A cable fault locator is described for sensing faults such as short circuits in power cables. The apparatus includes a plurality of current transformers strategically located along a cable. Trigger circuits are connected to each of the current transformers for placing a resistor in series with a resistive element responsive to an abnormally high current flowing through that portion of the cable. By measuring the voltage drop across the resistive element, the location of the fault can be determined

Cason, R. L.

Mcstay, J. J.

English

NASA Technical Reports Server

United States Patent 1191 
Cason et al. 
[I11 4,110,683 
[45] Aug. 29, 1978 
[54] APPARATUS INCLUDING A PLURALITY OF 
SPACED TRANSFORMERS FOR LOCATING 
SHORT CIRCUITS IN CABLES 
[75] Inventors: Robert L. Cason, Cape Canaveral; 
John J. McStay, Titusville, both of 
Fla. 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[73] Assignee: The United Stata of America as 
[21] Appl. No.: 814,004 
[22] Filed: Jul. 8,1977 
[51] Int. c1.2 ...................... 601R 31/08; GOlR 19/16 
[52] U.S. c1. ...................................... 324/52; 324/127; 
324/133; 340/650; 340/664 
[58] Field of Search ................... 324/51, 52, 127, 133, 
324/102; 340/248 C, 248 E, 253 A, 253 E, 227 
R, 409 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,099,828 7/1963 Kelley .............................. 340/248 E 
3,125,720 3/1964 
3,200,392 SA965 
3,356,939 12/1967 
3,477,019 11/1969 
3,579,280 5/1971 
3,588,689 6/1971 
3,771,049 11/1973 
3,832,678 SA974 
Swift ...................................... 324/51 
Chumakov ....................... 340/253 E 
Stevenson ........................ 324/127 X 
Hartmann .......................... 324/52 X 
Florance et al. ................ 324/127 X 
Crawford ............................... 324/52 
Piccione ................................. 324/51 
Gysell et al. ..................... 340/227 R 
Primary Examiner-Gerard R. Strecker 
Attorney, Agent, or Firm-James 0. Harrell; John R. 
Manning 
1571 ABSTRACT 
A cable fault locator for sensing faults such as short 
circuits in power cables. The apparatus includes a plu- 
rality of current transformers strategically located 
along said cable. Trigger circuits ,are connected to each 
of the current transformers for placing a resistor in 
series with a resistive element responsive to an abnor- 
mally high current flowing through that portion of the 
cable. By measuring the voltage drop across the resis- 
tive element, the location of the fa,ult can be determined. 
4 Claims, 2 Drawing Figures 
0 COMPUTER OR RECORDER 
https://ntrs.nasa.gov/search.jsp?R=19790010022 2020-03-20T18:10:37+00:00Z
U.S. Patent Aug. 29, 1978 4,110,683 
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4,110,683 
1 
APPARATUS INCLUDING A PLURALITY OF 
SPACED TRANSFORMERS FOR LOCATING 
SHORT CIRCUITS IN CABLES 
ORIGIN OF THE INVENTION 
The invention described herein was made by employ- 
ees of the United States Government and may be manu- 
factured and used by or for the Government for govern- 
mental purposes without the payment of any royalties 
thereon or therefor. 
BACKGROUND OF THE INVENTION 
The present invention relates to a cable fault locator 
and more particularly to an apparatus for locating cable 
faults in underground or overhead power cables by 
monitoring the ground fault current at preselected loca- 
tions and transmitting the data to a remote monitor or 
computer. 
The present mode of locating a fault in power cables 20 
requires disconnecting all ends of the cable, putting a 
signal consisting of high voltage pulses thereon, and 
then, by traveling parallel to the cable, the signal is 
traced to the point of the fault. This concept of fault 
location is rather time-consuming. 25 
Another fault indicator and locator for buried cables 
is disclosed in U.S. Pat. No. 3,771,049. In this particular 
patent, an underground cable fault indicator and locator 
is provided to indicate the location of a fault or short 
circuit in the cable. The cable is passed through the 30 
apertures of a plurality of spaced transformer sensors 
connected to indicating elements. Upon the occurrence 
of a fault in the cable, the indicating element provides a 
visual indication of the section of cable in which the 
fault has occurred. As illustrated in FIG. 1, lamps are 35 
usually provided above the surface for being illumi- 
nated responsive to a short occurring in that section of 
the cable. 
Another electrical ground fault detector is disclosed 
in US. Pat. No. 3,579,280. In this particular ground 
fault detector, a ring-type current transformer is utilized 
for triggering a silicon controlled rectifier for activating 
an alarm responsive to current flowing through the 
supply line to indicate a ground fault. 
SUMMARY OF THE INVENTION 
The invention relates to an apparatus for monitoring 
faults such as short circuits in an electrical power cable 
which produces an abnormally high current in the por- 
tion of the cable extending between a power supply 
connected to one end of the cable and the point where 
the cable is grounded. The apparatus includes a resistive 
element which is connected to a power supply. A plu- 
rality of current transformers are spaced at predeter- 
mined points along the cable. A plurality of resistors 
corresponding in number to the current transformers 
are provided. Trigger circuit means are connected to 
each of the current transformers for selectively con- 
necting one or more of the resistors in series with the 
