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Maintenance & Testing – Fault Finding Mission

When the unexpected happens, and cables are cut or damaged, it is crucial to have sound procedures in place to efficiently locate optical fiber faults and direct buried optical fiber cables.


September 1, 2001  


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Regardless of how well an outside plant optical fiber cable is installed, it is never completely safe. Accidents happen — buried cables can be cut by earth-moving equipment, while aerial cables might have trees fall upon them.

Once a cable is damaged, there are two major problems to deal with: restoring service to the cable and repairing the cable quickly enough to minimize impact on customers. It is critical to have solid procedures in place to locate optical fiber faults and direct buried optical fiber cables, and to perform restorations as soon as possible so that service can get back on-line in a timely manner.

Most types of loose tube optical fiber cables are manufactured with controlled overlength to assure that external forces applied to the cable do not directly transfer to the optical fibers. Different cable designs and constructions require varying amounts of excess fiber length — which means a precise value for this excess fiber length is not obtainable. However, a preferred method for determining very accurate fiber/sheath distance to a fault is available.

The method described below is an accurate method that accounts for variations in excess fiber length due to such factors as differences in cable design and manufacturing tolerances. This procedure does not require that a troubleshooter know cable parameters, such as fiber count and number of buffer tubes. In addition, the procedure’s measurement is specific to the cable being measured, regardless of the manufacturer. Finally, this method is insensitive to the index of refraction used by the optical time domain reflectometer (OTDR), as long as the same index of refraction is used when determining the fiber overlength correction factor and locating the fault or event.

DETERMINING SHEATH DISTANCE TO A FIBER FAULT

The equipment needed to determine the distance to a fiber fault includes a high resolution OTDR and an access jumper. Follow this procedure to determine the fault location:

STEP 1 – Calculate the fiber overlength correction factor.

a) System records should provide the optical fiber length to a system feature or event (splice point, demarcation point, etc.). If these records are not available, locate a splice point with the OTDR and record the fiber distance.

b) Measure the optical length to the splice point of one fiber in each of three different buffer tubes to obtain an average fiber length. If this cable is a high-density dual-layer buffer tube design (for example, 216-fiber cable), use fibers in the same buffer tube layer. If the cable design incorporates fibers segregated by threads in a single tube, measure three fibers from the same thread grouping. If the cable design incorporates ribbon fibers in the same tube, measure three fibers from the same ribbon.

c) System records or route diagrams should provide the cable length mark at each splice point. Knowing this length mark will allow the cable sheath distance to be determined.

d) Calculate the fiber overlength correction factor.

STEP 2 – Locate the fiber fault with the OTDR.

CorrectionFactor =Cable Sheath Length / OTDR Measured Fiber Length

a) It is essential to establish landmarks when measuring fiber distance to a fault. Measure the fiber distance to a fault from the nearest feature on the OTDR (splice point, demarcation point, connector peak or other fiber anomaly). This allows for a more accurate distance measurement than measuring the fiber distance from the beginning of the OTDR trace.

STEP 3 – Calculate the cable sheath distance to the fiber fault.

Cable Sheath Length = Fiber Length x Correction Factor

STEP 4 – Determine the cable sheath marking at the fault.

a) Calculate the cable sheath marking at the fault location by adding or subtracting (as appropriate) the distance to the fault by the sheath mark at the chosen landmark location. Locate this cable sheath mark in the field and inspect the cable carefully in this vicinity for any signs of damage.

OTDR OPERATION GUIDELINES

The following are recommendations for OTDR operation. (Note: these are not ranked in order of importance or in sequential step order).

1. Vertical Scale – When adjusting the vertical scale, use the minimum dB/div, which maintains the desired trace on the OTDR display. Typically, vertical scale increments of one dB/div or less are used for acceptance testing.

