A METHOD OF ESTIMATING THE LOCATION OF A CABLE BREAK INCLUDING A MEANS TO MEASURE RESISTIVE FAULT LEVELS FOR CABLE SECTIONS

METHODE DE LOCALISATION DES BRIS DE CABLE COMPRENANT UN MOYEN DE MESURER LES NIVEAUX DES DEFAUTS RESISTIFS DANS DES SECTIONS DE CABLE

English Abstract

A method and apparatus are used for locating cable breaks and resistive faults in cables, including fibre optic cables. The conductive shield or armour of the cable is divided into sections, usually at a splice. A step function voltage is applied to one end of the conductive shield. Remote sensors at the end of each section monitor the voltage and current as a function of time and at steady state. The measured data are encoded as current pulses and transmitted along the armour to the end of the cable. A computer at the cable end calculates from the measured voltages and currents the capacitance of each section of the shield. A broken section is identified by comparing the calculated and original capacitances of the sections and the distance along the broken section to the break is calculated from the calculated and original capacitances of the broken section. Resistive faults in the cable are located by calculating a fault resistance of each section of the shield from the measured steady state voltages and currents. A section containing a resistive fault is identified from the magnitude of the calculated fault resistance and the distance to the resistive fault is calculated from the measured steady state voltages and currents of the faulted section.

French Abstract

Une méthode et un appareil sont utilisés pour localiser les bris de câble et les défauts résistants dans les câbles, y compris les câbles à fibres optiques. Le blindage conducteur ou l'armature conductrice du câble est divisé en sections, généralement à une épissure. Une tension à fonction en escalier est appliquée à une extrémité du blindage conducteur. Des capteurs à distance à l'extrémité de chaque section surveillent la tension et le courant en fonction du temps et dans un état stable. Les données mesurées sont codées en impulsions de courant et transmises le long de l'armature à l'extrémité du câble. Un ordinateur situé à l'extrémité du câble calcule à partir des courants et des tensions mesurées la capacité de chaque section du blindage. Une section brisée est identifiée en comparant les capacités calculées aux capacités originales des sections et la distance le long de la section brisée jusqu'au bris est calculée à partir des capacités calculées et originales de la section brisée. Des défauts résistants dans le câble sont trouvés en calculant une résistance de défaut pour chaque section du blindage à partir des courants et tensions mesurés à l'état stable. Une section contenant un défaut résistant est identifiée à l'ampleur de la résistance de défaut calculée et la distance au défaut résistant est calculée à partir des courants et tensions mesurés dans un état stable de la section défectueuse.

Claims

Claims:
1. A method of locating a cable break in a cable having a conductive shield
extending therealong, wherein the conductive shield is divided into a
plurality of
sections sequentially along the cable, with each section having an original
capacitance, said method comprising:
applying a step function voltage to one end of the conductive shield;
measuring voltage and current as a function of time at each end of
each section of the shield;
calculating from the measured voltages and currents a calculated
capacitance of each section of the shield;
identifying a broken section containing the break by comparing the
calculated and original capacitances of the sections;
calculating the distance along the broken section to the break from the
calculated and original capacitances of the broken section.
2. ~A method according to Claim 1 comprising:
providing a base station at said one end of the cable;
providing a plurality of remote sensor stations at junctions of respective
pairs
of the cable sections;

actuating each remote sensor station to perform the step of measuring
voltage and current in response to the application of the step function
voltage to the
shield.
3. ~A method according to Claim 2 comprising transmitting data representing
the
measured voltages and currents from the remote sensor stations to the base
station
and performing the steps of calculating the calculated capacitance of each
section of
the shield and the distance along the broken section to the break at the base
station.
4. ~A method according to Claim 3 comprising transmitting the data as
electrical
pulses on the cable shield.
5. ~A method according to Claim 2 or 3 comprising transmitting the data from
the
respective remote sensor stations at different times.
6. ~A method according to Claim 5 comprising transmitting the data from the
remote sensor stations in sequence along the cable, starting with the remote
sensor
station farthest from the base station.
7. ~A method according to Claim 1 further comprising locating a resistive
fault in
the cable where each section of the shield has an original series resistance,
said
method comprising:
measuring steady state voltage and current at each end of each
section of the shield;
calculating from the measured steady state voltages and currents a
calculated fault resistance of each section of the shield;
16

