BATTERY

[1] What are Batteries used for On-Board Ships?

They are used for supplying essential power to: Radio Equipment, Telephone Exchange, Fire Detection, General Alarm Circuits, etc. These Systems are often supplied from two sets of batteries worked on a regular charge/discharge cycle.

They are also used as Emergency Supplies, i.e. for Emergency Generator Start-up and Emergency Lighting. Being used in this case in a “stand-by” role to give power when Main Supplies fail.

[2] How are the emergency Batteries maintained?

The connections to be checked for tightness. Casing surfaces should be kept clean. Terminals are protected with petroleum jelly or Vaseline. Specific gravity should be checkecl at reqular intervals and the batteries to be topped up with distilled water as required. Batteries are ro be kept on trickle charging after attaining its full charge. Load test to be carried out at specified Periodic intervals.

[3] Describe the circuit sketched, making special reference to how battery charge is maintained and how it operates upon loss of main power.

The circuit sketched shows a standby battery system which is used as a back up system in the event of total power failure, so that power is maintained to important sources such as alarm panels, communications etc.

In the event of power failure, a two-pole switch which is held on contacts 1 and 2 by an energized Coil in normal operating condition ie. From the mains supply will change over due to loss of Power onto contacts 3 and 4, now power is supplied from the back up batteries.

The battery change over switch is normally changed over weekly to ensure both sets of batteries Are fully charged. The batteries off load being charged, while the batteries those are not, are on Stand-by for loss of power.

[4] How do you ensure that Batteries are always in good working order?

Where Lead Acid Batteries are used, they require a constant trickle charge, but for both types of Battery a regular charge up is necessary.

Check state of Battery using a Hydrometer; Lead Acid – S G 1270®1285 Alkaline is 1190 but S G for Alkaline Batteries does not change much during charge or discharge.

Ensure cells are covered with electrolyte, if not, top up with distilled water.

Keep batteries dry and clean

Terminals should be smeared with Petroleum Jelly.

[5] Describe how a battery of alkaline cells may be tested for its usefulness after long storage and if found deficient, how it can be rendered fit for service.

A battery of alkaline cells may be tested for usefulness by measuring the voltage across the cells to give an indication of charge. If the voltage is low, but the electrolyte S.G is okay ( taken using a hydrometer ), the battery may be recharged. But if the S.G is found to have dropped to @ i.i60, the electrolyte solution should be replaced and then the battery recharged.

[6] State why one of the following battery types requires a trickle charge and the other does not;

Lead acid

Alkaline nickel cadmium

A lead acid battery requires a constant trickle charge as it tends to loose its charge, a trickle charge keeps it fully charged. An alkaline nickel cadmium battery usually only requires a full charge and is able to hold its charge. Also there is a danger of putting a lead acid battery on full charge as a lot of heat is produced and as a result of this the battery could be destroyed.

CABLES

[7] What are the factors that are to be complied by the installed cables?

The cables installed must comply with current ratings and voltage drop limitations’ passage of Current results in voltage drop along the length of the cables,’ hence voltage drop becomes A problem in very long -cables. Excessive Voltage drop can seriously affect the proper operation of electronic equipment. Motor can give starting problem if voltage drop at the starting current is excessive. The voltage drop in cables from the main switch board to the appliance must not Exceed 6% (in practice it is about 2%)

[8] How the cables are grouped or’ bunched’?

When several cables are run close together or are touching, the heat from one cable affects others, and thus requires lowering rating. For example, if more than 6 cables are running bunched together, it is usual to reduce the current ratings to 85%.
Where possible, cables intended for different operating temperatures should not be bunched In common clips, glands, conduits or pipes’

[9] What is the short circuit rating?

Under short circuit condition there is rapid rise in the conductor temperature, affecting cable Insulation. Mechanical forces due to magnetic effect and longitudinal expansion of conductors Due to heat have also to be considered. It is therefore necessary to match the cable size with the Short circuit protection equipment (fuses or circuit breakers) which may take certain time (a fraction of a second to several seconds) to trip.

