At a recent industry meeting, manufacturers complained that installers were testing fiber optic cable plants incorrectly. Their stories were horrifying quite frankly and surely indicated that these techs were not properly trained. Let’s weave some of their horror stories into this month’s quiz. Correct answers and explanations are in red.
1. To test the loss of an installed fiber optic cable plant, you will need a light source, power meter and _________.
A. An optical time-domain reflectometer (OTDR)
B. One reference test cable with a mating adapter
C. Two reference test cables with two mating adapters
D. A flashlight
Testing the installed cable plant requires a light source, power meter, two reference cables—launch and receive—and two mating adapters to connect to the cable under test.
2. Reference test cables must have _________.
A. The same fiber type as the cable plant being tested
B. Connectors that can be mated to those on the cable plant
C. Connectors in good condition
D. All of the above
Reference test cables must meet all these requirements to correctly test an installed cable plant.
3. The reference cables you need for testing _________.
A. Can be any cable
B. Should be random patch cables used for connecting equipment to the cable plant
C. Should be known good cables regularly tested for low loss
D. Must be special reference-grade test cables purchased from the test equipment manufacturer.
One complaint from the industry group was that techs did not know how to choose or care for reference test cables. Known good cables, defined by being low loss, certainly under 0.5 dB loss, are required to get good, consistent test results.
4. Connectors on reference test cables should be inspected with a microscope and cleaned _________.
A. Regularly, preferably between every measurement
B. When loss results start getting lower
C. Never, as it can ruin a good connector
D. When the ceramic ferrules turn black
Reference test cables should be cleaned between every test to prevent dirt contamination causing high loss. They should also be inspected with a microscope regularly to determine if they have been scratched or scuffed.
5. Reference test cables should be replaced or repolished _________.
A. Every 100 tests
B. When their loss exceeds a reasonable value
C. When the loss exceeds 1 dB
D. Test cables never need testing or replacing
Once test cables show a loss of more than 0.5 dB, they should be replaced, as high loss test connectors will give test results of all connectors of high loss, even if those connectors are very good.
6. You can test fiber optic cables with a source of any wavelength since the loss measured will always be the same.
The attenuation of the optical fiber is very sensitive to the wavelength of the test source, so use a source with the wavelength appropriate to the system.
7. Although you start by calibrating the source and power meter with the reference cables for “0 dB” loss, _________.
A. You need to recheck instrument calibration regularly
B. You must recheck calibration whenever changing reference test cables
C. You should recheck calibration when test results consistently get higher or lower
D. All of the above
Test set calibration problems will cause all loss tests to be invalid. Check the calibration regularly and always when changing reference test cables.
8. OTDR launch-and-receive cables are used to _________.
A. Allow the OTDR to measure the loss of the connectors on each end of the cable plant
B. Let the OTDR see beyond the overload from the initial test pulse
C. Protect the connector on the instrument from wear and dirt
D. All of the above
It’s amazing how many techs don’t know you even need reference test cables for OTDRs! Training on these expensive devices should be mandatory.
9. OTDR testing will generally not give the same test results as a light source and power meter.
OTDR testing uses an indirect method of testing loss while the light source and power meter uses a direct method that simulates the actual use of the link by a transmitter and receiver.
10. If a cable plant tests shows unacceptably high loss, the first thing the tech should do is _________.
A. Inspect and clean all connectors
B. Check the calibration of the test equipment
C. Test the reference cables for connector loss
D. Test again with a high-resolution OTDR
This is sort of a trick question! In fact the four choices listed are the steps you should use in most cases to troubleshoot problems encountered in testing a cable plant.
HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.jimhayes.com.