How to improve the accuracy of optical cable fault location


When the optical cable line is blocked due to obvious external forces such as natural disasters or external construction, the inspection and repair personnel will carefully inspect the optical cable route according to the fault phenomenon and approximate fault location provided by the tester, and it is generally easier to find the fault location. Otherwise, it is not easy for the inspectors to find the fault location from the abnormal phenomenon on the route. So, how to improve the accuracy of optical cable fault location?


Optical cable line failure

(1) Correct and proficient in the use of the instrument

Accurately set the parameters of the OTDR, select the appropriate test range file, apply the amplification function of the instrument, and accurately place the cursor on the corresponding inflection point, such as the inflection point of the fault point, the start point of the fiber, and the inflection point of the fiber end. Test Results.


(2) Establish accurate and complete original data

Accurate and complete optical cable line data is the basic basis for obstacle measurement and judgment. Therefore, attention must be paid to the collection, arrangement and verification of line data, and a true, credible and complete line data must be established.

GYTC8S cable

(3) Establish accurate line routing information

Including the benchmark stone (pole number) - slender length (cable length) comparison table (refer to the appendix), "optical fiber length accumulation" and "optical fiber attenuation" records, when establishing the "optical fiber length accumulation" data, it should be measured from both ends respectively. The distance from the station to each connector, in order to test the results accurately, the transition fiber can be used according to the situation. On-the-job acceptance personnel collect and record various reserved lengths. The more carefully registered, the smaller the error in obstacle determination.


(4) Establish complete and accurate line information

The building line information not only includes many data in the line construction, as-built technical documents, drawings, test records and pictures of the backscattered signal curve of the optical fiber in the relay section, etc., but also retains some original data of the optical cable and optical fiber provided by the manufacturer when the optical cable leaves the factory. (such as the shrinkage rate of the optical cable, the refractive index of the optical fiber, etc.), these data are the basis and comparison basis for future obstacle testing.


(5) Make the correct conversion

To accurately determine the location of the fault point, it is also necessary to convert the length of the fiber tested to the ground length from the test end (or a joint point) to the fault point.


The ground length from the test end to the fault point can be calculated by the following formula (the unit of length is m):

L = [(L1-L2)/(1+P)-L3]/( 1+a )

In the formula, L is the ground length from the test end to the fault point (in meters), L1 is the optical fiber length from the test end to the fault point measured by the OTDR (in meters), and L2 is the length of the optical fiber reserved in each splice box (unit is meter), L3 is the length (unit is meter) of the optical cable and all coils at each joint, P is the shrinkage rate (ie twist coefficient) of the optical fiber in the optical cable, it is best to use the value provided by the manufacturer, generally is 7‰, and a is the natural bending rate of the optical cable (0.5% for pipeline laying or overhead laying, and 0.7%-1% for direct burial laying). With accurate and complete original data, the faulty fiber length measured by the OTDR can be compared with the original data, and the location of the fault point can be accurately identified.


(6) Keep the obstacle test consistent with the test conditions on the data

During the fault test, try to maintain the consistency of the test instrument's signal, operation method and instrument parameter setting. Because the optical instrument is very precise, if there is a difference, it will directly affect the accuracy of the test, resulting in the difference between the two tests themselves, making the test results incomparable.


(7) Flexible testing, comprehensive analysis

Under normal circumstances, two-way fault testing can be performed at both ends of the optical cable line, and the location of the fault point can be calculated based on the original data. Then comprehensively analyze and compare the test and calculation results in the two directions to make the judgment of the specific location of the fault point more accurate. When there are no obvious features on the route near the obstacle point, and the specific obstacle point cannot be determined on site, methods such as measuring at the nearest joint can also be used, or excavation at the obstacle point of the preliminary test, and the test instrument of the terminal station is in a real-time measurement state. Find the change of the curve at any time, so as to find the exact fiber fault point.


The above is the whole content of how to improve the fault location accuracy of the optical cable line. Of course, we can check the distance from the fault point measured by the OTDR to the test end with the original test data to find out which two markers are the fault point ( or which two joints), after necessary conversion, find the specific location of the fault point. If conditions permit, two-way testing can be performed, which is more conducive to accurately judging the specific location of the fault point.

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