LLC Troubleshooting & Diagnostics 3 — Questions and Answers
Question 1: A technician suspects a wire nick caused by a lawn edger. Which tool is most appropriate for locating the exact fault point on a buried cable?
- Multimeter set to AC volts
- Clamp meter
- Time-domain reflectometer (TDR) or cable fault locator (Correct answer)
- Non-contact voltage tester
Correct answer: Time-domain reflectometer (TDR) or cable fault locator
A TDR sends a signal pulse and measures the time of reflection to pinpoint the exact distance to a fault in a buried cable.
Question 2: A 150-watt capacity transformer is powering fixtures that total 148 watts. Several fixtures are dim. The MOST likely cause is:
- The transformer is overloaded beyond safe capacity
- Voltage drop is occurring because the load is near the transformer's limit (Correct answer)
- The fixtures are incompatible with the transformer brand
- The timer is limiting output wattage
Correct answer: Voltage drop is occurring because the load is near the transformer's limit
Operating near maximum capacity causes increased resistance and heat in the transformer, leading to voltage drop and dim fixtures.
Question 3: After replacing a failed LED MR16 with the same wattage lamp, the new lamp flickers and fails within days. What should be investigated?
- Lamp packaging defect
- Incompatibility between the LED driver and the low-voltage transformer type (magnetic vs. electronic) (Correct answer)
- Excessive cable length on the zone
- Wrong color temperature selection
Correct answer: Incompatibility between the LED driver and the low-voltage transformer type (magnetic vs. electronic)
Some LED MR16 drivers require a minimum load or are incompatible with certain transformer types, causing flicker and premature failure.
Question 4: A pathway lighting zone is completely dark. The transformer's LED indicator shows the zone is active. Using a multimeter, the technician reads 12 VAC at the transformer terminal but 0 VAC mid-run. This indicates:
- All fixtures on the zone have failed simultaneously
- An open (broken) conductor exists between the transformer and the test point (Correct answer)
- The transformer's internal fuse has blown
- Excessive load has caused thermal shutdown
Correct answer: An open (broken) conductor exists between the transformer and the test point
Voltage present at the source but absent downstream confirms an open (break or disconnection) in the cable between those two points.
Question 5: During a service call, a technician finds that a hub-and-spoke connector has corroded wire crimp connections. The BEST long-term repair is to:
- Apply silicone sealant over the existing crimp
- Replace the connector hub with a direct-burial-rated splice with waterproof gel caps (Correct answer)
- Wrap each crimp with electrical tape
- Increase transformer voltage to overcome the resistance
Correct answer: Replace the connector hub with a direct-burial-rated splice with waterproof gel caps
Direct-burial waterproof gel-filled connectors provide a reliable, moisture-resistant connection that resists future corrosion.
Question 6: A homeowner reports that lights in one zone turn on randomly during the day. The timer is set correctly. What is the most likely cause?
- Transformer overvoltage causing phantom switching
- The photocell sensor is triggering from shadows or a dirty sensor lens (Correct answer)
- Wire insulation is breaking down and causing feedback
- Nearby radio frequency interference affecting the timer
Correct answer: The photocell sensor is triggering from shadows or a dirty sensor lens
A dirty or incorrectly positioned photocell can misread ambient light and trigger the zone during daylight hours.
Question 7: Voltage measured at a fixture splice reads 10.2 VAC, but the fixture socket reads only 9.6 VAC. This 0.6V difference most likely results from:
- The fixture lamp drawing excessive current
- Resistance at the connection between the main cable and the fixture's pig-tail lead (Correct answer)
- The fixture housing acting as a capacitor
- Incorrect multimeter probe placement
Correct answer: Resistance at the connection between the main cable and the fixture's pig-tail lead
A voltage drop between the splice and the socket indicates resistance—from corrosion or a loose connection—in the pig-tail lead or connector.
LLC Troubleshooting & Diagnostics 3
A technician suspects a wire nick caused by a lawn edger.
Which tool is most appropriate for locating the exact fault point on a buried cable?