Electrical

Question 1

Megger testing? Minimum values? Where is it listed?

Answer 1

Used to measure the insulation resistance of electrical equipment. It involves applying a high voltage between the conductors and the ground and measuring the resistance to current flow.

Min 1 megga ohm

The Institute of Electrical and Electronics Engineers (IEEE)
IEC (nternational Electrotechnical Commission standards

Question 2

Describe a megger test of your main alternators?

Answer 2

• Disconnect the main alternators
• Connect the terminals, one the the alternator winding, one to the alternator frame/ ground
• Apply the test voltage
• Record insulation resistance reading
• Compare to guidelines

Question 3

What is phasing?

Answer 3

In three-phase power systems, there are three alternating currents (AC) that are 120 degrees out of phase with each other. Each phase is represented by a sinusoidal waveform, and they are labeled as phase A, phase B, and phase C.

Question 4

What classification requirements are there for electrics?

Answer 4

• Compliance with international standards such as IEC (International Electrotechnical Commission) 60092 series, which covers electrical installations in ships
• Adequate insulation and protection against electrical shock hazards
• Protection against overcurrent, short circuits, and overvoltage conditions to prevent equipment damage and ensure system safety
• Requirements for emergency power systems
• Implementation of explosion-proof, flameproof, or intrinsically safe electrical equipment

Question 5

How to megger test a motor?

Answer 5

• Disconnect the motor
• Connect the terminals, one the the motor winding, one to the motor frame/ ground
• Apply the test voltage
• Record insulation resistance reading
• Compare to guidelines

Question 6

what types of earthing systems are fitted onboard a vessel?

Answer 6

• Return Earth
• Grounding to ships hull

Question 7

Earth fault alarms?

Answer 7

Earth fault alarms continuously monitor the electrical circuit for any imbalance in current flow between the phases and the earth. It then alarms once above the limit. Onboard ours are set at 7amps and high high is 10amps.

Question 8

Describe some common electrical faults?

Answer 8

• Short circuit
• Earth fault
• Overload
• Corrosion
• Blown fuses
• Connections vibrating loose
• Water

Question 9

How would you test for common electrical faults them?

Answer 9

• Visual inspections
• Insulation resistance testing with a meggar tester
• Continuity test with multimeter
• Earth fault half test
• Load testing equipment
• Thermal imagining for hot spots

Question 10

How would you know you have an earth fault?

Answer 10

Onboard we have alarms and a reading of how big our earth leak is. Some vessels will have earth fault lights that show there is a fault.

In some cases the breaker may trip, if this repeats its self multiple times, this could indicate an earth fault.

Question 11

Describe your main electrical system?

Answer 11

Onboard we have the MSB and ESB. Our vessels can take upto 318amp, 400v and 50hzs via shore power

Our generators are 250kw, 400v, 50hz, 450amp each, we have 2.

The emergency generator 106 kW

Question 12

What purpose does the neutral leg have in a 4 wire system?

Answer 12

In a 3-phase 4-wire system, which typically consists of three hot wires (phases) and one neutral wire, the neutral helps balance the load between the phases. this providing a factor of safety.

Question 13

How is an appliance earthed with a 3 phase 4 wire system?

Answer 13

In addition to the three phase wires (L1, L2, L3) and the neutral wire (N), there is often a separate earth (ground) wire. This earth wire is connected to the metal parts of the appliance and to the ground connection in the electrical panel or distribution board.

Question 14

Why do we use an isolation transformer?

Answer 14

It will isolate the vessels 400v power supply from the shore powers 400v supply. This removing any risk of galvanic corrosion occurring

Question 15

What is the acceptable limit for ground faults?

Answer 15

Onboard moonsand we have two alarm limits, high is set at 7amps and high high is at 10amps. But we keep a good practice of trying to keep it at zero or as close to it as possible.

Question 16

What is a continuity test?

Answer 16

A continuity test is a basic electrical test used to determine if there is a complete (unbroken) path for current to flow in a circuit or component. It checks whether an electrical circuit can carry current by measuring the resistance between two points.

Question 17

What electrical maintenance do we carry out on board?

Answer 17

• Visual inspections
• Checking connections
• Cleaning cabinet filters
• Checking calibration
• Testing preferential trips, reverse power, over speed, under and over voltage
• Battery tests
• Inspect and cleaning windings
• Lighting inspections
• AVIT inspections
• Alarm monitoring
• Ground faults

Question 18

How would you trace an earth fault?

