Locating breaks in fiber-optic networks

When a problem arises in a fiber-optic network, the source can usually be traced to human intervention. If your network goes down because of a break in a fiber cable or a defect in the thousands of feet of fiber that comprise most campus installations, certain tools are necessary to pinpoint the problem quickly. Without this equipment, identifying a break is virtually impossible.

Jan 1st, 1995

Norman Elsasser, 3M Telecom Systems Division

Problem

When a problem arises in a fiber-optic network, the source can usually be traced to human intervention. If your network goes down because of a break in a fiber cable or a defect in the thousands of feet of fiber that comprise most campus installations, certain tools are necessary to pinpoint the problem quickly. Without this equipment, identifying a break is virtually impossible.

Solution

With the proper equipment, however, you can locate faults in your fiber system quickly and effectively, minimizing downtime and inconvenience to LAN users. A basic set of test equipment includes a power meter, an optical time-domain reflectometer and a visual fault locator--available for less than $10,000.

Procedure

1) Go to a transaction point or the wiring closet and use an optical power meter to verify the presence of outside optical power. A transaction point is a place where cables are joined by connectors--for example, a wallplate or the end of an equipment cord. In horizontal cabling systems, breaks often occur at these connection points. The wiring closet is also a likely place to look for trouble because cables are often incorrectly jumpered or may be pinched during regular maintenance checks.

2) Use a logical approach to locate the trouble. If a whole floor is out, for example, the problem is probably in the wiring closet or in the riser cable leading to it. If one terminal is down, the trouble probably lies in the horizontal link between closet and terminal. Test outward from the closet to the workstation, or backward from the workstation to the closet. If you find that optical power is missing at some point in the horizontal link, you have localized the problem. If optical power is present throughout the link, proceed to Step 3.

3) To localize the break, attach an OTDR to the horizontal and riser links. An OTDR will supply a graphical trace of where the break occurs and detect high-loss splice points as far as 20 to 30 miles away. After the break is identified on the trace, you can pinpoint the physical location by cross-referencing wiring charts used during installation testing. If you cannot identify a break on the trace, the problem may be located in the dead zone of the OTDR; in this case, proceed to Step 4.

4) Having tested the incoming riser link and the outgoing horizontal link, you have now pinpointed the break within the wiring closet. Connect a visual fault locator to the appropriate cables and look for deformities such as cracks or breaks. An infrared beam going through the fiber will glow bright red at the point of the defect or break. Because an OTDR indicates the fiber distance to a problem and not its actual physical location, a visual fault locator may also be useful in zeroing in on a fault localized by the OTDR.

5) Once the fault is located, repair the cable break with a mechanical or fusion splice or, if the section of fiber is relatively short, you may simply want to pull a replacement section. In most premises troubleshooting, however, you will find that fiber problems are located at connectors. In these cases, reterminating the fiber usually solves the problem.

Norman Elsasser is marketing development manager for Photodyne products at the 3M Telecom Systems Division in Austin, TX.

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