Fiber Optic Cable Fault Point Test Formula

The fault point in a fiber optic cable is determined using the OTDR-measured distance adjusted for excess fiber length and sheath distance.

Fault Location Concept

To locate a fault in an optical fiber cable, the OTDR measures the time it takes for a reflected light pulse to return from the fault. This time is converted into a distance using the speed of light in the fiber, which depends on the fiber's index of refraction (IOR). However, due to excess fiber length in buffer tubes and variations in cable design, the raw OTDR distance must be corrected to obtain the actual sheath distance where the fault occurs .

Basic Fault Point Formula

The general formula to calculate the fault location is: Fault Distance (sheath) = OTDR Measured Distance − Excess Fiber Length + Sheath Offset Where:

• OTDR Measured Distance is the distance read from the OTDR trace to the fault reflection.
• Excess Fiber Length accounts for the extra fiber in buffer tubes or cable design, which can vary by manufacturer and cable type.
• Sheath Offset is the distance from the fiber to the cable sheath marking at a reference point, often measured at a system feature like a splice or connector .

Practical Steps

1. Set up the OTDR with a dummy fiber spool to avoid dead zone errors.
2. Select appropriate pulse width: shorter pulses for high resolution, longer pulses for long-distance cables.
3. Place cursors on the OTDR trace:
   • Cursor A at the linear portion of the trace near the start.
   • Cursor B at the linear portion near the fault reflection.
4. Measure the OTDR distance between cursors.
5. Adjust for excess fiber length by averaging fiber lengths from multiple buffer tubes if necessary.
6. Apply sheath offset to convert the fiber distance to the actual cable sheath distance .

Notes

• The method is independent of the OTDR index of refraction, as long as it remains consistent during measurement.
• For high-density or stranded cables, use fibers from different buffer tubes to obtain an average.
• This procedure ensures accurate fault location regardless of cable manufacturer or design variations . By following this formula and procedure, technicians can precisely locate faults in fiber optic cables for maintenance or troubleshooting.