Answer 18

• Isolate a distribution board and see if it disappears
• bring everything on the board back online one at a time till it returns
• Split that system in half, repeating till the component is found
• Rectify the problem
• Record findings into PMS

Question 19

Why is it important to fix earth faults quickly?

Answer 19

• Risk of electric shock
• Fire risk
• Preventing damage
• Maintaining system reliability and performance

Question 20

How to test reverse power trip and why?

Answer 20

Reverse power trip can be tested by increasing the load on one generator and decreasing on the other until the generator starts to become a motor

Question 21

What is the load of your bowthruster motor, how is it run?

Answer 21

150kw, 400v, 50hz by a slip ring motor/ squirrel cage motor. A Variable frequency drive is installed to allow the motor to gradually increase and decrees

Question 22

How does a frequency drive work? How do you calculate frequency?

Answer 22

An electronic device that controls the speed and torque of an electric motor by varying the frequency and voltage supplied to the motor.

• AC converts to DC power using the rectifier
• Capacitors then smooth out the DC power
• The smoothed DC is then converted back to AC power with a variable frequency and voltage using an inverter
• The control circuit then adjusts the values as needed
• The motor will now spin at a speed until changed

Frequency Counter: Use a frequency counter, an instrument that directly measures the frequency of an electrical signal. This is the most straightforward method and provides an accurate reading.

Question 23

Safety devices fitted to bow thruster / board?

Answer 23

• Circuit breaker
• Thermal overload relays
• Temperature sensors
• Oil level sensors
• Interlocking systems
• Emergency stop
• Ground fault protection

Question 24

Direct online starter / Star Delta start, what are the safeties, how do they work?

Answer 24

DOC
- Circuit breakers
- Thermal overload relays
- Emergency stop
- Under voltage protection

When the start button is pressed, the control circuit energizes the contactor, closing the main contacts and supplying full voltage to the motor. Safety devices like thermal overload relays continuously monitor the current and temperature.

Star-Delta Starter
- Time relay
- Circuit breaker
- Thermal overload Rely
- Interlock system
- Emergency stop
- Under voltage protection

When the start button is pressed, the control circuit energizes the star contactor, connecting the motor windings in star configuration, which reduces the voltage and current. After a preset time, the timer relay switches to delta configuration by de-energizing the star contactor and energizing the delta contactor, applying full voltage to the motor.

Question 25

Drawing of ammeter on one phase, what is it? How does it work?

Answer 25

see one note

Question 26

Want to check Shaft earth system, what would happen if it wasn’t working, potential faults etc?

Answer 26

If it wasn't working there is a possibility that the current being produced by the propeller will result in spark corrosion on the shaft. also know as galvanic corrosion. The faults could be caused by - Brush wear - Poor electrical contact - Corrosion and fouling - Broken earth cable

Question 27

Minimum requirement for Megger test?

Answer 27

1 meggar ohm

Question 28

How to find pump ground fault?

Answer 28

- Lockout system - Inspect for damage and connections - Isolate the motor - Conduct meggar test - Conduct continuity test - Check control circuit - Isolate sections of the circuit until found

Question 29

What causes the ground fault?

Answer 29

- Insulation breakdown - Salt - Vibration

Question 30

Earth fault, how to find?

Answer 30

- Lockout system - Inspect for damage and connections - Isolate the motor - Conduct meggar test - Conduct continuity test - Check control circuit - Isolate sections of the circuit until found

Question 31

Other types of test on an electrical motor?

Answer 31

- Megger test - Dielectric Absorption test - To assess the quality of the motor’s insulation over time by apply a DC voltage for a prolonged period and measure the resistance at different intervals. - High-Potential (Hi-Pot) Test - To check the insulation strength by applying a high voltage by apply a voltage much higher than the motor’s operating voltage for a short duration and check for insulation breakdown. - Winding Resistance Test - To measure the resistance of the motor windings by use a precision ohmmeter to measure the resistance of each phase winding. Compare with manufacturer specifications to detect issues like short circuits or imbalances.

Question 32

What information would you give when requesting shore power?

Answer 32

Voltage, Amps, Herts, Plug connections, Where connections are

Question 33

What if you were 50hz and supplied with 60hz?

Answer 33

- Equipment will run at faster speeds - Have reduced torque - Nosier due to increase speeds - Increased temperatures/ overheating - Possible malfunctions

Question 34

How do you check phase rotation?

Answer 34

By connecting the terminals to a phase rotation tester will be able to confirm if the rotation is correct

Question 35

What is an AVR and how does it work?

