4***Fault tracing.(OTDR)

OTDRs are used for fault tracing. Before measurement is begun, it should be checked that the transmitter and receiver have been disconnected from the fiber to be measured. This is necessary to avoid potential damage to the OTDR and to the transmitter or receiver.

 Network maps, splice plans and possibly overview diagrams are required to facilitate fault tracing. Measurements- primarily on fibers that have caused an alarm or fault registration are normally made in two stages: first approximate localization and then more accurate pinpointing of the fault.

 Approximate Localization.

The OTDR should be set up to show the entire cable length on the screen. The pulse duration normally used is 2 or 4 us.

 If the fiber is broken, the curve will show considerable reflection (end pulse) in the installed fiber and if there is greatly increased attenuation at an isolated point in the fiber, this will show as a sharp knee on the curve. Such points should then be expanded to provide better reading resolution. The approximate position of the fault is also determined.

 It should be noted that the fiber break is located in a cable section between two splices, the thixotropic jelly filling in the cable will reduce the end pulse.

Pinpointing the fault.

For the final pinpointing of the fault ©,  as short a pulse as possible should be used and the graph should  be expanded maximally around the area where the fault is suspected. After averaging, the distance to the fault is determined.

To determined the position of the fault more exactly, a known point in the cable such as a certain splice may be chosen as a reference point. The distance to the fault is measured with the OTDR set to reference measurement. In this way, the distance from the known point to the fault will be obtained.

Measurements are mode from both end points to check that the sum of the distances from A to C and from B to C is equal to the length of the fault cable.

 The distance to the known point is determined with the help of network maps and splice plans. The refractive index of the instrument should be adjusted so that the distance shown on the instrument is the same as the actual physical cladding length. The position of the fault is then measured with the adjusted refractive index value.