude detector 56 of the accom~anying Fig. 8 may be a con~entional comparator used similarly to coxresponding devices in my prior ~atent. Similarly, the secondary trip signal on line 66 may be produced in a manner similar to the secondary trip signal on line 187 in Fig. 4 of my above-mentioned patent. In the present invention, the open and short signals from amplitude detector 60 and phase detector 56 would be input signals, along with power-up and power~down signals commonly used in logic circuitry. If additional details are desired, reference is made to Figs. 4 and 5 of my prior patent, although one skilled in the art will be fully capable of instrumenti~g the block diagram illustrated in the accompanying Fig. 8.
The value of the comparison voltage V
that constitutes one input to amplitude detector 60 ~ ~,9 33Z~?d need not be an exact value but is selected to produce acceptable operation with the length of cable contemplated to be used.
From the Above description it may be understood that a simplified and more xelia~le ground fault monitor is possible as a result of the realization that the mutual inductance in the cable reduces the impedance of the test loop comprised of the pilot wire 18 and ground conductor 16 ~o that ~he impedance of that loop to the tes~
signal i~herently will be less than the impedance of a circui~ that includes an earth ground return path. Realization of this concept allows me to el ;m; n~te the costly and cumber~ome blocking impedancPs that were previously believed to be nece~sary in order to isolate the test loop from possible sneak parallel paths to ground.
In situations where a leng~h of cable or cables is connected to another length o cable or cable~ at a splice skid or cablP coupler, as is customary~ the above discussed teachings will hold since the contact resistance to earth of a splice skid, for example, is expected to be in excess of two ohms. If it is desired to be extra eautious, a ~locking impedance may be used at the splice skid to connect the ground conductor 16 to ~33~
the splice skid housing (ground), in the manner discussed in the Vernier U.S. Patent ~oO 4,321,643.
~ 12 -approximate~y 1.8 ohms for a 1, oao feet length of cable. The minimum value of contact resistance that one might expect between a switch house and earth is of the order of two ohms, and .in ~ractice probably will be ~v~n larger. With the parameter values given above it is seen that the impedance between term;~ls 1 and 2 is much small~r than the impedances between terminals 1 and 3~ or between terminals 2 and 3. This means that even if the earth ground ha~ zero impedance the ground conductor of the cable still will be a lower imPedance current path than the earth ground return path that has ~he mutual inductance M in series with it. In one actual example, the measured value of impedance between term;n~l.s 3 and 5 of Fig. 7 was 18 times greater than the measured value of impedance between te~minals 2 and 5. It will be remembered that the yround conductor 16 of Fig. 7 actually is two braided ground wires and the braided shields of the phase wires. Conse~uently, the resistance of the path between points ~ and 5 will be ~ess than the resistancP between points 1 and 4 and between 5 and 6. Assuming negligible impedance in the primary of transformer Tc, less than 5%% of the current on the ground monitor loop would flow through 1~933Z~
the earth ground return path e~en if it had zero impedance.
Ba~ed on my above cmalysis I made the discovery that with the design of my present ~ystem I do not need the large and expensi~e blocking impedances ~b that are included in the ground monitoring syst~m of my above mentioned patent since the leakage current that was being blocked actually was small enough to ignore.
A ground conductor monitoring ~ystem according to my present invention i5 illustra~ed in simplified form in Fig. 8. Ground monitor 3.S
include~ an oscillator and power amplifier 50 that produces a l kiloHer~z test ~ignal~ The signal passes through a l KHx series xesonant filter compri.sed of ~he pximary winding of transforme~ T2 and capacitor Cl The signal is further filtered in an ~MI and RFI filter 52 which provide~
transient and voltage spike suppressionO The thoroughly filtered tes~ signal then is directly connect~d to pilot wire 18 of the mine power cablP lO.
Pilot wire 18 and ground conductor 16 are connected together by a terminating impedance ~t at the far end of the cable where a machine i~
located. Terminating impedance ~ is comprised of the parallel connected capacitor Ct and the i~ductor.
'g l p ~ 14 ~
Lt. A test loop therefore i~ formed by pilot wire 18, ground conductor 16, ~erminati~g impedanee ~t~ and monitor 35.
The 1 kHz curren~ that flows ~o pilot wire 18 is sampled by sampling transformer ~2~
The current from the secondary of transformer T2 is coupled as the I~ input to phase detector 56. The other input to phase detector 56 is a voltage sample E~ of the 1 k~z ~est signal that is connected to pilot wire 18. The voltage sample E~ is filter~d by a l kHz parallel resonant filter 54. Because the ~urrent sample signal I~ is coupled through transformer T2, the input signals I~ and E~ to phase detector 56 are in phase auadratureD
The 1 k~z parallel resonant filter 54 filters ~0 Her$z ~ign~ls and transients. Filter 54 introduces a minimal phasa shift o not more than 10 degrees over the operating voltage and temperature ranges.
