MST Electrical Systems & Troubleshooting 3

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?

Accepted Answer

There is an open circuit between the fuse and the socket

Answer

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?

Accepted Answer

5%

Answer

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?

Accepted Answer

Disconnect the sender from the circuit before measuring resistance

Answer

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?

Accepted Answer

The ratio of insulation resistance measured at 10 minutes to resistance at 1 minute

Answer

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?

Accepted Answer

The shore power circuit breaker and reverse polarity indicator

Answer

The shore power circuit breaker at the main panel is the next logical component to check, and the reverse polarity indicator confirms correct wiring.

(MST) Marine Service Technician Practice Test

(MST) Marine Service Technician Practice Test

MST Electrical Systems & Troubleshooting 3