DC

[10] What happens when a DC motor is connected to AC supply?

Since on ac supply, reactance will come into the picture, The AC supply will be offered impedance (not resistance) by the armature winding. Consequently, with AC supply, current will be much less. The motor will run but it will not carry the same load as it would be on DC supply.

There would be more sparking at the brushes.

Though motor armature is laminated as a rule, the field poles are not. Consequently with AC supply, currents will cause the motor to heat up and eventually burn.

ELECTRO-MAGNETISM

[11] What is Fleming’s right hand rule?

This is used to determine the direction of emf induced. According to this, the thumb, fore finger

And middle finger of the right hand are kept at right angles to one another and if the fore finger

Represents the direction of magnetic lines of flux, the thumb indicates the direction of motion of the conductor, and then the middle finger will indicate the direction of induced emf.

[12] What is Fleming’s left hand rule?

This is used to determine the direction of the force acting on a current carrying conductor placed

in the magnetic field. According to this rule, the thumb, fore finger and middle finger of the left

Hand at right angles to one another and if the first finger represents the direction of magnetic field, the middle finger indicates the direction of current then the thumb will indicate the direction of force acting on the conductor.

[13] Compare magnetic and electric circuits.

In the electric circuit, electric current flows whereas in a magnetic circuit the magnetic flux does

Not flow. The reluctance of the magnetic circuit varies with flux density, whereas the resistance remains practically constant with current strength. Reluctance of the magnetic circuit is similar to that of resistance in electric circuit. The magneto-motive force in a circuit is similar to Electromotive force in electric circuit.

ELECTRONICS

[14] In a SCR define the following: (a) break over voltage (b) peak reverse voltage

Break over voltage is the minimum forward voltage at which SCR starts conducting (with Gate terminal open)

Peak reverse voltage: the maximum reverse voltage that can be applied to SCR in the Reverse direction (with conducting)

Holding current: the minimum value of anode current below SCR to off state from on state.

Forward current rating: the maximum anode current withstanding without getting damaged,

Latching current: it is the minimum value of anode current that an SCR must attain during Turn-on, to maintain conduction, when the gate signal is removed.

HULL PROTECTION

[15] Explain the principle of the impressed current system of cathodic protection.

The principal of the impressed current system of cathodic protection is to set up a flow of electricity, opposing that naturally generated by two metals, i.e.. The cathode and the anode. Care has to be taken though, as it is as harmful to supply too much current as it is too little. Changes in temperature, surface conditions of the metals and conductivity of the electrolyte demand that the current and therefore the PD should be changed, this automatically controlled and monitored by a thyristor control circuit.

INSULATION

[16] How is the insulation resistance of alternator is checked?

Disconnect any electronic circuit components which may be damaged by a 500v insulation test. Consult the wiring diagrams and manufacturer’s instructions before testing. Measure the Insulation of the stator winding and the rotor windings to earth, and between stator phases. A minimurn value is 1 Mohm, but a lower value may be acceptable to surveyor based on 1Kohm/volt. E.g. 450 Kohms for a 450 volt generator.

[17] How the insulation of pedestal bearing checked?

The pedestal insulation of a dismantled generator may be tested with a megger. Insulation of 20 Kohms and above is considered satisfactory
Condition of insulation can also be checked while machine is running by measuring mllivolts between the shaft and the bedplate. Two readings are taken, one with the jumper connecting shaft to pedestal and one without the jumper. If insulation is good, both readings will be alike. If the insulation is defective, the reading in (1) will be higher than in (2).

[18] How the earth faults are treated?

Earth faults should be eliminated when located. Damaged conductor insulation must be repaired. The method of repairing depends on the cause of earth fault and this is determined by visual Examination. A lamp fitting that is damaged must be replaced. Dampness or moisture in Insulation must be dried out by gentle heat and then precautions taken to prevent future ingress of moisture. Insulation that has been mechanically damaged or damaged by over heating must be made good again. If surface dirt is the cause, a thorough cleaning with electro cleaner solvent will probably cure the fault.