identifying a faulted section containing a resistive fault from the
magnitude of the calculated fault resistance;
calculating the distance along the broken section to the resistive fault
from the measured steady state voltages and currents of the faulted section.
8. ~A method according to Claim 7 comprising:
providing a base station at said one end of the cable;
providing a plurality of remote sensor stations at junctions of respective
pairs
of the cable sections;
actuating each remote sensor station to perform the steps of measuring
voltage and current in response to the application of the step function
voltage to the
shield.
9. ~A method according to Claim 8 comprising transmitting data representing
the
measured voltages and currents from the remote sensor stations to the base
station
and performing at the base station the steps of calculating the calculated
capacitance and calculated fault resistance of each section of the shield and
the
distances to the break and the resistive fault.
10. A method according to Claim 9 comprising transmitting the data as
electrical
pulses on the cable shield.
11. A method according to Claim 9 or 10 comprising transmitting the data from
the respective remote sensor stations at different times.
17

12. A method according to Claim 11 comprising transmitting the data from the
remote sensor stations in sequence along the cable, starting with the remote
sensor
station farthest from the base station.
13. An apparatus for locating a cable break in a cable having a conductive
shield
extending therealong, wherein the conductive shield is divided into a
plurality of
sections sequentially along the cable, with each, section having an original
capacitance, said apparatus comprising:
a power supply for applying a step function voltage to one end of the
conductive shield;
sensors for measuring voltage and current as a function of time at
each end of each section of the shield; and
a computer including:
means for calculating from the measured voltages and currents
a calculated capacitance of each section of the shield;
means for identifying a broken section containing the break by
comparing the calculated and original capacitances; and
means for calculating the distance along the broken section to
the break from the calculated and original capacitances of the broken section.
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14. An apparatus according to Claim 13 including a remote sensing station
between each two adjacent sections for generating a signal representing
voltage
and current data measured at the ends of the adjacent sections.
15. An apparatus according to Claim 14 wherein the remote sensing station
includes coding means for encoding the voltage and current data as electrical
pulses
and signal transmitting means for transmitting the encoded data to the
computer on
the cable shield.
16. An apparatus according to Claim 15 wherein the pulses are current pulses.
17. An apparatus according to Claim 15 or 16 including means for actuating the
remote sensing stations sequentially to transmit the encoded data.
18. An apparatus according to any one of Claims 13 to 17 further comprising
means for locating a resistive fault in the cable where each section of the
shield has
an original series resistance, wherein:
the sensors include means for measuring steady state voltage and
current at each end of each section of the shield;
the computer includes:
means for calculating from the measured steady state voltages
and currents a calculated fault resistance of each section of the shield;
means for identifying a faulted section containing a resistive
fault from the magnitude of the calculated fault resistance; and
19

means for calculating the distance along the faulted section to
the resistive fault from the measured steady state voltages and currents of
the
faulted section.

Description

CA 02298392 2000-02-14
v
A METHOD OF ESTIMATING THE LOCATION OF A CABLE BREAK INCLUDING
A MEANS TO MEASURE RESISTIVE FAULT LEVELS FOR CABLE SECTIONS
Field of the Invention
The present invention relates to the location of cable breaks and resistive
faults in fibre optic cable.
Background
In the past two decades the mass deployment of fibre optic cable has been
instrumental in increasing the reliability of the world wide telecommunication
network. This mass deployment has also resulted in the concentration of
communication circuits into long lengths of physically small and mechanically
vulnerable cables.
Fibre optic cables placed in the outside environment fall into one of three
general categories, aerial, buried and underground. Aerial installation
usually offers
the lowest cost for new cable placement particularly when the pole route
exists with
support capacity available. Direct buried is favored on long haul routes when
ploughing and trenching can be carried out in favorable right of way
conditions. In
new suburban areas buried cable construction is often a requirement to
eliminate
pole structures which compromise aesthetic appeal. Underground construction is
generally defined as cable placement in pre-built duct structures buried under
city
streets. Due to the high initial construction costs of the duct structures,
underground
is the most expensive placement method but necessary to avoid tearing up city
streets for the repair or addition of cable.

