CPCT Cable Testing & Troubleshooting 3 — Questions and Answers
Question 1: A technician uses a tone generator and probe to trace a cable. In a busy equipment room with many cables, the probe gives a faint signal on the target cable but also picks up tone on adjacent cables. What is the most likely cause?
- The tone generator battery is low
- Capacitive coupling between tightly bundled cables (Correct answer)
- The probe frequency is set too high
- The target cable has a short circuit
Correct answer: Capacitive coupling between tightly bundled cables
Capacitive coupling between adjacent cables in a bundle allows the tone signal to bleed onto neighboring conductors, producing false signals on untargeted cables.
Question 2: When certifying a fiber optic link, an OTDR trace shows a 'gainer' — a point where the backscatter level appears to increase. What does this indicate?
- An amplifier in the link
- A fiber section with lower backscatter coefficient preceding a higher-backscatter section (Correct answer)
- A broken fiber repaired with a mechanical splice
- Excessive bend radius at that point
Correct answer: A fiber section with lower backscatter coefficient preceding a higher-backscatter section
A gainer occurs when transitioning from a fiber with lower backscatter to one with higher backscatter, making the splice appear as a gain rather than a loss.
Question 3: Which test parameter specifically measures the signal strength at the far end of a cable pair after traveling the full link length?
- NEXT
- FEXT
- Insertion Loss (Attenuation) (Correct answer)
- Return Loss
Correct answer: Insertion Loss (Attenuation)
Insertion loss (also called attenuation) measures the reduction in signal amplitude from one end of the cable to the other.
Question 4: A newly installed Cat 6 horizontal run fails the ANEXT test. Which factor unique to alien crosstalk testing should the technician investigate first?
- The patch cord category used at each end
- The proximity and bundling of adjacent cables in the pathway (Correct answer)
- The connector type at the consolidation point
- The cable manufacturer's lot number
Correct answer: The proximity and bundling of adjacent cables in the pathway
Alien NEXT (ANEXT) is caused by crosstalk from cables outside the link under test; tightly bundled or parallel cables in the same conduit are the primary source.
Question 5: When performing a two-fiber OTDR test on a duplex multimode link, the technician notices that one fiber shows much higher loss than the other. The cables were installed in the same conduit. What should be checked first?
- Verify the OTDR wavelength matches the fiber specification
- Inspect and clean the connector on the high-loss fiber (Correct answer)
- Replace the entire fiber run
- Re-launch the OTDR from the opposite end
Correct answer: Inspect and clean the connector on the high-loss fiber
A dirty or damaged connector face is the most common cause of excessive loss on a single fiber when adjacent fibers in the same installation are fine.
Question 6: A link passes the insertion loss test but fails ELFEXT. What does this failure indicate?
- The cable is too long for the application
- Crosstalk energy is coupling from one pair to another and exiting at the far end (Correct answer)
- The near-end terminations are improperly done
- The cable's DC resistance is out of specification
Correct answer: Crosstalk energy is coupling from one pair to another and exiting at the far end
ELFEXT (Equal Level Far-End Crosstalk) measures the far-end crosstalk normalized to the insertion loss, indicating crosstalk interference arriving at the far end of the disturbed pair.
Question 7: A technician certifies a Cat 6 channel and notices the wiremap shows pairs 1-2 and 3-6 are correct, but pairs 4-5 and 7-8 are swapped end-to-end. What type of fault is this?
- Split pair
- Reversed pair
- Transposed pairs (Correct answer)
- Short circuit between pairs
Correct answer: Transposed pairs
A transposed pair fault occurs when entire pairs are swapped between their correct positions at both ends, distinguished from a split pair which intermixes conductors from different pairs.
CPCT Cable Testing & Troubleshooting 3
A technician uses a tone generator and probe to trace a cable.
In a busy equipment room with many cables, the probe gives a faint signal on the target cable but also picks up tone on adjacent cables.
What is the most likely cause?