MAINTENANCE

[19] State why the air gap needs to be checked occasionally.

Air gap checked with feeler gauge. It is need to be checked periodically so as to keep a record of the air gap taken approximately 90° apart. This to ensure that a worn out bearing (especially on the power take off side) does not permit rotor to rub against the stator.

Grinding the rotor lamination, filing the stator teeth, increasing in magnetizing current can definitely lower the power factor and air gap measurement is to keep a check so that it is not excessive.

MOTORS

[20] Describe its precautions necessary to ensure that the motor is not started inadvertently.

Precautions necessary to ensure that the motor is not started inadvertently are:

Remove the circuit fuses for the motor concerned.

Motor switch should be locked in open position.

Put sign to motor and switch indicating ‘do not start’ or ‘out of service’.

If necessary, disconnect the terminal at the motor junction box, tagging the terminal correctly and covering the exposed terminal with insulating tapes.

Switch off circuit breaker on the panel switch board in control room or on the respective motor control panel.

[21] Explain why it is important to ensure that supply connections to a three phase motor are made in the correct order.

It is important to ensure that the supply connections to a three phase motors are made in the correct order so that:

To ensure that the direction of motor is correct before couples to machinery, otherwise damage may be done to the driven machinery e.g. electro hydraulic steering pumps.

In case of lifts, hoists, all electric steering gear, the limit switches may not afford protection. It the lift travels up, instead of down, the top limit switch may open control circuit, which is not energized and the motor will not stop.

[22] Explain Regulations regarding Power Supplies for Steering Gears.

The Electrical Power Supply must be from two widely separate supplies i.e. one from the Main Switchboard and the other from the Emergency Board.

Electrical leads and fuses are to allow 100% overload, giving only short circuit protection.

If failure does occur on one system, the stand-by system will cut in within 45 seconds.

The Steering Motors will have sequential starting.

[23] What is ‘cogging’?

This is also known as magnetic locking. When the voltage is low, the motor does not start. This happens when the number of stator teeth is equal to the number of rotor teeth and due to the magnetic locking, the motor does not start. This can be overcome by making the number of rotor slots prime to the number of stator slots.

[24] What happens if single-phasing occurs when the motor is running? And when it is stationary?

If already running and carrying half load phase motor on the remaining single-phase blow normal fuses.
If motor is very heavily loaded, then it will stop under single-phasing since it can neither restart nor blow out the remaining fuses, the burn-out is very prompt.

A stationary motor will not start with one line broken. In fact, due to heavy standstill current, it is likely to burn-out quickly unless immediately disconnected.

[25] What are the causes of faulty starting of a synchronous motor?

It could be due to the following causes:

Voltage may be too low-at least half voltage is required for starting
There may be open-circuit in one phase-due to which motor may heat up.
Static friction may be large-either due to high belt tension or too tight bearings
Stator windings may be incorrectly connected
Field excitation may be too strong.

POWER GENERATION & DISTRIBUTION

[26] Which distribution system is more efficient in maintaining supply?

The distribution system that is more efficient at maintaining a supply is the insulated distribution system (used in most marine electrical supplies), if one earth fault occurred in this system, it would not cause any protective gear to operate and the system would continue to function normally. Important equipment still operates. The single earth fault does not provide a complete circuit, so no earth fault current can exist. It would take two earth faults on two different lines to cause an earth fault current.

[27] The short time rating; the rms value of the current that a circuit breaker is capable of carrying for the started time. What course of action needs to be taken when a generator is overloaded?

The course of action to be taken when a generator is overloaded is that the preference trip s should be operated; these will normally operate automatically by means of a timing relay disconnecting non essential loads in a definite order and at definite time intervals.

Example:

1^st trip Air conditioning, galley, laundry, ventilation.

