Last month, I discussed equipping installers with the tools needed for installations. Installers also need test equipment. There are many options in the sophistication and cost of fiber optic test equipment. Proper selection can reduce both equipment costs and testing labor costs. The types and quantities of test equipment required also will vary by job type.
(Ed. Note: Part 5 is here.)
All installation techs should carry a visual tracer or visual fault locator. The tracer is a visible flashlight or LED source used with multimode fiber to check continuity and trace fibers to ensure proper connections. A visual fault locator is a visible laser source with higher power that can be used with either single-mode or multimode fiber for tracing, but also will allow finding some faults like stress or breaks in most simplex or zipcord cables or plain buffered fibers. Either visual tracers or fault locators are inexpensive but invaluable during the installation and troubleshooting process.
Every fiber in a fiber optic cable plant requires loss testing with a light source and power meter, also called an optical loss test set (OLTS). The OLTS will confirm that the fiber was installed and terminated properly, by testing end-to-end loss and comparing it to the loss budget created during the design phase. Big jobs may require more than one set to finish the job in a timely manner.
Loss testers come in several configurations, including a separate light source and power meter usually sold as a test kit; an OLTS, which is a single instrument that includes both the light source and power meter; and modules for converting copper testers to an OLTS. The individual light source and power meter usually are the cheapest solution, especially for small jobs, since the meter and source can be separated to be carried by two techs to each end of the cable being tested. If an OLTS is used, two will be needed to test a cable end-to-end, but it can test two fibers at one time, saving labor costs. The OLTS adapters for copper testers usually are not cheap, but they can take advantage of the sophisticated data handling of the expensive copper tester and can produce complete reports. Contractors often choose these adapters if they have already invested in the copper testers.
Each loss test set needs reference test cables. These are just good 1–2 meter-long fiber optic patchcords that match the fiber size and connectors of the cables being tested. The reference cables do not have to be special cables, just ones that have been tested to have low loss. Bad patchcords will give bad test results, causing good fibers to fail testing. Reference cables need to be tested frequently to ensure they still are in good condition and have low loss. It makes good sense for each test set to have several sets of reference cables as they wear out or get damaged and need replacement.
Long outside plant runs with intermediate splices will require optical time domain reflectometer (OTDR) testing. OTDRs also are good troubleshooting tools for long cable plants, but are generally not designed to be used on short cables such as those common in premises applications. OTDRs are expensive, complicated instruments. Unless you use one often, it’s hard to justify the cost. Users not familiar with the quirks of interpreting OTDR data cause many problems, failing good cables and passing bad ones, often with expensive consequences. OTDRs can be rented, but considering the number of problems we see caused by inexperienced users, subbing OTDR testing to an experienced contractor could also be a wise move.
OTDRs need reference cables, too, especially a long launch cable, sufficiently long enough to allow the OTDR to settle down from the overload caused by the test pulse. A 1-kilometer launch cable is recommended for single-mode fiber. For most multimode OTDRs, 100 meters is adequate. New standards are calling for a cable on the other end of the cable under test to allow testing the connector on the far end, where 100 meters length is usually adequate.
The most important thing to remember about test equipment is to know how to use it and always check it out before taking it to a job. Batteries should be replaced or recharged, reference cables should be tested and, most importantly, the user should spend a few minutes refreshing his or her memory of how the instrument is used. The job site is not the place to find that the equipment is not ready for use.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.
(Ed. Note: Part 7 is here.)