Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
1998-06-08
2001-01-30
Metjahic, Safet (Department: 2838)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S528000
Reexamination Certificate
active
06181140
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the location of cable breaks and resistive faults in fiber optic cable.
BACKGROUND
In the past two decades the mass deployment of fiber 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.
Fiber 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.
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 fiber 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 fiber cable break employs an Optical Time Domain Reflectometer (OTDR). In this method short pulses of light are launched into the severed fiber. 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 fiber in every cable selected for monitoring. The main drawback of this method is that OTDR instruments are costly and a fiber 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 fibers. The present invention provides an effective means to detect and locate a break in a fiber optic cable which does not employ OTDR techniques and does not require access to a test optical fiber.
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:
measuring steady state voltage and current at each end of each sect ion o f the shield;
calculating from the measured steady state voltages and currents a calculated fault resistance of each section of the shield;
identifying a faulted section containing a resistive fault from the magnitude of the calculated fault resistance; and
calculating the distance along the broken section to the resistive fault from the measured steady state voltages and currents of the faulted section.
Thus, as an additional benefit, damage to the outer insulating cable jacket may be detected on a section by section basis, providing an indication of damage to the protective outer layers of the cable structure.
According to another aspect of the invention there is provided 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 method 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.
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.
REFERENCES:
patent: 4095174 (1978-06-01), Ishido
patent: 4317151 (1982-02-01), de Mesmacker et al.
patent: 4947469 (1990-08-01), Vokey et al.
patent: 5990689 (1999-11-01), Vokey et al.
patent: 0793112a1 (1997-09-01), None
patent: WO 98/08104 (1998-02-01), None
Loewen Myron
Vokey David E.
Battison Adrian D.
Metjahic Safet
Nguyen Vincent Q.
Norscan Inc.
Thrift Murray E.
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