CME Troubleshooting & Maintenance Techniques 2 — Questions and Answers
Question 1: A motor repeatedly trips its overload relay shortly after reset. The motor is running at rated load. What is the MOST likely cause?
- Oversized overload relay heater elements
- High ambient temperature around the motor
- Undersized supply conductors causing voltage drop (Correct answer)
- Defective overload relay contact block
Correct answer: Undersized supply conductors causing voltage drop
Undersized supply conductors cause voltage drop, which increases motor current draw and causes repeated overload trips under rated load conditions.
Question 2: When using a clamp-on ammeter to troubleshoot a single-phase 240V circuit, you clamp around both conductors together and read zero amps. What does this indicate?
- An open circuit exists in the load
- The circuit is operating normally with balanced current (Correct answer)
- A ground fault is present in the circuit
- The meter is defective and needs calibration
Correct answer: The circuit is operating normally with balanced current
Clamping around both conductors of a single-phase circuit cancels the magnetic fields, producing a zero reading when currents are equal and opposite — indicating normal balanced operation.
Question 3: A fluorescent lighting circuit has one lamp flickering while adjacent lamps on the same circuit are steady. What should be checked FIRST?
- The circuit breaker ampere rating
- The lamp, ballast, and lamp holders on the flickering fixture (Correct answer)
- The neutral conductor at the panel
- The voltage at the branch circuit breaker
Correct answer: The lamp, ballast, and lamp holders on the flickering fixture
A single flickering lamp while others are steady points to a localized fault — the lamp itself, its ballast, or the lamp holders — rather than a circuit-wide issue.
Question 4: During megohmmeter testing of a motor winding, what minimum insulation resistance reading (in megohms) is generally considered acceptable using the 1-minute test method per IEEE 43?
- 0.1 MΩ per kV of rated voltage plus 1 MΩ (Correct answer)
- 1 MΩ per kV of rated voltage
- 5 MΩ regardless of voltage rating
- 10 MΩ per kV of rated voltage
Correct answer: 0.1 MΩ per kV of rated voltage plus 1 MΩ
IEEE 43 recommends a minimum insulation resistance of 1 MΩ per kV of rated voltage plus 1 MΩ as an acceptable threshold for rotating equipment.
Question 5: A three-phase 480V motor is humming loudly and not starting. A voltmeter check shows 480V across two phases but zero volts on the third. What is the likely problem?
- The motor windings are shorted
- One phase of the supply is open (single-phasing) (Correct answer)
- The motor bearings have seized
- The overload relay has tripped
Correct answer: One phase of the supply is open (single-phasing)
Zero volts on one phase while the other two phases read correctly indicates an open phase (single-phasing), which prevents the motor from starting and causes the characteristic loud hum.
Question 6: What does a high resistance reading across a closed switch or contactor contact indicate?
- The switch rating exceeds circuit requirements
- The contacts are burned, pitted, or contaminated (Correct answer)
- The switch has adequate interrupting capacity
- The insulation is beginning to break down
Correct answer: The contacts are burned, pitted, or contaminated
High resistance across closed contacts indicates burned, pitted, or contaminated contact surfaces that impede current flow and cause voltage drop and heat.
Question 7: A GFCI receptacle will not reset and the test button does not trip it. What is the MOST likely cause?
- The GFCI is wired with line and load terminals reversed
- The circuit breaker protecting the circuit is undersized
- The GFCI device has failed and must be replaced (Correct answer)
- There is a ground fault downstream on the load side
Correct answer: The GFCI device has failed and must be replaced
A GFCI that will not reset and whose test button produces no response has an internal failure and must be replaced — the device's self-test circuitry is non-functional.
CME Troubleshooting & Maintenance Techniques 2
A motor repeatedly trips its overload relay shortly after reset.
The motor is running at rated load.
What is the MOST likely cause?