MEO Class 4: Function 5 (Electrical) – Questions and Answers part 1

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Function 5 – EECEOL Most frequently asked questions in MMD’s

Function 5: EECEOL(Electrical, Electronics, and Control Engineering at Operational Level)  generally called “Electrical” is one of the oral functions in class 4 exams. The surveyors are mostly focusing on,
  1. Safety aspects of the equipment such as Motor, Generator, Crane, MSBD, ESBD, etc. 
  2. Basic working principles of the equipment such as Alternator, Avr, motors, starters, etc.
  3. Regulations for Emergency Power supply.
  4. Maintenance of the equipment.
  5. The power distribution system of the ship that includes MSBD, ESBD, Emergency Generator, Batteries, etc.

So, in this article, we have collected the answers for most frequently asked questions in MMD’s.

 

Switch Board Safety Devices:

 

1.    Preferential trip.

2.    Low-frequency trip.

3.    Overvoltage trip.

4.    Under voltage trip.

5.    No voltage trip.

6.    Reverse current trip.

7.    Over-current trip.

8.    Fuses.

9.    Automatic circuit breaker, ACB.

10.   Earth lamp.

11.   Meters.

12.   Synchroscope.

13.   Emergency synchronising lamp.

14.   Ebonite handrail and Rubber footstep.

 

Preferential trip: 

 

1.    Operate after a fixed time delay, causing non-essential loads to be shed. 

2.    The usual setting for overload trip is 150% load[50% overload]. 

3.    When the generator load reaches 110%, Preferential Trip comes into operation as follows.

    

      First tripping at 5 sec:   

         Shut down non-essential loads [air condition, entertainment, accommodation fans,            cargo hold fans, amplifiers, etc.] to reduce the generator load.

         

      Second tripping at 10 sec:   

         Shut down essential loads [services required for running the ship properly, leaving loads of top priority services to maintain Propulsion and Navigation]           if generator load is still high.

 

     Third tripping at 15 sec:

         Shut down the main generator as the last action, if the load is still too high, it maybe due to short circuit or insulation breaking.

 

Earth lamps:  

A set of lamps, which show the presence of earth fault in the distribution system.

 

Earth lamps of 3 phase, 3 wire system, AC.

 

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Each lamp is connected to secondary connections of every single phase step-down transformer and primary connections are common to star the point, which is earthed to ship structure.

Normally 3 earth lamps burn with equal brightness if there is no fault.

If phase A is earth fault, lamp A becomes dark while the other two lamps burning with

extra brightness.

     =      Location of fault can be traced, by switching off the branch circuit, one at a time.

= When branch circuit with a fault is switched off, the dark lamp will become normal glowand all 3 lamps burn with equal brightness.

 

Synchroscope:

1.    An instrument, which indicates that two electrical supplies are in synchronism and can be paralleled. [OR]

2.    An instrument, which indicates that, voltages, frequencies and phase angle of two electrical supplies of running machine and incoming machine, are in synchronism and can be paralleled.

3.    Synchroscope should not be left in a circuit for more than 20 minutes, as it cannot continuously rotate.

 

Synchronising method:

~ Synchroscope is the main method.

~ Back-upmethods are:  1) Lamp dark method     2) Lamp bright method 

                                           3) Rotating lamp method or Sequence method [preferable].

 

Sequence method:

1.    One of the lamps known as the key lamp is connected in one phase.

2.    Other two lamps are cross-connected.

3.    If two frequencies differ, lamps will brighten up in the rotation. Clockwise indicates the incoming machine is fast, and counter-clockwise indicates it is slow.

4.  The synchonizing moment is when the key lamp is dark and the other two lamps equally bright.

5.    If phase rotation is wrong, all lamps will become bright and dark together. The remedy is to interchange any two phase-connections.

 

Reverse current protection:

Fitted in the main switchboard to trip and disconnect the main circuit breaker, in the event of a reverse current flow. It prevents the generator against running as a motor.

