Полезная информация для электромехаников / Topic for ETO
.docxTopic for ETO`s interview questions:
Motors : types, construction, principle of working, application depending on type, marine application, motor starters (DOL / star delta / auto-transformer), maintenance (insulation testing / winding resistance / overhaul).
Generator / Alternator : principle, construction, safeties. Brushless alternator.
Marine electrical distribution : basic single line diagram.
High voltage (on ships) : Advantages, safeties, PPE.
Propulsion. Electric propulsion (basic).
Ship’s grounding : insulated neutral, earthed neutral.
Various electrical faults : short circuit, open circuit, earth fault.
Various measuring instruments : Multimeter, insulation tester, leakage clamp meters etc.
Cables : construction, type, rating. How do you determine which cable to choose (type / thickness / rating / length) , if you know the wattage of equipment to be installed, fuse rating and voltage ? High voltage cables.
Semiconductor electronic components : various types, method of testing of diode / transistor etc.
Other topics : PCB, PLC, control tech (PID), frequency control : VFD, soft starter, transformers, sensors (pressure / temperature / level)
Electro Technical Officer Interview Questions
1. What is a phase?
Phase in A.C system is the fraction of time period which has elapsed since current/voltage last passed through zero position of reference. The unit is in secs or radians.
2. 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 a continuing 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.
3. How will you check a diode with multi meter?
Remove or disconnect the diode from the circuit. (If the power is turned off instead of disconnecting the diode, discharge all capacitors and remove all fuses in the circuit). Multi meter drives a small DC current (a few- mA) through the diode P-N junction, and acts as a voltmeter to measure the voltage across it. Connect the two probes across the diode. Read the voltage drop, it should be 0.5V - 0.8V (500 mV to 800 mV) for a healthy silicon diode. Reverse the probe connections and the display should indicate 'over range’. If the display indicates over range in both directions, means the diode is open circuited. If the display indicates less than lV in both directions, diode may be short circuited.
4. Why does a motor rotate?
DC Motor: when field is excited and armature is supplied with current from supply mains, they experience a force tending to rotate armature (direction given by Fleming's left hand rule). Each conductor experiences a force and all these forces collectively produce a driving torque which sets armature rotating.
6. What is an electric shock?
Electric shock is due to flow of current through the body. This is often from hand to hand or hand to foot. A shock current of 15mA AC or DC maybe fatal.
7. 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 5000П at 25V, falling to about 2000П 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.
8. What are the types of switch boards available ?
Switch boards maybe 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, ‘dead front’ type is mandatory' on AC system.
9. Which system of supply is more lethal, AC or DC ?
AC supply is more lethal than DC supply at similar voltages. On no account should work of even the simplest
nature be undertaken on live AC apparatus without the operator being fully protected.
10. What are the precautions to be observed before commencing work on a switch board?
Before any work is done on the electrical equipment, follow the following safety" procedures:
1. there should be an approved rubber matting on the floor, in front of the switchboard, for safety.
2. Rubber gloves should be specially made for the purpose and comply with regulation requirements.
3. portable hand lamps if used should be fully insulated without metal guards so that there is no risk of shock or short circuits.
4. Precautions must be observed to isolate and lock off the apparatus. Where fuses are provided, these should also be removed.
5. Where pilot lights, control circuits are concerned, these may still have power from separate source, even though main power circuit may be isolated.
6. If the draw-out type circuit breaker is being overhauled, it will become dead, when it is pulled out fully.
7. Before commencing work, it should be verified that the apparatus is actually dead using a live line tester. This test should be conducted between phases, and also between phase and earth.
8. A warning sign to be posted indicating the work is progressing in the apparatus.
9. A switch board cannot be considered dead unless all generators connected to it are off and all other supplies disconnected.
11. What do you understand by the term "dead switch board'?
A dead switch board is the one in which there is no supply" of power available, whatsoever.
12. What is the setting of reverse power relay?
• 10% of generator power rating with 5 secs time delay" for diesel driven alternator.
• 2 to 5% of generator power rating with 5 secs time delay" for turbo alternator.
13. What is understood by the term 'Insulation Resistance’?
Insulation resistance is a measure of opposition offered to the current by the insulating materials
14. What are the factors that affect insulation resistance?
Insulation is affected by many factors such as:
1. Moisture - results in a decrease in the measured value of insulation resistance.
2. Dust and dirt deposits - reduces insulation resistance and failure of insulation due to oxidation.
3. Oil and grease - prevents heat dissipation and cause dust and dirt to settle on the insulation, thereby reduces the insulation resistance and leads to failure.
4. Ageing - over a period of time the insulation deteriorates due to it getting affected by temperature variation, mechanical stresses, vibrations, moisture, dirt deposits, chemical contacts etc Some varnishes tend to become hard with age and crack during the operation.
5. Temperature - excessive temperature dehydrates and oxidizes the insulation, making it brittle and disintegrate under vibration and shock.
