⚡ Marine Electrical

Oral Questions & Practical Answers

Part 5 – Batteries, DC Systems & Measurements

DC systems explained clearly — the part most engineers underestimate.

1. What is Direct Current (DC)?

Direct Current is the unidirectional flow of current.
It flows in one direction only.

2. Where is DC mainly used onboard ships?

DC supply is commonly used for:

• Navigation systems

• Control circuits

• Alarm systems

• Emergency lighting

• Communication equipment

• Battery-backed safety systems

3. Why is DC preferred for control and safety systems?

Because:

• Reliable

• Stable

• Can be battery backed

• Works during blackout

4. What is the most common battery type used onboard?

Lead-acid batteries

Used for:

• Emergency power

• Control systems

• Starting systems

5. What is the voltage of one lead-acid battery cell?

2 volts DC

6. How do we obtain 24V DC battery supply?

By connecting:

• 12 cells in series

Calculation:

12 × 2V = 24V DC

7. What is the function of batteries onboard?

• Provide emergency power

• Supply DC loads

• Ensure continuity of critical systems

8. What routine maintenance is done on batteries?

• Check cell voltage

• Measure specific gravity

• Inspect terminals

• Clean and lubricate terminals

• Perform periodic load tests

9. What safety precautions are taken during battery maintenance?

• Proper ventilation

• Protective gloves and goggles

• No sparks or naked flames

• Avoid short-circuiting terminals

• Disconnect negative terminal first

10. Why is spark-proof lighting used in battery rooms?

Because:

• Explosive gases may be released during charging

• Sparks can cause explosion

11. What is a battery charger?

A battery charger:

• Converts AC to DC

• Maintains battery charge

• Prevents overcharging

12. What is float charging?

Float charging:

• Maintains battery at full charge

• Supplies small current continuously

• Prevents battery discharge

13. What is the difference between AC and DC?

14. What is RMS value?

RMS – Root Mean Square value

It is the effective value of AC voltage or current.

For sine wave:

RMS = Peak value / √2

15. What is the meaning of power factor?

Power factor is:

• Cosine of angle between voltage and current

• Indicates efficiency of power usage

16. What is the usual power factor onboard ships?

Typically:

• 0.8

17. What is the best possible power factor?

• 1.0

• Achieved with purely resistive load

18. What is the benefit of improving power factor?

• Reduced current

• Lower I²R losses

• Improved efficiency

• Better voltage regulation

19. How is power factor improved onboard?

• Capacitor banks

• Synchronous condensers

• VFD usage

20. What is an electrical instrument?

Electrical instruments measure:

• Voltage

• Current

• Resistance

• Frequency

• Power

21. What is a multimeter?

A multimeter measures:

• Voltage (V)

• Current (A)

• Resistance (Ω)

Also known as:

• AVO meter

22. How do you check continuity using a multimeter?

• Set multimeter to resistance mode

• Connect probes across circuit

• Low or zero resistance = continuity

• Infinite resistance = open circuit

23. What is a Megger?

A Megger is used to:

• Test insulation resistance

• Apply high DC voltage

• Detect weak insulation

24. What voltage is used in a Megger?

Typically:

• 500V DC

• Higher voltages for larger equipment

25. Why is Megger preferred over multimeter for insulation testing?

Because:

• Megger applies high voltage

• Reveals insulation weakness

• Multimeter voltage is too low

26. What happens to insulation resistance when temperature increases?

• Insulation resistance decreases

• High temperature accelerates insulation failure

27. What happens to conductor resistance when temperature increases?

• Resistance increases

• Causes higher losses

28. What is earthing onboard ships?

Earthing:

• Provides safety path

• Protects personnel

• Prevents dangerous voltage rise

29. What is the difference between earth and neutral?

30. What is safe voltage to prevent electric shock?

Generally:

• Below 50V

Depends on:

• Body resistance

• Environment

• Insulation