 

Intrinsically Safe:

1.  An electrical circuit or part of a circuit is intrinsically safe if any spark or thermal effect produced normally (e.g. by breaking or closing the circuit) or accidentally (e.g. by a short circuit or earth fault), is incapable of igniting a prescribed gas mixture, under prescribed test condition.

2.  An equipment, which cannot release sufficient electrical or thermal energy, under any condition to ignite a particularly flammable vapor in its vicinity.

 

Emergency Power Sources:

 

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All passenger and cargo vessels shall be provided with emergency sources of electrical power, for essential services under emergency conditions. 

The emergency source may be a generator or batteries, but must comply with the rules.

Emergency sources must be installed in a position such that they are unlikely to be damaged or affected by any incident, which has caused to main power.

Emergency source of power should be capable of operating with a list of up to 

         22 ½° and trim of up to 10°.

 

        =       Emergency generator with its switchboard, is located in a compartment which is:   

                           À  Outside and away from main and auxiliary machinery space.   

                           Á  Above the uppermost continuous deck, and  Not forward of the collision bulkhead.

 

= Batteries: The above same rules applied, but must not be fitted in the same place as an emergency switchboard.

 

Rules for Emergency Power Sources:

Passenger Ship:

» Emergency Generatorshall be automatically started and connected within 45 sec.

» Capable of supplying simultaneously at least the following services        for the period of 36 hours.

1.    Emergency lighting [at alleyways, stairways and exits, muster and embarkation stations, machinery space, control room, main and emergency switchboard, firemen’s outfits storage positions, steering gear room, fire pump, emergency bilge pump starting positions]

2.    Navigation lights, 

3.    Internal communication equipment, 

4.    Fire detection and fire alarm system, 

5.    Daylight signalling lamp, Ship’s whistle,

6.    Navigation equipment, 

7.    Radio installations, [VHF, MF, MF/HF] 8.              One of the fire pump, Emergency bilge pump.

 

» A set of automatically connected Emergency Batteriesmust be capable of carrying certain essential services for a period of 30 min.

1.    Emergency lighting, 

2.    Navigation lights, 

3.    Internal communication equipment, 

4.    Fire detection and fire alarm,

5.    Daylight signalling lamp, ship’s whistle. 

 

Cargo Ship:

» Emergency power source, Emergency generatormust be sufficient to operate certain essential services at least for the period of 18 hours.  

1.    Emergency lighting [at alleyways, stairways and exits, muster and embarkation stations, machinery space, control room, main and emergency switchboard, firemen’s outfits storage positions, steering gear room, fire pump, emergency bilge pump starting positions]

2.    Navigation lights, 

3.    Internal communication equipment, 

4.    Fire detection and fire alarm system, 

5.    Daylight signalling lamp, Ship’s whistle,

6.    Navigation equipment, 

7.    Radio installations, [VHF, MF, MF/HF].

8.    One of the fire pump, Emergency bilge pump.

       

» Where the emergency source of electrical power is an accumulator battery, it shall be capable of carrying loads without recharging and battery voltage throughout the discharge period must be maintained within 12% above or below its nominal voltage.

      »      Battery system is automatically connected upon loss of main power.

      »      Batteries are required as a transitional power source for 30 min. for the following items:

1.    Emergency lighting

2.    Navigation lights

3.    Internal communication equipment

4.    Fire detection and fire alarm.

 

Essential Services from Emergency Power Source:

 

1.    Emergency Lighting.

2.    Navigation Lights.

3.    Communication Equipment.

4.    Fire Detection and Fire Alarm.

5.    Daylight Signalling Lamp and Ship’s Whistle.

6.    Navigation Aids.

7.    Emergency Fire Pump.

8.    General Alarm.

9.    Manual Fire Alarm.

10.   Steering Gear.

11.   Watertight Doors.

 

Emergency Switchboard Distribution:

 

1.  There are 2 sections:  440V and 220V.

2.  Under normal condition, 440V Supply is taken from ER Main Switchboard, through a Circuit Breaker.

3.  When the main power is lost, this Circuit Breaker is tripped [opened]. 

4.  Emergency The generator comes into action and supplies power through another Circuit Breaker.

5.  An interlockis provided, to prevent simultaneously closing of both Breakers; [both Main and Emergency Generator maybe running, simultaneously].