resistive element responsive to an abnormally high cur- 
rent flowing through the respective current transformer 
connected thereto. Voltage sensing means is provided 
for measuring the voltage drop across the resistive ele- 
ment so as to determine the number of transformers 
through which the abnormally high current flow trav- 
els. Thus, the location of the grounded point within the 
cable can be determined from the magnitude of the 
voltage drop across the resistive element. 
40 
45 
50 
55 
60 
65 
2 
Each of the trigger circuits includes a silicon con- 
trolled rectifier that is connected to a respective trans- 
former so as to place a respective resistor in series with 
the resistive element responsive to an abnormally high 
current flowing through that particular transformer. 
Accordingly, it is an important object of the present 
invention to provide a low cost system for locating 
cable fault in an underground or overhead power cable 
system by monitoring the ground fault current at prese- 
lected locations and transmitting the data to a remote 
monitor or computer. 
Another important object of the present invention is 
to provide a relatively simple fault detector for power 
cables which is simple in operation and sensitive to 
faults in power cables. 
Another object of the present invention is to provide 
a fault indicator and locator which is safe in operation 
even when the cable that is being monitored has an 
abnormally high current flow therethrough. 
These and other objects and advantages of the inven- 
tion will become apparent upon reference to the follow- 
ing specification, attendant claims, and drawing. 
BRIEF DESCRIPTION OF THE DRAWING 
FIG. 1 is a schematic diagram illustrating the cable 
fault detector constructed in accordance with the pres- 
ent invention. 
FIG. 2 is a more detailed schematic diagram illustrat- 
ing a portion of the cable fault detector illustrated in 
FIG. 1. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENT 
Referring now in more detail to FIG. 1 of the draw- 
ing, there is illustrated a conventional cable 10 which 
may be of any length. The cable 10 normally has a 
power supply connected to one end thereof so as to 
produce a current flow therethrough. If a ground devel- 
ops along the cable, it will cause a large increase in 
current from the power supply to that ground point. 
For purposes of illustration, the ground point will be 
described as being at point 11. Therefore, if or when 
point 11 becomes grounded, the current flow through 
the portion of the cable which includes transformers T1 
and T2, increases substantially. 
Transformers T1, T2, T3 and T4 are strategically 
spaced along the cable. When the current flow through 
the portion of the cable which includes transformers T1 
and T2 increases abnormally high as a result of a short 
circuit, a larger voltage is produced on the secondary 
windings 12 and 14 of transformeus T1 and T2. There is 
no increase in voltage on secondary windings 16 and 18 
since transformers T3 and T4 are not located between 
the shorted point 11 and the power supply 20 connected 
to the other end of the cable. Monitoring circuits 20,22, 
24 and 26 are each connected to a respective output 
winding 12, 14, 16 and 18 of the current transformers. 
These monitoring circuits include a triggering silicon 
controlled rectifier 28 which is triggered on responsive 
to a predetermined voltage being applied thereto for 
closing a circuit that includes one of the resistors R1, 
R2, R3, or R4 for placing it in series with a resistive 
element R5. The resistive element R5 is, in turn, con- 
nected to a power supply 30 by means of an on-off 
switch 32 and a DC rectifier 34. The power supply may 
be any suitable AC power supply. Connected across the 
resistive element R5 are voltage measuring devices 32, 
34,36, and 38. These voltage measuring devices 32-38 
4,110,683 
3 4 
may be any suitable voltage detectors such as volt me- 1. An apparatus for monitoring faults such as ground- 
ters or electronic circuits that are triggered “on” upon ing in an electrical power cable which produce an ab- 
a predetermined voltage being applied thereto. These normally high current in that portion of said cable ex- 
trigger circuits in one particular embodiment illuminate tending between a power supply connected to one end 
lamps responsive to a predetermined voltage being ap- 5 of said cable and the point where said cable is grounded, 
plied thereto. For example, the voltage detector circuit said apparatus comprising: 
32 will be activated when the voltage drop across resis- 
tor R5 is of a value that is produced by placing resistor 
R1 in series therewith. Voltage detector 34 is, in turn, 
activated when both resistors R1 and R2 are placed in 10 
series with resistive element R5. Similarly, voltage de- 
tector circuit 38 is activated wherein all four of the 
resistors R1 through R4 are placed in series with resis- 
(a) a power supply, 
(b) a resistive element connected to said Power SUP- 
(C) a Plurality of current tramformers spaced at Pre- 
determined points along said cable and coupled 
thereto for sensing current flowing through said 
cable, 
Ply, 
tive element R5. The outputs of the voltage detector 
circuits 32 through 38 can be connected to a computer 15 
(d) trigger circuit means connected to each of said 
current transformers for increasing the voltage 
and/or recorder. 
Referring in more detail to FIG. 2 of the drawing, a 
more detailed schematic diagram of the monitoring 
circuit and, in particular, monitoring circuit 20 is illus- 