2^nd trip Refrigeration plant

3^rd trip Deck equipment

[28] Describe how Synchronizing Lamps are used to parallel a Generator.

this is normally used as a back-up or alternative way to the Synchroscope, the Lamps are connected between the Incoming Generator and the Bus-bars. The “Sequence Method” is the preferred one as it displays a rotation of Lamp Brightness, which indicates, whether the incoming machine is running fast (clockwise) or slow (anti clockwise). As with the Synchroscope, the Lamp Sequence must appear to rotate slowly clockwise. Correct synchronization occurs when the top or “Key” lamp is dark and the two bottom lamps are equally bright.

[29] State what would be the effect on the current, voltage and frequency of the machine if an AC generator slowed down slightly during operation.

If one machine running alone:

Speed slow down, frequency decreases. N a f/p.

Voltage is not affected as it controlled by AVR.

Impedance reduces (XL = 2PfL) with voltage being constant, current would therefore increase.

If two alternators in parallel:

If one generator slows down slightly, it would drop / shed part of its kW loading which would be taken up immediately by the other one. However, there will be no change in the frequency, voltage and current in the system as both are locked electrically.

Ø Emergency Power

[30] Comment on the sequence of events following a mains failure ?

Control and monitoring equipment of emergency generator on sensing a large drop in voltage or Frequency sends a starting signal to emergency generator. Bus tie breaker between the main Switch board and emergency switch board opens. In 45 secs the emergency generator starts and comes on load during this time the supply for emergency, lighting, alarms etc. is maintained by the emergency batteries. In case emergency generator fails to come on load, these batteries can Supply power to selected lighting, communication and alarm circuits for minimum period of Half an hour.

[31] Explain how the Emergency Generator starts up in the Event of Total Power Failure.

The Start-up of the Emergency Generator is initiated by an Electrical Relay, which monitors the normal Mains Power Supply. Falling Mains Frequency or Voltage causes the “start-up” relay to operate the Generator Starting Equipment. The Prime Mover may be electrically cranked from its own 24V battery and starter motor or air started from its own air reservoir fitted local to the Generator Engine. A manual start-up may also be initiated by push buttons in the Main Control Room and in the Emergency Generator Room.

Ø Generators

[32] Describe how a Self Excited A C Generator would work

The Self Excited A C Generator basically consists of a Synchronizes Generator, where the 3-phase supply is generated, an A C Exciter and a Rotary Rectifier and a Static Excitation Device.

The Rotating Diodes change A C into D C for the D C Excitation Current, which is supplied through Brushes. The Exciter in conjunction with the A V R is used to monitor the correct Voltage under Load Changes.

PROTECTION

[33] State what protective devices are carried on, and peculiar to AC switchboards.

The protective devices that are fitted on AC switchboards are:

overload trip

under voltage protection

reverse power relay

Preferential trip.

For marine application, provision of protection for each phase is very rare. The most important reason concerning the earthing of the system neutral. Normal internal generator faults are between one phase and earth. On a typical marine system, with an insulated neutral these faults can be tolerated until appropriate action can be taken.

[34] What is meant by Protection Discrimination?

This is the ability of a Protection System to disconnect only Faulty Circuits and to maintain the Electrical Supplies to Healthy ones.

Discrimination is achieved by co-coordinating the Current Ratings and the Time Settings of the Fuses and Over-current Relays used between the Generator and the Load.

The Devices nearest the Load having the lowest Current Rating and shortest Operating Time; those nearest the Generator having the highest Current Rating and longest Operating Time.

i.e. a Motor Fault on a Centrifugal Pump would not cause the Generator Breaker to Trip.

[35] How is protection discrimination achieved?

Protection discrimination is achieved by coordinating the current ratings and time delays of fuses and over current relays used between the generator and load. The devices nearest the load having the lowest current rating and shortest operating time, those nearest the generator having the highest current rating and the longest operating time. This will allow the protection system to disconnect only faulty circuits and maintain the electrical supply to healthy ones.