6.  From 440V section:  

a)      Emergency Bilge pump,

b)      Sprinkler System,

c)       One of two Steering gears.

 

7.  From 220Vsection:

a)      Navigation Equipment,

b)      Radio Communication,

c)       Transformed and rectified supply to Battery Systems.

    

     Transitional Emergency Power Battery:  [Emergency Lights for 30 minutes.]

       Low power DC system Battery:  [Alarms and Control system.] Emergency Generator and its maintenance:

 

1.    Provided with independent means of automatic starting, [compressed air, batteriesor hydraulic] and repeated starts of at least 3 times, and further attempt can be made within the 30 minutes temporary battery life.

2.    Adequate and independent supply of fuel with a flashpoint of not less than 43°C [109.4°F]. 

3.    Must be able to be started in cold condition, up to 0°C [32°F].

4.    For cold weather, JCW system must be treated with anti-freeze agent, and heating arrangement provided.

 

Maintenance:

1.    Every Saturday, the Emergency generator must be tested-run.

2.    Air bottlepressed-up or starter battery fully charged, at all times.

3.    Changeover the selector switch to the local position before starting.

4.    Make sure breaker switch at ‘off’ position before starting [an interlock between ER Main Switchboard breaker and Emergency Switchboard breaker is provided to prevent simultaneously closure of both breakers].

5.    During testing, check frequency, voltage and ampere.

6.    Fuel tank, always checked to ensure an adequate level.

7.    Air filterof the generator, regularly cleaned.

8.    Required tools and spares kept in a container.

9.    Emergency light for this room should be always checked.

 

Emergency Generator Starting, when Black-out:

1.    Normally cut-in automatically when main power fails.

2.    Starting is initiated by Start-up relay.

3.    Falling of frequency or voltage of Main Power, cause the Start-up relay to operate generator-starting equipment.

4.    If this system fails, after switching the MODE selector to Manual(Local) position, the generator can be started manually by means of Back-up Starting Equipment within 30 minutes of Transitional Emergency PowerBattery lighting.

 

Battery room inspection and maintenance:

1.    Battery installation and its charging rectifier checked.

2.    Battery room environmentmust be dry and well ventilated.

3.    Battery tops shall be clean and dry, and terminal nuts must be tight and a smear of petroleum jelly applied to prevent corrosion.

4.    Electrolyte at a proper level, and shall have the correct value of specific gravity by checking with a hydrometer.

5.    Rubber gloves and goggles used when handling electrolyte.

6.    Charging equipment checked for dirt, overheating, loose connection, and correct the functioning of indicators.

7.    Ventilation arrangement for battery locker checked. Battery installation of both lead-acid and alkaline needs good ventilation.

8.    Since both types generate hydrogen gas during charging, no smoking and naked light allowed.

9.    Steelworks and decks adjacent to lead-acid battery should be painted with acid-proof paint. 

           [For Cad-Ni cell, alkaline resistance paints.] 

Battery Room Safety Arrangement:

Safety is provided by:  

À Proper ventilation

Á   Prevention of heat source for ignition.

 

Ventilation:

1.    Independent exhaust fan provided.

2.    Inlet ductshould be below battery level and outletat top of the compartment. 

 

Prevention of heat source for ignition:

1.    No naked light and no smoking.

2.    Uses of externally fitted light or flameproof light.

3.    Cables of adequate size and they are well connected.

4.    Never placed Emergency Switchboard in this room.

5.    Use insulated spanner and plastic jug for distilled water, to prevent short-circuit.

6.    Room temperature maintained at  15 ~ 25°C.

 

Emergency Lights Maintenance:

» Every Saturday, routine testing of emergency lights carried out by E/E.