The operating principle underlying the ~bove mentioned phase comparison is as fol~ows.
Under normal conditions the combined reactance of the ground conductor 16 and pilot wire 18 of the type of cable under con~ideration is inductive~
ThP reactance at 1 kHz of terminating impedance ~t is capacitive, being about 10 microfarads in one embodiment of the invention~ The i~ductance elemen_ Lt pr~vides a low impedan~e, hi~h current carrying path at 60 Hz. ~t SO Hæ, terminating impedance ~t appear~ as a 2.5 milliHenry ~nduc~or. This prevents high voltag~ build-up on the pilot wire in the event of a phase wire to pilst wire short, and a~so assures that the phase wire curr~nt will no$ be li~ited, thereby assuring that the circuit breaker will trip on an overcurrent.
Even with as many as ten 1,000 foot ca~les connected in seri~s, the reactance of ter~;nating impedance ~t at lkH~ will ~ppear to be capacitive to ground monitor 35 if the ground con~
ductor moni~oring loop is in accepta~le condition.
If a short circuit exists between ground conductor 16 and pilot wire 18, as illustrated at 61 in Fig. 8, ground monitor 35 no longer will "see' the capacitive reactance of terminating impedance ~t but will "see~ the inductive reactance of the conductors 16 ana 180 Phase detector 56 senses this change from a negative ~o a positive reactance and produces a fault signal in response thereto.
The current of the 1 kHz test signal flowiny in ground conductor 16 of cable lO i5 sampled ~y current transformer Tc that is located at the ground monitor end of cable 10, i.e., remote from the machinery. The primary of this transformer is a 2~
straisht conductor that is series connected 1:o the c~ble ground conductor 16. The secondary winding is comprised of multiple ~"indings on a ferromagnetic toroid that encircles the straight primary wire. Because the primary winding of trans:Eormer Tc is a straight conductor, its impedance will be negligible compared with the actual value of the mutua~ inductance M. This is consistent with the theory and explanation given above in connection with Figs. 3 ~ 7r The sampled ground conductor current Igc is coupled to EP~I and RFI ilter 58 which functions to re~ect 60 and 120 Hertz signals and provide~ .
transient and voltage spike protection. This f iltered ground conduc:tor current sample Igc then ,:
i~ coupled to amplitude detector 60 where it is compared with a reference voltage Vlref . If the ground conductor monitoring loop is in acceptable operating condition the voltage Vgc corresponding to current Ig~ is greater in magnitude than th~
reference voltage Vlr~f and the output of comparatvr 60 will be high. If an o~en circuit or high resis-tance fault condition exists in the ground conductor monitoring loop, the current in the loop will fall and voltage Vgc will be smaller in magnitude than reference voltage VlreEO W.it~ this condition, comparator 60 will produce a fault signal~ which in one embodiment is a low signal.
The u~e of the current sensing transformer Tc for monitoring the current of the test signal on ground conductor 16 provides a number of advantages.
First, it is failsafe in that if the test current is bypas~ed around ~he transformer or if the transformer should become open circui~ed in i~s primaxy or secondary, the out~ut of the transformer will all and a fAult signal will be produced by ~mplitude detector 60. Additionally, because the primary of transformer is a straight conductor with substantial current carrying capacity, ~h~ use of transformPr Tc does not appre~iably increase the impedance to 60 ~z current. The system of this invention also may be used in a power distribution sy~tem that is carrying d.c. power.
As further illustrated in Fig. 8, the outputs of phase detector 56 and amplitude detector 6Q are coupled to logic circuitry 64 which responds to a fault signal on either of its inputs to produce 2 corre~ponding output signal that actuates a respective one of the circuit breaker~ CBl ox CB2.
In r~sponse to an open circuit fault signal or a short circuit fault signal logic cixcuitry 64 also produces a secondaxy trip signal on output line 66.
3~
~ ~ 8 --Secondary trip line h6 incllldes the two conta~ts CBl-l and CB2~1 of circuit breakexs CB1 and CB2. These contactC; are closed, as illustrated, when the circuit breakers are set and a xespective one of the contacts opens when a fault signal is coupled from logic circuitry 64 to one of the ciruit breakers CBl or CB2. Secondary trip line 66 is energized in the absence o~ a fault condition on the ground conductor monitoring loop and the coils of relays Kl and K2 normally are energized. The energization of the,se relay coils-causes contacts Kl~1, K1~2 and K2~1, K2-2 to be closed. The closed contacts energize a holding coil, or coils, in the cable circuit breakers to connect the phase wires of cable 10 to the substation.
