⚡ Generator & Switchboard

Oral Questions & Practical Answers (Marine Electrical)

Written by Seafarers, for Seafarers ⚓
Focused on real onboard practice, not book definitions.

1. What does the different position of the synchroscope needle mean?

What is the difference between 6 o’clock and 12 o’clock?

• 6 o’clock → Generator is out of synchronisation.
  Voltage, frequency or phase sequence do not match.
  ❌ Paralleling is not allowed.
• 12 o’clock → Perfect synchronisation.
  Voltage, frequency and phase sequence match.
  ✅ Generator can be paralleled.

2. Why do we close the breaker at 11 o’clock and not exactly at 12 o’clock?

Because of mechanical closing delay.

• When the needle approaches 12 o’clock, slip is very small.
• The breaker takes a short time to close.
• If we press close at 11 o’clock, the breaker actually closes near 12 o’clock.

👉 This ensures smooth synchronisation without shock loading.

3. What is slip during synchronisation?

Slip is the

difference in frequency between:

• incoming generator
• bus bar

Slip becomes zero at 12 o’clock position.

4. Why is a Megger used instead of a multimeter for insulation testing?

• Megger uses high DC voltage (usually 500V or 1000V).
• Multimeter uses very low voltage (2–3V DC).

Low voltage cannot detect weak insulation.
Megger gives a real insulation resistance value.

5. What are the main safeties provided on the Main Switchboard (MSB)?

• Circuit breakers
• Fuses
• Overcurrent protection relays
• Reverse power protection
• Earth fault indication

MSB panels are dead-front type, meaning:

• You cannot access live parts during normal operation.

6. What is reverse power flow?

Reverse power occurs when:

• Generator terminal voltage drops below bus voltage
• Generator starts acting like a motor
• Power flows from bus bar to generator

This condition is dangerous for the prime mover, not the generator.

7. What is the harm caused by reverse power?

• Generator absorbs power instead of producing it
• Sudden step increase of load on the system
• Possible frequency drop
• Risk of prime mover damage (especially steam turbines)

8. How is protection against reverse power provided?

By Reverse Power Relay:

• Detects reverse power condition
• Trips the generator breaker
• Cuts fuel supply to the prime mover

👉 Purpose: anti-motoring protection

9. How do you test reverse power trip onboard?

When two generators are running in parallel:

1. Slowly reduce load on one generator using governor
2. When load becomes very small, breaker trips
3. Fuel supply cuts off

Alternatively:

• Test relay using relay test button / simulation mode

10. What is ACB and where is it used?

ACB – Air Circuit Breaker

• Used for high current applications
• Commonly used as generator breaker
• Typically for 400V systems

11. What happens if you press the ACB close button on an idle generator?

Normally nothing happens.

Because:

• Synchronisation conditions are not met
• Under-voltage protection prevents closing

This avoids accidental paralleling.

12. What is under-voltage protection?

It prevents:

• Closing breaker when generator voltage is low
• Damage to equipment
• Accidental closing during parallel operation

Also protects loads during voltage failure.

13. What is preferential trip and why is it provided?

Preferential trip:

• Automatically disconnects non-essential loads
• Keeps essential services running

Examples:

• Trips: galley, air conditioning
• Keeps: steering gear, navigation, pumps

👉 It is a critical safety feature.

14. What is the purpose of earth fault indication on switchboard?

• Detects phase-to-earth fault
• Gives alarm without immediate blackout
• Allows controlled fault finding

15. What should you do if you get an earth fault alarm?

1. Identify high-risk areas (deck lights, pantry, wet spaces)
2. Isolate circuits one by one
3. When alarm clears → faulty circuit identified
4. Repair before re-energising