[36] How the under voltage relay is checked and calibrated?

Under voltage protection can be electronic or electromagnetic. Checking and calibration of Generators’ under voltage relays can only be done accurately by voltage injection. A known Variable voltage is directly applied to the under voltage relay to check:

The voltage at which Relay pulls in (2) the voltage at which relay drops out. Generator under voltage relays have time delay to prevent false tripping during transient voltage dips (typically 15%), caused by large Motor starting currents.

[37] Describe where overload protection arrangement associated with a shore supply connection

The overload protection arrangements associated with a shore power supply connections are:

a circuit breaker or a switch and fuses to protect the cable linking the connection box to main switchboard with data plate showing the ship’s voltage and frequency and methods of connecting the shore supply cable is displayed at the connection box.

The connection box must have suitable terminals to accept shore supply cable, including an earthing terminal to earth the ship’s hull to the shore earth.

Ø Fuses

[38] Compare the effectiveness of filled cartridge and rewire able fuses for three phase circuit protection.

Effectiveness of cartridge fuses in AC 3 phase circuit protection are:

Cartridge fuses or high rupturing fuses can be used for very high fault current levels.

Time / current fusing characteristic are very consistent.

Cartridge fuses are sized to ensure that the correct fuses are inserted.

While rewire able fuses :

Rewire able fuses tend to deteriorate with use.

They are corroded by the surrounding atmosphere.

They do not have any accurate time – current characteristic.

[39] How will you come to know whether a fuse is blown out or not, explain two methods?

Live circuit: By using multi-tester in voltage mode check the voltage at either ends respectively w.r.t. to neutral line or other phase. If voltage is available on both ends fuse is OK else it is blown off.
Cold check; By using the multi-tester in ohm mode connect the leads on both ends of the fuse. If it showing no resistance it is OK else it is blown off.

[40] How a fuse is selected for circuit protection ?

Rating of fuse for lighting or heating circuit: it is the current which a fuse will carry continuously. For a 50A circuit, a 50A fuse would be appropriate, unless it was a motor circuit.
Rating of fuse for motor circuit: in motor circuits, fuse should be large enough to carry starting current for the time necessary to start the motor plus necessary margin.

SAFETY

[41] What would you do if you found an Engineer in Electric Shock?

First raise the alarm then isolate or switch off the current.

If this is not possible, pull or push the victim away from the source, taking care not to make electrical contact with the victim or the electrical source, i.e. use something non-conductive.

Once the victim has been removed from the electrical source, if the breathing is feeble or ceased, artificial respiration should be started at once. Delay in starting artificial respiration can prove fatal.

Note: If artificial respiration is started at once, 70% of the victims recover. If there is 3 (three) minutes delay, only 20% of victims recover.

[42] What is the difference between shock received due to dc and that due to AC?

Current from a steady dc source in passing through the skin, will tend to cause muscular contraction at the initial contact and when the contact is broken. Ac produces continue spasm in the muscles through which current passes, with its change from forward to reverse flow either at 50 hz or 60 hz rate. Ac has the ability to stimulate nerves directly.

[43] What are the conditions that affect the electric shock?

The size of electric shock current is related to following factors: 1. Applied voltage (2) body Resistance. The size of human body resistance depends on other factors also, like state of health, Degree of contact with live wires, perspiration or dampness of skin. Typical dry full contact Body resistance is about 5000 ohms at 25V, falling to about 2000 ohms at 250V. This indicates that human body resistance goes down as the applied voltage goes up, giving rise to the increased Current at high voltages.

[44] What is meant by intrinsically safe?

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 short circuit or earth fault), is incapable of igniting a prescribed gas mixture, under prescribed test condition.

An equipment, which cannot released sufficient electrical or thermal energy, under any condition to ignite a particular flammable vapour in its vicinity.

[45] What are the causes for short circuits?