Upon occurrence of a fault signal from logic circuitry 64, circuit breaker CBl or CB2 is tripped to open a corr~sponding set of contacts CBl-l or CB2-1 to open secondary trip line 66. Addition~lly, secondary trip 66 is deenergized upon occurrence of a fault signal to logic circuitry 640 Consequently, relay coils Kl and K~ are deen~rgized and their corresponding contacts Kl-l, Kl-2 and K~-l and K2-2 opPn to deenergize the holding coil of the cable circuit breakers~ thereby causing them to open and disconnect cable 10 from the substation~
~9 ~,3Z~
The particular circuitry that is included in logic circuitry 64 may be conventional and may be similar to that illus~ra~ed in Figs. 4 and 5 of my above identi~ied patent, and as in Figs. 4 and 5 of the above-mentioned Vernier application. In the accompanying Fig. 8, phase detector 56 would include the signal pakh comprised o squaring amplifiers 111, 112 exclusive or circuit 113, and integrator R18 C18 of my above identified patent. Amplitude detector 56 of the accom~anying Fig. 8 may be a con~entional comparator used similarly to coxresponding devices in my prior ~atent. Similarly, the secondary trip signal on line 66 may be produced in a manner similar to the secondary trip signal on line 187 in Fig. 4 of my above-mentioned patent. In the present invention, the open and short signals from amplitude detector 60 and phase detector 56 would be input signals, along with power-up and power~down signals commonly used in logic circuitry. If additional details are desired, reference is made to Figs. 4 and 5 of my prior patent, although one skilled in the art will be fully capable of instrumenti~g the block diagram illustrated in the accompanying Fig. 8.
The value of the comparison voltage V
that constitutes one input to amplitude detector 60 ~ ~,9 33Z~?d need not be an exact value but is selected to produce acceptable operation with the length of cable contemplated to be used.
From the Above description it may be understood that a simplified and more xelia~le ground fault monitor is possible as a result of the realization that the mutual inductance in the cable reduces the impedance of the test loop comprised of the pilot wire 18 and ground conductor 16 ~o that ~he impedance of that loop to the tes~
signal i~herently will be less than the impedance of a circui~ that includes an earth ground return path. Realization of this concept allows me to el ;m; n~te the costly and cumber~ome blocking impedancPs that were previously believed to be nece~sary in order to isolate the test loop from possible sneak parallel paths to ground.
In situations where a leng~h of cable or cables is connected to another length o cable or cable~ at a splice skid or cablP coupler, as is customary~ the above discussed teachings will hold since the contact resistance to earth of a splice skid, for example, is expected to be in excess of two ohms. If it is desired to be extra eautious, a ~locking impedance may be used at the splice skid to connect the ground conductor 16 to ~33~
the splice skid housing (ground), in the manner discussed in the Vernier U.S. Patent ~oO 4,321,643.
Claims (7)
1. Apparatus for monitoring the condition of a ground conductor that extends throughout the length of an electrical cable that has a plurality of power carrying wires and at least one other wire, said apparatus comprising means at one end of said ground conductor for earth grounding said ground con-ductor, said ground conductor having no other earth grounding means connected thereto in the absence of a fault condition, an a.c. test signal source coupled between said one end of the ground conductor and a corresponding end of said other wire, impedance means connected between the opposite ends of the ground conductor and said other wire to form a ground monitoring loop, said impedance means having a reactance value at the frequency of said test signal that is opposite to, and greater in magnitude than, the reactance value of said ground conductor and other wire without the impedance means connected thereto, means at said one end of the cable for sampling the voltage and current of the test signal that is coupled onto said monitoring loop, means for comparing the phases of said sampled voltage and current and for producing a fault signal when the comparison indicates that the reactance of the monitoring loop changes from the reactance value of a properly operating monitoring loop with the terminating impedance connected therein, means at said one of the ground conductor for sampling the test signal current flowing thereon, said last-named means for sampling the test signal current flowing on said ground conductor presenting an impedance to said test signal that is small compared to the impedance represented by the mutual inductance between said ground conductor and other wire at the frequency of said test signal, and means for producing a fault signal when the test signal current sampled on the ground conductor is different from a predetermined magnitude.
2. Apparatus for monitoring for short circuits on a ground conductor that extends throughout the length of an electrical cable that has a plurality of power carrying wires and at least one other wire, said apparatus comprising means for earth grounding said ground conductor only at one end thereof, said ground conductor having no other earth grounding means connected thereto an a.c. test signal source coupled between said one end of the ground conductor and the corresponding end of said other wire, terminating impedance means connected between the opposite ends of the ground conductor in said other wire to form a ground monitoring loop on which said test signal propagates, said impedance means presenting at the frequency of said test signal a reactance value that is opposite to, and greater than, the reactance value of said ground conductor and said other wire without the impedance means connected thereto means at said one end of the cable for sampling the voltage and current of the test signal that is coupled onto said monitoring loop from said test signal source, means for comparing the phases of said sample voltage and current and for producing a fault signal when the comparison indicates that the reactance of the monitoring loop changes from the reactance value of a properly operating monitoring loop with the terminating impedance connected therein.