Answer 35

Automatic Voltage Regulator - Their purpose is to regulate and stabilize the output voltage of the generator within its limits. This is due to when load is increased, voltage will drop so the AVR will measure this drop and in response increase the excitation.

Question 36

The earth fault would not go after finding?

Answer 36

You have a second earth fault

Question 37

Why does a synchroscope go clockwise?

Answer 37

A visual indication to the operator that the generator's speed is leading the grid frequency ( Going faster)

Question 38

If the online gen is at 1500rpm, would the incoming gen be faster or slower? Why?

Answer 38

It would be slightly faster because when the load is applied it will dip slightly while the governor reacts to increase the speed to 1500

Question 39

A motor won’t start, everything points to the motor, what would you do?

Answer 39

- Visual inspection of the power supply, wiring and motor - Check voltage, continutiy and capacitor is appliacalbe - Rotate shaft manually - No mechanical obstructions - Check starter circuit - Check overloads and trips - Perform meager test

Question 40

How does star delta work and components?

Answer 40

When the start button is pressed, the control circuit energizes the star contactor, connecting the motor windings in star configuration, which reduces the voltage and current. After a preset time, the timer relay switches to delta configuration by de-energizing the star contactor and energizing the delta contactor, applying full voltage to the motor.

Question 41

Tag out procedure for electrical?

Answer 41

Follow our onboard SMS electrical checklist procedure - Inform affected personnel - Stop and secure equipment - Locate and lock power feed - Install lock and tag - Engineer witness lockout - Reverse for un-isolating

Question 42

Switchboard safety devices?

Answer 42

- Circuit breakers - Residual Current Devices (RCD) - Earth fault - Fuses - Surge protection - Thermal overloads - Reverse power - Preferential trips - Over and under voltage - Synchronizes - Emergency stops

Question 43

How do we get 220v for lighting systems from 400v supply?

Answer 43

Step down transformers

Question 44

Set points for reverse power trips?

Answer 44

Around 20% of its full load

Question 45

Motor protection devices?

Answer 45

- Thermal overloads - Circuit breakers - Motor protection relays - Phase failure relays - Earth faults - Surge protection - Unload and overload protection - Temperature sensors

Question 46

Singe phasing and how to fault find?

Answer 46

Single phasing refers to a condition in a three-phase electrical system where one of the phases is lost or disconnected, leaving the motor or equipment operating with only two phases instead of three. This condition can lead to several issues, including reduced motor torque, overheating, increased current draw, and mechanical stress on the motor. Use a multimeter to measure voltage across each phase of the motor. Compare the voltage readings for all three phases.

Question 47

What is 3 phase and what are its benefits?

Answer 47

Three-phase power is a type of electrical power transmission and distribution system that utilizes three alternating currents (phases) of equal magnitude and frequency, with a phase difference of 120 degrees between them. - Increased power capacity - Balanced load distribution - Smoother power delivery - Efficient power transition - Motor performance - Better harmonic profile

Question 48

Motor components?

Answer 48

- Stator - Rotor - Windings - Bearings - Shaft - Housing - End covers - Fan - Terminal box - Name plate

Question 49

Motor starting arrangements?

Answer 49

- DOL The motor is directly connected to the power supply (typically through a contactor) without any additional starting devices. This arrangement is suitable for small to medium-sized motors with low starting torque requirements. - Star/ Delta Starter The motor windings are initially connected in a star configuration for starting, and then switched to a delta configuration once the motor reaches a certain speed. This arrangement reduces the starting current and torque. - Soft starter A soft starter is a solid-state device that gradually ramps up the voltage supplied to the motor during starting, reducing the starting current and torque. It provides smooth acceleration and minimizes mechanical stress on the motor and connected equipment. - VFD A VFD controls the speed and torque of the motor by varying the frequency and voltage of the power supply. It allows for precise control of motor speed, acceleration, and deceleration, as well as energy savings and improved efficiency.

Question 50

What can be damaged caused by reverse power trip failing to operate?

Answer 50

- Mechanical damage to the generator - Rotor damage - Overheating

Question 51

Most likely places to find earth faults?

Answer 51

- Junction boxes - Motors - Transformers - Sacrifical anodes - Navigation lights - Winches - Crane remotes

Question 52

How does a thermocouple work?

Answer 52

If two dissimilar metal wires are joined together to form a closed circuit and one junction is maintained at a higher temperature than the other, then a small e.m.f. is generated and a milliamp current will flow through the wires. The size of the current is directly related to the difference in temperature between the two junctions.