CA 02298392 2000-02-14
Each installation method has reliability and maintenance issues. The threat to
the cable structures comes from two sources, man made problems and
environmental conditions. Excluding craft error, man made failures include dig
ups,
collision, fire, and gunshot damage. Environmental threats include, rodents,
lightning, floods, ice and power line failure.
Major outages are usually the result of a complete cut through of the cable
structure. Unintentional cable dig ups are responsible for 50 percent or more
of
outages. Due to the high capacity of a fibre optic cable, a single cut cable
can result
in thousands of dollars a minute in lost revenue. It is therefore of critical
importance that the location of the cable cut is determined quickly and a
repair crew
dispatched to the scene to restore the cable.
A well known method of locating a fibre cable break employs an Optical Time
Domain Reflectometer (OTDR). In this method short pulses of light are launched
into the severed fibre. A portion of the pulse is reflected back at the break
and the
time difference from the moment of launch to the return of the reflected light
pulse is
measured to estimate the distance to the break. For maximum benefit, an OTDR
must be constantly measuring a test fibre in every cable selected for
monitoring.
The main drawback of this method is that OTDR instruments are costly and a
fibre
must be made available for the measurement. Deploying OTDR systems to actively
monitor a large network of cables is therefore is costly and frequently
involves
complex interfacing to active optical fibres.
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CA 02298392 2000-02-14
The present invention provides an effective means to detect and locate a break
in a
fibre optic cable which does not employ OTDR techniques and does not require
access to a test optical fibre.
Summary
According to one aspect of the present invention there is provided a method of
locating a cable break in a cable having a conductive shield extending
therealong,
wherein the conductive shield is divided into a plurality of sections
sequentially along
the cable, with each section having an original capacitance, said method
comprising:
applying a step function voltage to one end of the conductive shield;
measuring voltage and current as a function of time at each end of
each section of the shield;
calculating from the measured voltages and currents a calculated
capacitance of each section of the shield;
identifying a broken section containing the break by comparing the
calculated and original capacitances of the sections;
calculating the distance along the broken section to the break from the
calculated and original capacitances of the broken section.
In preferred embodiments, the method further comprises locating a resistive
fault in the cable by the steps of:
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CA 02298392 2002-05-06
measuring ste$dy state voltage and current at each end of each
section of the shield;
calculating from the measured steady state voltages and currents a
calculated fault resist:~rtce of each section of the shield;
ir~entifying a faulted section containing a resistive fault from the
magnitude of the calculated fault resistance; and
calculating the di$tance along the broken section to the resistive fault
from the measured steady state voltages and currents of the faulted seetlon.
Thus, as an additional benefit, damage to the outer insulating cable jacket
may be detected on a section by section basis, prr~viding an indication of
damage to
the profiectlve outer layers of the cable structure.
According to another aspect of the invention there is provided an apparatus
far locating a cable break in a cable having a conductive shield extending
thereatQn~, wherein the conductive shield is divided int4 a plurality of
sections
sequentially along fihe cable, with each section having an original
capacit~anre, said
apparatus comprising:
a power supply for applying a step funckion voltage to one end of the
conductive shield;
sensors far measuring voltage and cun-ent as a function of tune at
each end of each section of t>1e shield; and

CA 02298392 2000-02-14
a computer including:
means for calculating from the measured voltages and currents
a calculated capacitance of each section of the shield;
means for identifying a broken section containing the break by
comparing the calculated and original capacitances; and
means for calculating the distance along the broken section to
the break from the calculated and original capacitances of the broken section.
In preferred embodiments the apparatus, further comprises means for
locating a resistive fault in the cable, wherein:
the sensors include means for measuring steady state voltage and
current at each end of each section of the shield;
the computer includes:
means for calculating from the measured steady state voltages
and currents a calculated fault resistance of each section of the shield;
means for identifying a faulted section containing a resistive
fault from the magnitude of the calculated fault resistance; and
means for calculating the distance along the faulted section to
the resistive fault from the measured steady state voltages and currents of
the
faulted section.
Brief Description of the Drawings