3. The combination claimed in claim 2 wherein said terminating impedance comprises a parallel connected capacitor and inductor, said inductor presenting a low enough value of impedance at the frequency of the electri-cal power carried on said power wires that current undesirably flowing from a power wire to said ground conductor will not be impeded to the extent that the operation of overcurrent circuit breakers in the power wires will be prevented from tripping upon occurrence of said undesired current flow.
4. The combination claimed in claim 3 wherein said terminating impedance presents a capacitive reactance at the frequency of said test signal, said capacitive reactance being greater than the combined inductive reactance of of the ground conductor and said other wire at the frequency of said test signal.
5. Apparatus for monitoring for open circuits in a ground conductor that extends throughout the length of an electrical cable that has a plurality of power carrying wires and at least one other wire, said apparatus comprising means for earth grounding said ground conductor only at one end thereof, said ground conductor having no other earth grounding means connected thereto an a.c. test signal source coupled between said one end of the ground conductor and the corresponding end of the other wire, impedance means connected between the opposite ends of the ground conductor and said other wire to form a ground monitoring loop, means at said one end of the ground conductor for sampling the test signal current flowing thereon, and means for producing a fault signal when the test signal current sampled on a ground conductor is different from a predetermined magnitude.
6. The apparatus claimed in claim 5 wherein, said means for sampling the test signal current flowing on the ground conductor presents an impedance to the test signal that is negligable compared to the impedance represented by the mutual inductance between the ground conduc-tor and other wire at the frequency of said test signal, whereby the impedance of said test loop that includes the ground conductor is less to the test signal than the impedance of a parallel ground return path from said loop that includes an earth ground path.
7. The method of monitoring the condition of the ground conductor of an electrical power cable that also includes a plurality of power or phase wires and at least one other wire, all of which extend from one end to the opposite end of the cable, said method comprising the steps earth grounding said ground conductor only at said one end, coupling an a.c. test signal between said one end of the ground conductor and the corresponding end of said other wire, terminating the ground conductor and the other wire at said opposite end of the cable in an impedance that presents at least a predeter-mined capacitive reactance at the test signal frequency when monitored at said one end of the cable, monitoring the reactance of said terminating impedance and producing a fault signal when the reactance of the cable as monitored at said one end changes from said predetermined capacitive reactance, sampling the test signal current flowing in said ground conductor at said one end of the cable, and producing a fault signal when the sampled test signal current is below a predetermined magnitude.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000403539A CA1193322A (en) | 1982-05-21 | 1982-05-21 | Ground conductor monitoring system |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000403539A CA1193322A (en) | 1982-05-21 | 1982-05-21 | Ground conductor monitoring system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1193322A true CA1193322A (en) | 1985-09-10 |
Family
ID=4122837
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000403539A Expired CA1193322A (en) | 1982-05-21 | 1982-05-21 | Ground conductor monitoring system |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1193322A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102183707A (en) * | 2011-02-21 | 2011-09-14 | 孙映金 | High-pressure grounding on-line monitoring warning device |
| IT201700053259A1 (en) * | 2017-05-17 | 2018-11-17 | Tecnikabel S P A | MONITORING SYSTEM OF THE STATUS OF AN INDUSTRIAL WIRING SYSTEM AND ITS RELATED INDUSTRIAL WIRING SYSTEM |
-
1982
- 1982-05-21 CA CA000403539A patent/CA1193322A/en not_active Expired
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102183707A (en) * | 2011-02-21 | 2011-09-14 | 孙映金 | High-pressure grounding on-line monitoring warning device |
| IT201700053259A1 (en) * | 2017-05-17 | 2018-11-17 | Tecnikabel S P A | MONITORING SYSTEM OF THE STATUS OF AN INDUSTRIAL WIRING SYSTEM AND ITS RELATED INDUSTRIAL WIRING SYSTEM |
| EP3418755A1 (en) * | 2017-05-17 | 2018-12-26 | Tecnikabel S.p.A. | Monitoring system to monitor the condition of an industrial wiring system and corresponding industrial wiring system |
| US10910831B2 (en) | 2017-05-17 | 2021-02-02 | Tecnikabel S.P.A. | Monitoring system to monitor the condition of an industrial wiring system and corresponding industrial wiring system |
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