» Ensure that batteries are fully charged and ready for use.

» Burnt-out bulbs replaced at once. 

 

Requirements for Navigation Light panel: 

1.    Navigation lights should be connected to a distribution board, which does not supply other services. 

2.    There should be a changeover switch so that it can be transferred to another source of power supply.

3.    Visual and audible alarms required for individual Navigation Light failure.

4.    Fuse protection provided.

 

Secondary Cells: 

» Secondary cell or accumulator is an apparatus, which utilises chemical action to store up electrical energy.

           »  Secondary cells are 1) Nickel Cadmium storage battery.    2)  Lead-acid battery.

 

Nickel Cadmium battery [alkaline battery]


» + ve plate:          Nickel hydroxide + graphite.

» – ve plate:          Cadmium + Iron.

» Electrolyte:        Potassium hydroxide solution [strong alkaline]. 

                  Normal Sp.Gr.[1.21] does not change with charging or recharging.

                  But Sp.Gr. of Electrolyte gradually decreases, and Electrolyte should be renewed when Sp.Gr. becomes [1.160]    

» It is a sealed battery, thus no gassing during charging.

» Very low open-circuit losses, but requires 67% more cells than Lead Acid battery.

                 [1.2 V per cell:   when fully charged 1.7 V per cell.] » Not harmful when overcharged.

» Left for a long period, either fully charged or fully discharged, without deterioration.

» Better mechanical strength and durabilitythan a lead-acid battery. » High initial cost but longer life.

Lead Acid Battery:

» + ve plate:       Lead Peroxide [chocolate brown].

» – ve plate:        Spongy Lead  [slate gray color].

» Electrolyte:     fully charged  HSO₄,  SG 1.28, Renew when SG is 1.110.

      [1.8 V per cell:   when fully charged 2 V per cell.] 

      When undercharged, + ve plates are pale brown or yellowish instead of deep chocolate and very difficult to convert back to normal form.

» Efficiency [watt hour efficiency] is higher than Alkaline Cells.

 

When Battery is Overcharged: [Lead Acid battery]

1.    Overheating cause buckling of plates.

2.    Internal short-circuit.

3.    Sludgeformed at the bottom of the cells [lead peroxide].

4.    Battery may be ruined.

5.    Lower the capacity.

 

When Battery is Undercharged: [Lead Acid battery]

1.    Over discharging.

2.    + ve plates are pale brown or yellowish, instead of deep chocolate. 

3.    – ve plates, almost white in color.

4.    Falling of voltage:  1.8 V / cell, and SG of HSO  1.15

 

Depolarisation:  

Liberation of  H+  at   – ve electrode [cathode] and that will decrease the current flow.  

 

Why AC is the popular onboard ship?

1.    Smaller, lighter and compact machine size, for a given kW.

2.    High power and high voltage AC generator can be easily manufactured.

3.    Voltage can be raised or lowered by the transformer.

4.    AC can be easily converted to DC.

 

Electrical repair job hazard:  Electric shock prevention:   

1.    Switch off the main switch

2.    Put a mechanical lock-on

3.    Take out fuse

4.    Put a signboard “Man Working on Line”.

 

Squirrel cage induction motor:   

1.    Most widely used of all types of AC motors, due to simplicity, the strength of construction and ease of maintenance.

2.    Made up of two main parts, rotor and stator and no direct electrical connection between them. No wire winding or slip rings. 

3.    Rotor has a series of plain bars ( copper or Al ) running in slots the length of the iron core.

4.    Each end of the bar is brazed into 2 heavy copper rings, one at each end. Those bars form a cage, that looks like a squirrel cage.

5. Stator has 3 separate windings supplied from a 3-phase AC supply. 

6.    Phase difference 120°and the number of poles 2,4,6 or more, depending on the speed required. 

Difference between Synchronous Motor and Induction Motor:

  •  Synchronous motor is almost exactly the same as an alternator.
  •  An induction motor cannot run normally at synchronous speed. It has a slip.

