CEP Troubleshooting & System Testing 2 — Questions and Answers
Question 1: A clamp-on ammeter reads zero current on a single-phase 120V circuit that should be energized. What is the most likely cause?
- The load is operating normally at no load
- An open neutral conductor
- The clamp is placed around both conductors simultaneously (Correct answer)
- The circuit breaker is undersized
Correct answer: The clamp is placed around both conductors simultaneously
Clamping around both the hot and neutral conductors cancels the magnetic fields, resulting in a net zero reading even when current is flowing.
Question 2: When performing a megohmmeter test on motor windings, a reading of 1 MΩ at 1000V DC after one minute is generally considered:
- Excellent insulation — no action needed
- Acceptable for most motors above 1 kV
- Marginal — further analysis or drying may be required (Correct answer)
- A failed winding requiring immediate replacement
Correct answer: Marginal — further analysis or drying may be required
The general rule of thumb is 1 MΩ per kilovolt of rated voltage plus 1 MΩ; a 1 MΩ reading is at the minimum acceptable threshold and warrants further evaluation.
Question 3: A three-phase induction motor draws excessive current on all three phases but runs slower than rated speed under full load. The most probable cause is:
- Single-phasing condition
- Low supply voltage on all phases
- Open rotor bar in a squirrel-cage rotor (Correct answer)
- Faulty overload relay contacts
Correct answer: Open rotor bar in a squirrel-cage rotor
Broken rotor bars cause high slip, reduced speed, and increased stator current draw as the motor attempts to compensate for reduced torque.
Question 4: During a ground-fault circuit interrupter (GFCI) test, pressing the TEST button does not trip the device. The FIRST step a technician should take is:
- Replace the GFCI immediately
- Check for a reversed line and load wiring connection (Correct answer)
- Increase the supply voltage to the GFCI
- Test the downstream receptacles first
Correct answer: Check for a reversed line and load wiring connection
A GFCI wired with line and load terminals reversed will power the outlet but the self-test circuit will not function, preventing the TEST button from tripping the device.
Question 5: A power quality analyzer shows significant 5th and 7th harmonic distortion on a commercial distribution panel. The most likely source is:
- Resistive heating elements
- Single-phase fluorescent lighting ballasts
- Variable frequency drives (VFDs) or switch-mode power supplies (Correct answer)
- Synchronous motors at full load
Correct answer: Variable frequency drives (VFDs) or switch-mode power supplies
Six-pulse VFDs and switch-mode power supplies are classic generators of 5th and 7th order harmonics due to their rectifier front-end topology.
Question 6: An arc flash hazard analysis indicates an incident energy level of 42 cal/cm² at the panelboard. A technician must work on energized bus bars. What PPE category is required?
- PPE Category 1 (4 cal/cm² arc rating)
- PPE Category 2 (8 cal/cm² arc rating)
- PPE Category 4 (40 cal/cm² arc rating minimum)
- No PPE is required above 40 cal/cm² — energized work is prohibited by NFPA 70E (Correct answer)
Correct answer: No PPE is required above 40 cal/cm² — energized work is prohibited by NFPA 70E
NFPA 70E 130.7 states that when incident energy exceeds 40 cal/cm², additional PPE requirements apply and energized work may be restricted unless engineered controls reduce the hazard.
Question 7: A technician uses a digital multimeter to measure continuity on a de-energized 480V three-phase motor feeder. Before testing, the correct safety step is:
- Verify voltage is absent using a two-probe test with an appropriate CAT III/IV meter (Correct answer)
- Discharge capacitors in the panel using a screwdriver
- Set the meter to AC voltage and touch both probes to ground
- Rely on the open circuit breaker as sole proof of de-energization
Correct answer: Verify voltage is absent using a two-probe test with an appropriate CAT III/IV meter
NFPA 70E requires verifying the absence of voltage with an appropriately rated meter before declaring a circuit safe to work on; an open breaker alone is insufficient proof.
CEP Troubleshooting & System Testing 2
A clamp-on ammeter reads zero current on a single-phase 120V circuit that should be energized.
What is the most likely cause?