MEO Orals on Marine Electro Technology Function 5- Part 1

What is shaft generator ?

• It is an alternator driven by main propulsion unit through clutch and gearing to produce electricity.
• It can be used while vessel at sea running with a sea speed.
• It must be able to supply normal sea load.

What are the advantages and disadvantages of shaft generator on ships ?

Advantages

• Slightly reduce total fuel consumption.
• Reduced running hours of auxiliary generator.
• Reduced noise in E/R.
• Can made overhauling the G/E at sea.

Disadvantages

• Cannot be used at rough sea.
• Increase load on main engine
• More complicated mechanism.
• Complete black on main engine slow down.

What is earthing device on propeller shaft ?

• It is a propeller shaft grounding device.
• Its function is to ground the static electricity to the ship hull caused by propeller rotation.

What is earth lamp ?  How will you know when earth fault occur and how to trace it ?

• It is a set of lamps, which show the presence of earth fault in distribution system.
• Each lamp is connected between one phase and common neutral point.
• If earth fault occurs the lamp will show dim light or goes out because potential is zero.
• The location of fault can be traced by switching off the individual branch circuit breaker and check the condition of earth lamp.
• When the branch circuit with the fault is switched off, the earth lamp will return to normal brightness.

Explain earth lamps of a 3 phase, 3 wire ac system ?

• Each lamp is connected to secondary connections of each single phase step-down transformer, and primary connections are common to star point, which is earthed to ship structure.
• Normally 3 earth lamps burn with equal brightness if there is not fault
• If phase ‘R’ is earth fault, lamp ‘R’ becomes dark while the other two lamps burn with extra brightness
• Location of fault can be traced, by switching off the branch circuit, one at a time
• When branch circuit with fault is switched off, dark lamp will become normal glow and all 3 lamps burn with equal brightness.

Why earth lamp is fitted ?

• Earth lamp is fitted to give visual signal when there is an earth fault occurs in the system.

What is megger insulation test ?

• The instrument used for measuring high resistance in million of ohms. It is used for measuring the insulation resistance of cables, electrical equipment wiring insulation.

Why megger reading is taken ?

• To verify insulation resistance 
• To detect insulation fault

Write a note on meggar testing ?

• Megger tester [generally a 500V set] is used for measuring high resistance, like insulation resistance of cable, electrical equipment and wire installation in mega ohms.
• The test voltage is produce either by an internal hand driven generator or by a battery and electronic voltage changer.
• A measurement of the insulation resistance gives one of the best guides to the state of health of electrical equipment.
• The resistance should be measured between insulated conductors and earth, and between conductors.
• To get more accurate results, using the large instrument, it is important that the terminal marked earth, which is the + ve pole, shall be connected to the earth.
• For AC machines measurements are done as phase to phase, phase to earth and rotor winding to earth.
• For DC machines measurements are done as pole to pole, each pole to earth, field to earth and armature to earth.

How insulation resistance of a motor is tested ?

1. Switch off power from main switchboard
2. Take out the fuse
3. Tag the label on the switch board, Don’t start
4. Disconnect the connection from starter
5. Test with megger
6. Insulation Resistance is tested while at hot condition because it is minimum at that time

Difference between earthed and short circuit ?

Earthed 

• A fault in an electric circuit where a conductor come into contact with the ship hull or a metal enclosure. ( leakage of conductor due to insulation failure to ground.)

Short circuit

• A fault condition where a low resistance connection occurs between two points in a circuit, a large current flow will usually occurs. (contact to each other or leakage of line conductor or leakage of line to neutral.)

What are switchboard safety devices ?

• Overload trip
• Preferential trip
• Under voltage trip
• Over voltage trip
• Reverse power relay (No voltage trip)
• Reverse current trip
• Fuse 
• A .C .B( Air circuit breaker )
• Earth lamps.
• Low frequency trip
• Meters
• Synchroscope
• Emergency synchronizing lamp
• Ebonite handrail and Rubber footstep

What are the safeties on switch board ?

• See “Switch Board Safeties on Ships”

What is switch board survey ?

1. Performed during dry-docking. (G.E black out)
2. Check volt meter, ammeter, watt meter all calibrators and accuracy.
3. Check all trips over current trip, reverse power trip, preferential trip.
4. Synchronizing test. (load sharing test)
5. Switch inside each terminal to be checked for insulation resistance min: 1Mega ohms, all contact must be clean.
6. Tightness of loose joint and connection, bus bar support, fuse holder.
7. Check automatic circuit breaker A.C.B and automatic voltage regulator A.V.R.
8. Earth detecting lamp and alarm system.

What are the meters fitted on main switchboard ?

• Ammeter
• Voltmeter
• Wattmeter
• Frequency meter
• Ohm meter
• Synchroscope.

Reference:

Oral Guide by – MIN ZAR TAR

MEO Orals on Marine Electro Technology Function 5- Part 1