correspond thereto. The current transformer includes a 
U-shaped coil of rectangular cross section with the 
power cable 10 occupying the semi-circular end of the 
U. Between the ends of the U is clamped a core of 25 
circular cross-section which will carry the secondary 
winding 12. Both pieces are of low carbon steel. 
As the current changes as a result of a ground devel- 
oping within the cable 10, the current transformer T1 
will detect this abnormally high current flow through 3o 
12. This voltage is coupled to the input of a full-wave 
rectifier generally designated by the reference character 
40. The output of the full-wave rectifier is taken across 
terminals 42 and 44 and fed to a voltage divider circuit 35 
including resistors 46 and 48. An input terminal 50 of 
silicon controlled rectifier 28 is connected between the 
resistors 46 and 48 for being activated upon a predeter- 
mined voltage being applied thereto. 
“on”, current flows from the Source of Power 30 
through resistors R5 and R1. The voltage sensor 32 is 
connected across resistor R5 for detecting the voltage 
drop thereacross. The voltage drop across resistor R5 
varies according to the number of resistors R1 through 45 
pi4 that are placed in series therewith. 
A zener diode 52 is connected across the Output ter- 
minals 42 and 44 of the full-wave rectifier 40 for pro- 
tecting the rectifier circuit from excessive voltages that 
may be produced by the current transformer T1. 
The voltage that is rectified by the full-wave rectifier 
4Q is applied across a resistor 54 which is connected 
between the output terminals 42 and 44. The voltage 
drop across this resistor 54 is applied through the resis- 
tor 46 to trigger the silicon rectifier 28. 
Each of the monitoring circuits 20, 22, 24 and 26 
includes all of the structure shown in FIG. 2 and, in 
and the voltage divider network which includes resis- 
tors 46 and 48. This portion of the circuit is enclosed 60 
within the boxes identified by the reference character 
56. cable, 
While a preferred embodiment of the invention has 
been described using specific terms, such description is 
for illustrative purposes only, and it is to be understood 65 
that changes and variations may be made without de- 
parting from the spirit or scope of the following claims. 
drop across said resistive element a predetermined 
amount responsive to an abnormally high current 
flowing through the current transformer that it is 
connected to, 
(i) a resistor, and 
(ii) means for placing said resistor in series with 
said resistive element responsive to an abnor- 
mally high current flowing through the respec- 
tive transformer to which the respective trigger 
circuit is connected, and 
(f) means for measuring the voltage drop across said 
resistive element for determining the number of 
current transformers said abnormally high current 
flows through, 
cable can be determined from the magnitude of said 
voltage drop across said resistive element. 
2. The apparatus as set forth in claim 1 wherein said 
trated. Each of the monitoring circuits 22, 24 and 25 2o (e) said trigger circuit 
the cable 10 and produce a On Output winding whereby the location of said grounded point of said 
for said resistor in series with said 
tive element includes a silicon controlled rectifier, 
a circuit including said silicon controlled rectifier, 
said resistor and said resistive element, 
means for connecting said silicon controlled rectifier 
to the respective transformer, and 
means for triggering said silicon controlled rectifier 
responsive to an abnormally high current flowing 
through the respective transformer for connecting 
said resistor in series with said resistive element. 
3. The apparatus as set forth in claim 2 further corn- 
a full wave rectifier interposed between each of said 
silicon controlled rectifiers and each of said current 
transformers. 
4. An apparatus for monitoring faults such as short 
circuits in an electrical power cable which produce an 
abnormally high current in the portion of the cable 
extending between a power supply connected to one 
end of the cable and the point where said cable is 
When the silicon controlled rectifier 28 is triggered 40 
prising: 
50 
55 grounded, said apparatus comprising: 
(a) a power supply, 
(b) a resistive element connected to said power sup- 
(c) a plurality of current transformers spaced at pre- 
determined points along said cable and coupled 
thereto for sensing current flowing through said 
(d) a plurality of resistors corresponding in number to 
said current transformers, 
(e) trigger circuit means connected to each of said 
current transformers for selectively connecting one 
of said resistors in series with said resistive element 
responsive to an abnormally high current flowing 
particular, the full-wave rectifier 40, the zener diode 52, Ply, 
What is claimed is: 
5 
4,110,683 
6 
mine the number of transformers through which 
said abnormallv high current travels. through the respective current transformer con- 
nected thereto, and whereby the lockon-of said grounded point of said 
cable can be determined from the magnitude of the 
voltage across said resistive element. (0 voltage sensing means for measuring the voltage 5 - * * * * *  drop across said resistive element so as to deter- 
10 
15 
20  
25 
30 
35 
40 
45 
50  
55 
65 


Apparatus including a plurality of spaced transformers for locating short circuits in cables

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