 

AC motor overload protections:

1.    Magnetic overload relay.

2.    Thermal overload relay.

3.    Built-in protective device. 

 

Difference between Circuit Breaker and Fuse:

Circuit breaker:  

1.Has switching actions to close the circuit or to open the circuit, and has a trip circuit if the load current exceeds the set value. 

2.After tripping, the circuit breaker can be reused without replacing any part. Fuses:   

1.    Have only breaking function, and fitted in the circuit to protect the circuit from damaging effect of high current flow. 

2.    It breaks the circuit by melting the fuse metal itself. 

3.    After breaking, the blown fuse must be renewed.

 
Static Electricity:  

1.    Electricity at rest instead of in motion 

2.    Electric charges may be induced by friction or atmosphere effect.

 

Single phasing: 

 

1.    Single phasing occurs when one of the 3 phase circuits is opened, hence the remaining circuits carry excess current. 

2.    One phase of the circuits becomes open, due to blown fuse, faulty contactor, or broken wire.

3.    It prevents a motor from starting, but a running motor may continue to run with this fault.

4.    For a running motor, it can be detected by an overloaded device in the supply line or overheating.

5.    For an idle motor, it cannot be started.

6.    Due to single phasing, overheating in a stalled or the running motor will cause, burnt-out overloaded coil.

 

Residual Magnetism:

 

1.    Magnetism remaining in the fields of a generator, after the exciting current is cut-off. 

2.    Residual Magnetism is essential for initial generation of current, necessary for further a build-up of shunt field strength.

3.    Generator may fail to excite, if there is loss or reversal of residual magnetism of the pole.

 

Remedy, when the generator fails to excite:

 

1.    Pass a currentthrough shunt field coil incorrect direction.

2.    Correct direction means the current will re-magnetise the iron core in the right way. 

3.    Currentfor restoration can be obtained from another DC generator or from a battery.

     If the battery is used for re-magnetizing:       A 12 V  is connected [exclusively]

                 across shunt field coil with the machine stopped. 

                 Current flow in the right direction, for a few seconds only will establish the field.

4.    During this time faulty generator must be in the stopped position.

 

Excitation:   

1.    Production of an electromagnetic field of a generator by supplying exciting current for magnetising the field magnet. 

2.    For excitation, DC is used, because DC produces a constant rate of magnetic flux.

3.    AC generator sometimes lost excitation due to reverse current.

 

Exciters:    The source, which generates the field current for excitation of field magnets.

 

Rotary converter:    A rotating diode to convert AC to DC current for alternator excitation.

 

Equalizing bar:   

1.    Equalizer is a low resistance circuit, connected across armature ends of series coils of parallel compound generators, via a special bar in the switchboard. 

2.   An Equilizer is fitted to stabilize the parallel operation of compound DC generators.

 

AVR:    Automatic Voltage Regulators are used in conjunction with the generator for controlling the terminal voltage to give a steady voltage under varying load.

 

Types of AVR:

1.      Carbon pile

2.      Rotating sector

3.      Vibrating contact

4.      Multi-contact

5.      Magnetic amplifier

6.      Electronic amplifier

 

Megger testing:

1.    Megger tester [generally a 500V set] is used for measuring high resistance, like insulation resistance of the cable, electrical equipment and wire installation, 

        in million ohms.

2.    To get more accurate results, using the larger the instrument, it is important that the terminal marked earth, which is the + ve pole, shall be connected to the earth.

 

Difference between DC and AC generators:

DC generator has a commutator and the AC generator has a slip ring.

 

Slipping Clutch:

 

1.    Slipping clutch is commonly fitted between the prime mover and gearing.

2.    It is incorporated with the motor, magnetic brake and drive shaft.

3.    Set to slip at approximately 133% of full load torque.

4.    Letting go or dropping speed is controlled by a friction brake.

5.    Hauling speed is  0.15 m / sec.

 

Why fitted?

