CME Troubleshooting & Maintenance Techniques 3 — Questions and Answers
Question 1: When performing predictive maintenance using infrared thermography on electrical panels, what temperature differential between similar components typically warrants immediate attention?
- Greater than 1°C above ambient
- Greater than 10°C above a similar component under similar load (Correct answer)
- Greater than 25°C above ambient temperature
- Any reading above the equipment's nameplate temperature
Correct answer: Greater than 10°C above a similar component under similar load
A temperature differential greater than 10°C between similar components under similar load conditions is the commonly accepted threshold for immediate corrective action in electrical thermography.
Question 2: A branch circuit shows correct voltage at the panel but low voltage at the last receptacle in a long run. No load is connected. What does this indicate?
- Excessive voltage drop due to conductor resistance under load
- A high-resistance splice or connection in the circuit (Correct answer)
- An undersized circuit breaker limiting current flow
- Normal voltage variation within NEC tolerance
Correct answer: A high-resistance splice or connection in the circuit
Voltage drop with no connected load indicates a high-resistance connection (loose splice, corroded terminal) rather than conductor size, since resistive drop under no-load should be negligible.
Question 3: Which test instrument is MOST appropriate for verifying the operation of an arc fault circuit interrupter (AFCI)?
- A megohmmeter set to 500V DC
- A dedicated AFCI circuit tester with a test button (Correct answer)
- A clamp-on ammeter measuring leakage current
- A phase rotation meter
Correct answer: A dedicated AFCI circuit tester with a test button
A dedicated AFCI tester simulates the specific arc fault waveform signature required to trip an AFCI breaker, which standard outlet testers cannot do.
Question 4: During transformer maintenance, a technician notices the mineral oil is darker than normal and has a burnt odor. What does this MOST likely indicate?
- Normal aging of the transformer insulation system
- Internal arcing or overheating of windings or core (Correct answer)
- The oil needs to be topped off with fresh mineral oil
- Moisture contamination from a failed gasket
Correct answer: Internal arcing or overheating of windings or core
Darkened oil with a burnt odor in a transformer indicates internal arcing or overheating, which produces carbon particles and pyrolysis products that contaminate the oil.
Question 5: A disconnect switch blade shows significant discoloration and pitting at one phase only. After replacing the blade, what additional step is CRITICAL before returning to service?
- Apply anti-oxidant compound to all switch blades
- Identify and correct the root cause of the overheating (Correct answer)
- Megohmmeter test all three phases of the downstream circuit
- Verify the correct fuse size is installed in all three phases
Correct answer: Identify and correct the root cause of the overheating
Replacing only the damaged component without finding the root cause (loose connection, overloaded circuit, poor contact pressure) will result in recurrence of the same damage.
Question 6: What is the purpose of performing a dielectric absorption ratio (DAR) test during insulation resistance testing?
- To measure the exact capacitance of cable insulation
- To identify moisture or contamination in insulation by comparing 30-second and 60-second readings (Correct answer)
- To determine the conductor resistance of the cable under test
- To calibrate the megohmmeter before the main insulation test
Correct answer: To identify moisture or contamination in insulation by comparing 30-second and 60-second readings
The DAR compares the 30-second to 60-second megohmmeter readings; a ratio near 1.0 indicates moisture or contaminated insulation, while clean dry insulation shows significantly higher readings at 60 seconds.
Question 7: A technician measures 120V to ground on both legs of a 240V single-phase panel. When checking a 240V-only circuit (no neutral), one conductor reads 120V to ground and the other reads 240V to ground. What does this indicate?
- The circuit has a properly balanced load
- The neutral conductor of the system is open (Correct answer)
- The two hot conductors are connected to the same leg
- A double-pole breaker with one failed pole
Correct answer: The neutral conductor of the system is open
When system neutral is open, equipment connected between hot legs can create a voltage divider that shifts ground reference, causing one leg to read higher and the other lower than normal to ground.
CME Troubleshooting & Maintenance Techniques 3
When performing predictive maintenance using infrared thermography on electrical panels, what temperature differential between similar components typically warrants immediate attention?