CA 02298392 2000-02-14
In the accompanying drawings, which illustrate the theoretical basis and an
exemplary embodiment of the present invention:
Figure 1 is a low frequency cable shield equivalent circuit;
Figure 2 is a low frequency cable shield equivalent circuit with resistive
faults;
Figure 3 illustrates the voltages and currents in one cable section;
Figure 4 is a chart of the line voltages at the beginning and end of a cable
section as
a function of time;
Figure 5 is a chart of the line currents at the beginning and end of a cable
section as
a function of time;
Figure 6 is an equivalent circuit for locating a resistive fault to ground;
Figure 7 is a block diagram of one embodiment of the apparatus; and
Figure 8 shows the voltage sequence applied to the cable to measure segment
characteristics.
Detailed Description
General Description
The applicants' method makes use of the metallic armour or shield of a fibre
optic cable to estimate the location of a cut or break in a fibre optic cable.
The
metallic armour and surrounding insulating plastic jacket form a coaxial
circuit when
referenced to earth ground.
Figure 1 shows a low frequency model which can be used to electrically
6

CA 02298392 2000-02-14
represent the coaxial circuit formed by the cable armour or shield 10 and the
plastic
insulating jacket with an earth return 12. The cable circuit is divided into n
discrete
sections which represent the individual cable lengths spliced together. Each
section
has a series resistance RX and a shunt capacitance CX, with the subscript x
designating the section 1 to n.
Damage to the insulating jacket could result in faults to ground parallel to
the
shunt capacitances. Section fault to ground resistances R~ are added to the
electrical equivalent model as shown in Figure 2.
The method for determining the distance to a cable break including resistive
faults for every cable section requires that the fault to ground resistance R~
and
shunt capacitance CX be determined for every section. In simple terms, the
method
involves the application of a step function voltage to the circuit at the
beginning of
the cable under test and the measurement of the voltage and current as a
function of
time at the beginning and end of every section as illustrated in Figure 3. The
resistive and capacitive parameters R~ and CX are then calculated from the
steady
state and transient responses respectively.
Detailed Mathematical Description
To determine the individual shunt capacitance CX and fault resistance R~ to
ground for every section, a step input DC voltage is applied and the input and
output
current and voltage response measured for every cable section. The measured
parameters are used to calculate the transient and steady state response of
every
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CA 02298392 2000-02-14
section. The general response of the network at any point along the line is
given by:
v(t) = Vo + (Vf-Vo) (~-e~-~~~°~)~
Where: Vo is the initial line voltage
Vf is the final line voltage
rc is the circuit time constant
and i(t) = to + (Ir-lo) (~-e~-~~rc))
Where: to is the initial line current
If is the final line current
Figure 4 shows the general voltage response and Figure 5 shows the general
current response of the nth section to a step voltage input. From the steady
state
conditions reached when the capacitive component of the line is charged, the
nth
section fault resistance is estimated by:
R~ -_ VS/IS . (3)
where VS = (V~+V~+~)/2 (4)
It"+'
and V~ is the steady state voltage at the beginning of the
8

CA 02298392 2000-02-14
xth cable section
V~+~ is the steady state voltage at the end of the xth
cable section
I~ is the steady state current at the beginning of the
xth cable section
I~+~ is the steady state current at the end of the nth
cable section
The total capacitive charge for the xth cable section is calculated from the
transient and steady state line currents as follows:
QX = ~iX(t~dt-IfXT-,~iX+,~t~+IfX+,T.............(6)
where QX is the electrical charge of the xth cable section
ix (t) is the current into the xth cable section
The capacitance for the xth cable section is calculated from the charge as
follows:
Cx = QxN
9

CA 02298392 2000-02-14
where CX is the capacitance of the xth section in farads
V is the average voltage across the cable section
and U = (V~ - Vn+1 )/2
Equations (6) and (8) can be numerically evaluated by measuring the input and
output voltages and currents as functions of time for each cable section.
The fault resistance and capacitance for every cable section are thereby
determined. In the event of a cable cut only ix (t), vx (t), are reported from
the last
section. In this case all of the steady state current is fault current and all
of the
charge current is into the remaining cable section.
As the capacitance per unit length is assumed to be constant the distance to
the
cut is estimated:
D = L (Cf/CX) (9)
where L is the original section length
Cf is the calculated capacitance after the cut
Cx is the original capacitance of the section before the cut
If the series resistance for a section is known, then referring to Figure 6,
the
series resistance along the cable shield or armour to the location of a
resistive fault
to ground can be estimated from the steady state conditions by:

CA 02298392 2000-02-14
Ran = (VX - VX+~ - IX+~ RI)/(IX-IX+~) (10)
where: R~ is the total series resistance of the cable section (Ran + Rbn)
The distance to the fault from the beginning of the section is calculated by:
D = Dt Ran/Ri (11 )
where: D is the distance to the fault
Dt is the total length of the cable section
In summary, by mathematically analyzing both the transient and steady state
response of a cable section to a step input function, the distance to any
cable open
and the distance to a resistive fault can be determined accurately.
Circuit Description
The basic circuit for the measurement system including a remote sensor is
shown in Figure 7. A computer controlled measurement system 14 is connected to
the metal armor or shield 10 of the cable 16 under test. The armor is
insulated from
ground by an insulating plastic jacket 18. The measurement system has a
digital
voltmeter 20 and ammeter 22 to collect transmitted data from remote sensors 24
(one shown) which are often located at distances of 100 km or more away from
the
start of the cable. The data generated by the remote sensors is sent over the
cable
sheath as a series of current pulses. Encoded in the data are the charge and
voltage readings from each remote location. A power supply 26 in the
measurement
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CA 02298392 2000-02-14
system generates ~48 volts which is applied to the line to power the remote
sensors
and initiate the test sequence. Once the data is collected from all of the
remote
sensors, the computer calculates the line parameters using equations (7), (9),
(10),
and (11 ).
The most convenient method of dividing the cable into measurement sections is
to place remote sensing and measurement devices 24 at the cable splice
locations.
For fibre optic cables the typical cable section lengths range from 5 to 10
km. At the
splice points the optical fibres 28 from one section are joined to the
corresponding
optical fibre in the next cable section. The entire spice arrangement is
enclosed in a
mechanical case which typically has sufficient space to allow placement of a
small
electronic device inside the case.
The circuit in Figure 7 details the measurement system and one remote
measurement device between cable sections 1 and 2. Resistor Rsense is in
series
with the cable armour 10 of the two sections and electrically completes the
circuit
through the splice. RSense is a shunt resistor of a few ohms and converts the
current
passing from one section to the next into a proportional voltage. Two voltage
dividers 30, consisting of resistors R1 and R2, and 32, consisting of
resistors R3 and
R4, are connected to opposite sides of resistor Rse~se. The positive summing
inputs
of operational amplifiers U1 and U2 are connected across RSer,se by the
voltage
dividers 30 and 32 and step down the line voltage and buffer the downstream
analogue to digital (A/D) converters from the line. The output of U1 is
applied
through a voltage divider 34 consisting of resistors R5 and R6 to the positive
input of
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CA 02298392 2000-02-14
an amplifier U3. The output of U2 is applied through a divider 36 consisting
of
resistors R7 and R8 to the negative input of U3. U3 forms a differential
amplifier
which scales the voltage drop across the current sense resistor Rsense. The
output of
U3 is applied to a current measurement input 38 of the AID converter of a
controller
40. The output of U2, which is proportional to the line to ground voltage, is
applied
to a voltage measurement input 42 of the A/D converter of the controller. The
readings collected are processed by the controller and the data is transmitted
over
the cable sheath to the measurement system via current pulse modulation of a
transistor Q1.
The timing diagram in Figure 8 shows the sequence of line voltages to complete
the line charge current and the steady state current and voltage readings. The
line
voltage between tests is -48V and when it switches to +48V all the sensors
power
up. The measurement system begins the test by toggling the line voltage. This
charges and discharges the armour through all the sensors. The sensors
approximately synchronize on the first pulse after the command and begin
logging
readings for several charge and discharge cycles. When the test is over the
sensors
send back the results specified by the power up command. Data is encoded by
the
controller as current pulses which are detected by the measurement system.
To ensure that data sent by the remote sensors is not corrupted, only one
sensor at a time may transmit. This is accomplished by assigning each remote
sensor a unique code that represents a specific time slot during report phase
of a
test. The sensors are numbered so that the farthest from the start of the
cable
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CA 02298392 2000-02-14
reports first. This order increases the speed of detecting opens. The
capacitance of
the open segment can be converted to a distance and reported first.
While one embodiment of the present invention is described in the
foregoing, it is to be understood that other embodiments are possible within
the
scope of the invention, which is to be ascertained solely by the scope of the
appended claims.
14

Representative Drawing