MST Electrical Systems & Troubleshooting 2 — Questions and Answers
Question 1: What does a voltage drop test across a circuit component indicate when the reading is higher than specified?
- The component has too little resistance
- Excessive resistance exists in that part of the circuit (Correct answer)
- The battery voltage is too high
- The circuit is operating normally
Correct answer: Excessive resistance exists in that part of the circuit
A high voltage drop across a component indicates excessive resistance, which can be caused by corrosion, loose connections, or a failing component.
Question 2: When testing a marine alternator's output, you should connect the voltmeter's positive lead to the alternator's output terminal and the negative lead to:
- The alternator case ground (Correct answer)
- The battery positive terminal
- The battery negative terminal
- The engine block
Correct answer: The alternator case ground
The negative lead goes to the alternator case ground to measure the actual output voltage at the alternator itself.
Question 3: A marine engine's starter motor cranks slowly. After fully charging the battery, the problem persists. What is the MOST likely cause?
- Faulty ignition switch
- High resistance in the starter circuit (Correct answer)
- Defective fuel pump
- Weak battery cells
Correct answer: High resistance in the starter circuit
High resistance from corroded cables, loose connections, or a failing solenoid causes excessive voltage drop, reducing current flow to the starter.
Question 4: What is the primary purpose of a bilge pump float switch circuit's fuse protection?
- To prevent the bilge pump motor from overloading and overheating
- To protect the boat's wiring from short circuits in the bilge pump circuit (Correct answer)
- To limit the maximum run time of the bilge pump
- To prevent reverse polarity damage to the float switch
Correct answer: To protect the boat's wiring from short circuits in the bilge pump circuit
The fuse protects the vessel's wiring from damage caused by a short circuit in the bilge pump circuit, preventing fire hazards.
Question 5: A marine technician is diagnosing an intermittent electrical fault. Which tool is BEST suited for capturing the fault as it occurs?
- Standard analog multimeter
- Digital multimeter with MIN/MAX recording function (Correct answer)
- Continuity tester
- Hydrometer
Correct answer: Digital multimeter with MIN/MAX recording function
A digital multimeter with MIN/MAX recording captures brief voltage spikes or drops that occur during intermittent faults.
Question 6: What is the correct wire gauge to use when replacing a 20-foot (one-way) 12V DC circuit that must carry 20 amps on a marine vessel?
- 16 AWG
- 14 AWG
- 10 AWG (Correct answer)
- 8 AWG
Correct answer: 10 AWG
ABYC standards require 10 AWG for a 20-amp circuit with a 20-foot run to keep voltage drop within the 3% maximum allowance.
Question 7: On a marine vessel, why are bonding wires connected to through-hulls and metal underwater fittings?
- To provide an additional current return path for DC systems
- To equalize electrical potential and prevent galvanic corrosion (Correct answer)
- To ground the vessel's AC electrical system
- To reduce electromagnetic interference from the engine
Correct answer: To equalize electrical potential and prevent galvanic corrosion
Bonding equalizes the electrical potential between underwater metal fittings, preventing galvanic corrosion caused by dissimilar metals in an electrolyte.
MST Electrical Systems & Troubleshooting 2
What does a voltage drop test across a circuit component indicate when the reading is higher than specified?