MST Electrical Systems & Troubleshooting 3 — Questions and Answers
Question 1: A vessel's navigation lights fail but the fuse is intact. A technician measures 12V at the fuse but 0V at the light socket. What does this indicate?
- The bulb has an open filament
- There is an open circuit between the fuse and the socket (Correct answer)
- The ground connection is good but the positive lead is shorted
- The battery is discharging rapidly
Correct answer: There is an open circuit between the fuse and the socket
Voltage present at the fuse but absent at the load indicates an open circuit — a broken wire or failed connector — between those two test points.
Question 2: What is the maximum allowable AC voltage drop for shore power circuits on a marine vessel per ABYC standards?
- 3%
- 5% (Correct answer)
- 10%
- 15%
Correct answer: 5%
ABYC E-11 allows a maximum 5% voltage drop for AC circuits on marine vessels, compared to 3% for DC circuits.
Question 3: When a marine technician performs a resistance test on a fuel sender unit, what must be done FIRST?
- Connect the ohmmeter with power applied to the circuit
- Disconnect the sender from the circuit before measuring resistance (Correct answer)
- Measure resistance with the fuel tank completely full
- Apply 12V to the sender before testing
Correct answer: Disconnect the sender from the circuit before measuring resistance
The component must be isolated from the circuit before resistance testing; measuring resistance in a live circuit will give inaccurate readings and may damage the meter.
Question 4: What does the term 'polarization index' refer to when testing marine electrical insulation?
- The ratio of insulation resistance measured at 10 minutes to resistance at 1 minute (Correct answer)
- The maximum voltage rating of the insulation material
- The measure of how well insulation resists UV degradation
- The percentage of current leakage per foot of cable
Correct answer: The ratio of insulation resistance measured at 10 minutes to resistance at 1 minute
The polarization index is the 10-minute to 1-minute insulation resistance ratio; a ratio above 2.0 generally indicates good insulation condition.
Question 5: A shore power inlet on a vessel shows no power at the main panel despite shore power being connected. The technician confirms 120V at the inlet receptacle. What should be checked NEXT?
- The galvanic isolator
- The shore power circuit breaker and reverse polarity indicator (Correct answer)
- The battery charger output voltage
- All DC circuit breakers
Correct answer: The shore power circuit breaker and reverse polarity indicator
The shore power circuit breaker at the main panel is the next logical component to check, and the reverse polarity indicator confirms correct wiring.
Question 6: What is the function of a galvanic isolator installed in a shore power system?
- It prevents AC current from flowing back through the green safety ground wire
- It blocks low-voltage DC galvanic current on the AC safety ground while allowing AC fault current to flow (Correct answer)
- It converts AC shore power to DC for battery charging
- It isolates the vessel's AC system from the DC system
Correct answer: It blocks low-voltage DC galvanic current on the AC safety ground while allowing AC fault current to flow
A galvanic isolator uses diodes to block low-voltage DC galvanic current on the safety ground, preventing underwater corrosion while still allowing AC fault current to pass safely.
Question 7: A marine technician is testing a suspect relay. With the relay coil energized, the normally open (NO) contacts should show:
- Open circuit (OL) on an ohmmeter
- Near zero resistance on an ohmmeter (Correct answer)
- 12 volts when connected in circuit
- Infinite resistance when tested in circuit
Correct answer: Near zero resistance on an ohmmeter
When the relay coil is energized, the normally open contacts close and should show near zero (continuity) resistance.
MST Electrical Systems & Troubleshooting 3
A vessel's navigation lights fail but the fuse is intact.
A technician measures 12V at the fuse but 0V at the light socket.
What does this indicate?