Short circuit may occur because of breakdown of insulation of equipment due to overheating or ingress of water. Sometimes terminal connections become loose, causing two power supply cables to make electrical contact. Short circuit may also occur in the windings of a transformer because of insulation breakdown due to overheat. Electrical cables exposed to fire, mechanical damage. Cuts etc. can cause short circuit.”

[46] Why the equipments are earthed on board the ship?

In order to protect against the dangers of electric shock and fire that may result from earth Faults, the metal enclosures and other non current carrying metal parts of electrical equipment Must be earthed. The earthing conductor connects the metal enclosures to earth (ship’s hull) and prevents such metal parts from attaining dangerous voltage with respect to earth in the event of an earth fault occurring.

STARTERS

[47] When would a Star/Delta Starter be used on a Motor?

Where a slow moving high inertia load is involved the starting time must be considered because of the heating effect of the starting current and in this case a Star/Delta Starter would be used.

The Star/Delta Starter first connects the Starter Windings in Star and when running changes over to Delta. The Star connection results in about 58% Line Voltage being applied to each Phase with therefore a reduction in Starting Current. The Starting Torque is also reduced to about 1/3 of its direct on-line valve.

Why star delta starter is preferred with induction motor?

Star delta starter is preferred with induction motor due to following reasons:
• Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses.
• Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.
• In addition, starting torque is increased and it prevents the damage of motor winding.

SWITCH GEARS

[48] How does a magnetic relay operate?

Magnetic relay overloads are of the solenoid type, consisting of an iron plunger surrounded by a coil, one for each phase protected. The coil carries line current, or a proportion of line current if transformers are used. At a predetermined value, the plunger is attracted to the centre of the coil and a pushrod activates a trip bar which opens an auxiliary contact in the operating circuit and activates a mechanical latch.

[49] What are the four fault ratings of a circuit breaker?

The four fault ratings of a circuit breaker are;

The symmetrical breaking current; the rms value of the AC component of the breaking current.

The asymmetrical breaking current; the rms value of the total breaking current which includes both AC and the DC components.

The making current; the peak value of maximum current loop, including the DC component in any pole during the first cycle of current when the circuit is closed.

SWITCHBOARDS

[50] What are the types of switch boards available?

Switch boards may be open type in which all essential switch gear is exposed on the front of the Panel, or ‘dead front’ type in which all the live parts are concealed behind sheet steel panels. Only the operating handles and instruments appear on the front. While dc switch boards may Employ open type panels (of voltage less than 55v),’dead front’ type is mandatory for voltages More than 55v and on ac system.

TRANSFORMERS

[51] What effects are produced by change in voltage?

Iron loss – varies approximately as V²

Cu loss – it also varies as V² but decreases with an increase in voltage if constant kva output is assumed.

Efficiency – for distribution transformers, efficiency at fractional loads decreases with increase in voltage while at full load or overload it increases with increase in voltage and Vice versa.

Regulation – it varies as V² but decreases with increase in voltage if constant kva output is assumed.

Heating – for constant kva output, iron temperatures increase whereas cu temperatures decrease with increase in voltages and vice versa.

[52] Distinguish between an induction coil and a transformer.

Induction coil such a mutual induction in its simplest form, a transformer consist of two inductive coils which are electrically separate by magnetically linked thru a path of low reluctance (a path that offers low resistance to the flow of magnetic flux).

A transformer of AC system is the ease with which an alternating voltage can be increased or reduced. The transformer may be an air cooled type

[53] State the normal care and attention needed for a transformer in operation.

Normal care and attention needed for a transformer in operation:

At regular specific interval, transformer must be disconnected cover removed and all accumulated dust and deposits remove by a vacuum cleaner and suitable brushes.

Windings must be inspected for any sign of damage or overheating.

Windings continuity resistances are measured.

Insulation resistance of al winding must be measured with respect to earth and to other phase windings.

Cable connections must be checked for tightness and covers must be secured.

Transformer generally air cooled, thus ensure no obstruction at that area.

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