2. Horizontal Scale – When adjusting the horizontal scale (distance), use the lowest meter/div or feet/div that maintains the desired trace on the OTDR display. In other words, you want to expand your trace to maximize the use of the OTDR display grid. (Note: your entire span does not need to fill the grid; the span up to the fault does.)

3. Index of Refraction – Input the optical fiber group indices of refraction into your OTDR per the cable manufacturer’s recommendation for accurate fiber length measurements.

4. Pulse Width – Choose the shortest possible pulse width that allows for the smoothest OTDR trace.

5. Scan Mode or Averaging Time – Use a sufficient averaging time to achieve a smooth trace. (Typically, less than one minute averaging time is sufficient).

6. Access Pigtail – Use an access pigtail that is long enough to encompass the OTDR initial reflection (“dead zone”). Typical access pigtail lengths are shown below:

Multimode Fiber -* 25 meters

Single-mode Fiber – * 75 meters

7. Cursor – Place cursor “A” near the beginning of the trace to be measured or near a fiber landmark, if available. Ensure cursor “A” is on a linear portion of the trace and not within the connector or splice reflection. Place cursor “B” as closely as possible to the fiber fault reflection. Ensure cursor “B” is on a linear portion of the trace and not within the fault reflection.

LOCATING DIRECT BURIED FIBER CABLES

Several variables can affect the procedure of locating a buried optical fiber cable, including cable depth, type and quality of the cable locator used, the presence of other buried objects near the cable, and operator proficiency.

Most modern cable locators are simply transmitters and receivers. The transmitter applies an AC radio wave to the cable. The metallic armour of the cable is generally used for this purpose, although other metallic components within the cable may also be employed. The AC radio wave will produce a time-varying magnetic field around the cable. The receiver, a copper wire antenna, is used to locate the cable by measuring the induced voltage as it cuts through the magnetic field of the cable.

The strength of the magnetic field is reduced as the antenna is moved further away from the cable. As a result, cables become harder to locate the more deeply they are buried. The cable locator also looks for peaks and nulls in the received signal. A peak or null will be generated, depending on the orientation of the antenna relative to the cable, when the antenna cuts through the most magnetic lines of flux. If the magnetic field around the conductor is round, this point occurs when the antenna is directly over the cable and the cable can be accurately located. If the magnetic field is not perfectly round, the peak or null will not occur directly over the cable and the cable’s position will be improperly marked to one side.

A number of factors can distort the magnetic fields around cables. For example, if the cable is buried near a metal water pipe, the pipe will distort the magnetic field. This occurs because the metal in the pipe has a higher magnetic permeability than the surrounding soil. The distorted magnetic field will cause the locator to incorrectly identify both cable location and depth.

ADDITION LOCATION METHODS

Other means of locating buried cables exist. One method is to simply use a receiver to look for the magnetic field around a cable. However, this method requires the cable to be carrying AC current and is therefore not applicable to fiber optic cables.

Another method utilizes an audio tone instead of a radio frequency tone. The basic detection theory is
the same, and this method is susceptible to the same error and interference problems as the radio frequency detection method. Additionally, the audio frequency will propagate a finite distance in the air. If the receiver is too close to the transmitter, audio tones received through the air may cause the operator to inaccurately locate the cable.

Because of the large number of variables involved, there is no way to guarantee the ease and accuracy with which fiber optic cable may be located. Operator proficiency is perhaps the single most important variable, and is the factor over which the contractor has the most control. But having sound procedures in place to efficiently locate optical fiber faults and direct buried optical fiber cables will ensure that service does get back on-line in a timely manner and customers will not be adversely affected.CS

Doug Coleman is Manager of Technology and Standards, Private Networks, at Corning Cable Systems in Hickory, NC. Mr. Coleman is active in the development of optical specifications at the Insulated Cable Engineering Association (ICEA), the Society of Cable Telecommunications Engineers (SCTE) and TIA/EIA, IEC, IEEE and Fiber Channel standards groups. He has been awarded four domestic and twelve international patents, and currently has four patents pending.


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