·Table of Contents
·Methods and Instrumentation
The Design Scheme and Realization Method of Telecommunication Cable Fault Observation SystemKe Wang Yukuan Ma
Jilin University of Technology, Changchun, Jilin, 130025, China. Email : email@example.com
Changchun Institute of posts and Telecommunications ,Changchun, Jilin, 130022, China
The fault of telecommunication cable mainly includes the short circuit, broken and mix line as well as insulation bad of cable etc. There are many method of testing cable fault being used in now. For example, distribution capacity detection method, pulse reflection detection method, the bridge detection method of direct current, the induction current detection method and detection method of atmospheric pressure etc. In which, the detection method of atmospheric pressure is used in internal gas pressure cable mainly; Induction current detection method applies to the armored cable in tunnel that lies; Line distribution capacity detection method only can be used in the broken state of cable. Because of the much difference of detection method, the detection equipment of cables fault are varied. According to its function, the detection equipment can be divide into mainly some kinds as following:
2.1 The basic principle of the pulse reflection method
When the cable work as normally, terminal load impedance Zl and line property impedance Zc match, that is Zl = Zc, shown as Fig.1. The electromagnetism wave online will reach load all right without reflection. As the line fault occur on certain point, the input impedance becomes the terminal load before fault point as Fig.2 shows. That is Zx ¹ ZC and Zx ¹ Zl. The reflection coefficient r is shown as follow:
|Fig 1:||Fig 2:|
When electromagnetism wave transmission goes to fault point, part or all electromagnetism waves will return to excite terminal. According to the different reflection of fault nature, the size of coefficient is also different.
As the broken fault with single line or two- line occur, impedance Zx = ¥.
|Fig 4:||Fig 5:|
The reflection coefficient
From this we know, the phase of the reflection pulse of starting point and the phase of excite pulse are opposite.
The impedance in founder terminal point from increase to, reflection coefficient is
So, the phase of the reflection pulse of founder terminal point and the phase of excite pulse are identical.
From above-mentioned we can know, we can judge the type of fault according to the phase of reflection pulse.
2.2 Measurement distant and distinguishing ability.
Because of the spread speed of electromagnetism wave in certain kind cable, if spread time and the speed of reflection pulse have been known, the fault point can be found.
|Where Lx------ Measurement distant|
|V ------ Speed of reflection pulse|
|Dt------ March time|
|Where S------ Distinguishing ability|
|t------ Pulse width|
From above we can know, if reduce pulse width, the distinguishing ability will increase. However, the spread speed of pulse will, transmit loss also will increase.
|Fig 6: Frame of the system|
3.2 Design of the system software
The system software includes program of Testing Font Terminal and program of Management Terminal. In the program of Testing Font Terminal, there are Measurement Subprogram and Frame of Communication Subprogram, as Fig.7 and Fig.8 as well as Fig.9 shown.
|Fig 7: Frame of main program||Fig 8: Frame of Measurement Subprogram||Fig 9: Frame of Communication Subprogram|
The design of management terminal software adopts the window scheme with Visual Basic language. Which is consisting with program of database and program of series communication as well as cable distributes picture etc.
4.1 The basic function of system
a. Automatic fault warning.
Detection system can keep on the online working state of detection telecommunication cable. When telecommunication cable occurs fault, the system can quickly and accurately make sure the location of fault, fault category, concurrent make the fault warning system.
b. Data is defended.
Cable fault Detection system possesses 4 kinds of databases including basic equipment installation database, data installation database, fault point warning database, fault historical database.
c. Cable distribute picture is glanced over and can be printed.
The system can give the cable distribute picture of normal state and the picture of contain fault points and print them.
d. Data is transmitted.
The fault information goes to the management terminal of backstage supporter automatically through telephone net transmission and be processed by center computer.
4.2 Criteria of the system
The biggest count of test front high-speed count circuit is FFFF.
The counting frequency with high speed circuit is 100 MHz.
The minimum test scope is 37Km.
The minimum of measure blind area is about 1.98 m.
The measure capacity is 128 couple wires.
The maximum distinguish capacity is to be 0.989 m.
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