Every installation requires confirmation that components are installed properly. The installer or contractor wants to ensure the work is done properly to satisfy the customer and to ensure callbacks for repair are not be necessary. Customers generally require test results as well as a final visual inspection as part of the documentation of a proper installation before approving payment.
In my experience, however, there often is confusion about exactly what should be tested and how documentation of test results is to be done on fiber optic projects. These issues should be agreed on during the design phase of the project. Project paperwork should include specifications for testing, references to industry standards and acceptable test results based on a loss budget analysis done during the design phase of the project.
The process of testing any fiber optic cable plant may require testing three times: testing cable on the reel before installation, testing each segment as it is installed and testing complete end-to-end loss. Practical testing usually means testing only a few fibers on each cable reel for continuity before installation to ensure there has been no damage to the cable during shipment. Then each segment is tested as it is terminated by the installers. Finally the entire cable run is plugged together and tested for end-to-end loss for final documentation.
One should require visual inspection of cable reels upon acceptance and, if visible damage is detected, test the cable on the reel for continuity before installing it to ensure no damage was done in shipment from the manufacturer to the job site. Since the cost of installation usually is high—often much higher than the cost of materials—it makes sense to ensure that one does not install bad cable, which would then have to be removed and replaced. It is generally sufficient to test continuity with a fiber tracer or visual fault locator. However, long spools of cable may be tested with an optical time delay reflectometer (OTDR) if damage is suspected and one wants to document the damage or determine if some of the cable needs to be cut off and discarded (or retained to get credit for the damaged materials).
After cable installation and termination, each segment of the cable plant should be tested individually as it is installed to ensure each connector and cable is good. Finally, each end-to-end run (from equipment to equipment connected on the cable plant) should be tested for loss as required by all standards. Remember that each fiber in each cable will need to be tested, so the total number of tests to be performed is calculated from the number of cable segments multiplied by the number of fibers in each cable. This can be time-consuming.
Testing the complete cable plant requires insertion loss testing with a source and power meter or optical loss test set (OLTS) per TIA standard test procedure OFSTP-14 for multimode or OFSTP-7 for single-mode. The test plan should specify the “0 dB” reference method option (one, two or three reference cables), as this will affect the value of the loss. TIA 568 calls for a one-cable reference, but this may not be possible with all combinations of test sets and cable plant connectors. The required test methods need to be agreed on by the contractor and user beforehand.
OTDR testing is not required, nor is OTDR testing alone acceptable for cable plant certification. However, long lengths of outside plant cabling that include splices may be tested with an OTDR to verify splice performance and look for problems caused by stress on the cable during installation. While there are advocates of using OTDRs to test any cable plant installation, including short premises cables, it is not recommended for premises cabling nor is it required by industry standards. The shorter lengths of premises
cabling runs and frequent connections with high reflectance often create confusing OTDR traces that can cause problems for the OTDR autotest function and are sometimes difficult for even experienced OTDR users to interpret properly.
The test plan should be coordinated with the cable plant documentation. The documentation must show what links need testing and what test results are expected based on loss budget calculations. The test plan should also specify how the test data are incorporated into the documentation for acceptance of the installation and for reference in case of future cabling problems that require emergency restoration.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.