1.   In windlass, undue stresses must not be applied to chain cable and machinery.

2.   Without slipping clutch, excessive stresses could be applied to the cable, by armature momentum, by sudden obstruction when heaving, or when bringing the anchor into hawsehole.

3.   Fitted also to avoid inertia of prime moverbeing transmitted to windlass machinery, in the event of shock loading on cable, when the anchor is being housed.

4.   When the ship is riding at anchor, bow stopperprevents the strain for the windlass.

 

Windlass safety devices:  

  1. Overload [thermal switch] 
  2. Over speed 
  3. Slipping clutch      

 

Electric Braking System:

1.   When electric deck motor is used for lowering or lifting load, the electric brake system is used. All brakes are failsafe types.

2.   In the event of power failure, brake automatically applied, thus preventing the load from running back. A number of brake pads [free to move] are located in the carrier, which is keyed to the motor shaft. 

3.   Armature plate applies pressure to brake pads, by means of a number of springs, to force against friction surface of the backplate, so that it prevents the motor shaft from turning.

4.   When energized, armature plate is attached to a magnet, thus releasing the thrust pressure and allowing the shaft to rotate.

 

Winch brake adjustment:   Adjust the distance between the pressure plate and friction plate. 

 

Drying out of Electrical Machine:

It is essential when the machine has been exposed to weather, or when accidentally immersed.

 

When Electrical Machine is flooded with SW:

1.    Machine is disconnectedfrom power sources and dismantled.

2.    All salt deposits washed out with FW.

3.    If deposited with oily bilge water, wash out with Electro Cleaner.

4.    Should be heated with lamp, and enclosed or covered up to retain the heat.

5.    Moisture should be escaped, by lifting the cover continuously or periodically.

6. The hottest part of the machine shall not exceed  90°C, while heating.

7.    IR readings and temperatures have taken regularly, until a constant value reach about 1 m Ω.

8.    Then, spray the machine with insulation varnish.

9.    Assemble and put in service with low load, if possible.

 

Short, open and grounded circuits:

Short circuit:   A low resistance path that actually shortens the intended path for the flow of current. 

Open circuit:  The path for the flow of current is broken. A switch is one method of creating an “open” intentionally.

Grounded circuit:  A circuit that has come in contact with the earth, by coming in contact somewhere in itself, or by a conductor which is connected with the earth.  


Shore connection:

 

The shore connection box is provided at a convenient position, to receive ashore power supply, so that ship’s generators can be shut down, in port or Dry Docking.

 

Lloyd’s Requirements:

 

1.   Connection box contains a circuit breaker or isolating switch with fuses.

2.   Provided full information of supply system, normal ship voltage and frequency for AC current.

3.   Main Switchboard must be provided with a link switch or a circuit breaker, voltmeter,ammeter, and an indicator to show that the cable is energised.

4.   For 3-phase supply with earth neutral, an earthing terminal must be provided, for connecting the hull to shore side.

5.   A phase sequence indicator is necessary to ensure correct connections.

6.   Means shall be provided for checking polarity, and terminals should be labelled. 

 

Diode:    

1.    A thermionic tube consisting of cathode and anode and heating elements. 

2.    Electric current can pass the diode in only one direction.

3.    It is used as a half-wave rectifier in an electronic circuit, because electronic current cannot flow back to the cathode.

 

Thermionic:   Transmission of an electron by heating.

 

Transistor:   A small electronic device used for rectification and amplification of the current.

 

Dash pot:   

1.   Dashpots are fitted for overload trips to get time delay action, so that breaker will not be opened, due to momentary current surge. 

2.   When load current is in excess, it attracts the plunger of the solenoid.

3.   Plunger or the piston moves up against the displacement of viscose oil or silicone fluid, through a small hole on the piston.

4.   Time lagdepends upon hole size, and viscosity of the oil.

5.   Load current setting for a trip is about 25% above the maximum, but should not exceed 50%.

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