15. What is understood by the term ‘permissible temperature rise' in case of insulation?
Permissible temperature rise is the difference between the ambient temperature and maximum permitted temperature for the class of insulation in use.
16. What is the relationship between the insulation and the temperature?
The insulation resistance falls with temperature rise. Life of insulation will be halved for even" 10°C above the permitted temperature. For example a machine designed for continuous operation at 70°C will have its useful life cut in half when operating at 8o°C.
17. Why the insulation tests are carried out on electrical equipments?
A measurement of the insulation resistance gives the best guide to the state of health of the electrical equipment. The marine environment is particularly arduous for electrical equipment, due to damp salt laden atmospheres, extreme temperatures and vibrations. The continuous operation of equipment on board ship demands high operating efficiency' and optimum economy in order to keep down costs.
18. What is the most preferred condition for measuring insulation resistance?
Insulation resistance becomes more leaky at high temperatures. So insulation test should be done while the machine is hot just after it has stopped. This will give the realistic reading at near working temperature.
19. What is “megger ?
The instrument used for measuring insulation resistance in Mega Ohms.
20. What is armouring in a cable ?
Extra mechanical protection provided by armouring with woven wire braid made of galvanized steel or tinned phosphor bronze. Steel armouring is not preferred for single core cables carrying heavy alternating currents because of magnetic properties.
21. What is meant by current rating of a cable ?
The passage of current through a cable conductor results in rise of temperature due to resistance of the conductor. Therefore maximum current rating is based upon maximum operating temperature of the insulation and sheathing.
22. 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.
23. 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. Motors 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% (practice it is about 2%)
24. Comment on the requirements of conduits in regard to cable installation
Cable outlets from conduits should be bushed to prevent abrasion at the ends.
Total cross sectional area of cables should be less than 40% of the cross section area on conduit.
Ventilation of conduits should provided, and it should be ensured that water does not collect in the conduit.
The conduit should be earthed.
25. How the cables are grouped or ‘bunched" ?
1. 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%.
2. Where possible, cables intended for different operating temperatures should not be bunched in common clips, glands, conduits or pipes.
26. What are the sources of electrical power on ship?
• Main alternator
• Emergency generator
• Emergency batteries
• Shore power
27. What are the functions of circuit breakers, fuses relays ?
The circuit breakers and switches are the means of controlling the flow of electric current. The fuses and the relays protect the distribution system from large fault currents.
28. What is understood by "essential loads' ?
Essential loads or services are those required for the safety' of personnel and for safe navigation and propulsion of ship.
29. Why emergency source of electrical power is provided?
Emergency source of power supply is provided so that in the event of an total power failure, a supply will still be available for emergency lighting, alarms, communications, water tight doors and services necessary' to maintain safety'.
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, alarm etc are 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- How the emergency power supply is tested ?
Correct functioning of the auto start equipments is obviously vital to the production of emergency7 power. Regular, weekly testing of the emergency7 generator should include simulation of the loss of normal power. The start up equipment may provide a push button to interrupt the normal voltage supply to the panel which then triggers the start sequence. Loss of main power supply can easily be simulated by pulling a fuse in the auto start panel which supplies the under voltage or under frequency7 relay.
Emergency generators should be regularly checked and run up for short test runs to comply with safety7 regulations. These no load running checks should, when practicable, be supplemented occasionally7 by7 a proper load test. This requires the disconnection of normal mains power while the emergency generator is loaded up to near its rated value. Only7 a proper load test will prove the performance of generator and its prime mover, together with the circuit breaker operation.
32. What is the reason for using insulated neutral system in ships ?
The priority7 requirement on board ship is to maintain continuity7 of the electrical supply7 to equipment in the event of a single earth fault occurring.
33. What are the causes for short circuit ?
Short circuit may7 occur because of breakdown of insulation of a equipment due to overheating or ingress of water. Sometimes terminal connections become loose, causing two power supply7 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.
34. Why the equipments are earthed on board the ship?
In order to protect against the dangers of electric shock and fire that may7 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.
35- What are the common locations of earth faults ?
Earth faults are found at lamp fittings and terminals etc., where moisture has found its way in.
1. Lamp fittings on open deck, exposed to rough weather are prone to ingress of moisture.
2. In accommodation, washing machines drenched with water area another source of earth faults.
3. In galley, the ovens, hot plates etc., are also exposed to earth faults.
4. Dripping water over electrical machinery.
5. Motors standing idle for long time - their insulation gets affected by condensed moisture.
6. Overheated insulations of motor and cables cause drop in insulation resistance.
7. Dirty electrical apparatus with surface tracking (leaking current)
8. Old electrical apparatus whose insuslation has aged has become brittle.
9. Terminal wire which has become loose from the terminal and is touching the metal casing of electrical machinery.
36. 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 overheating must be made good again. If surface dirt is the cause, a thorough cleaning with electro cleaner solvent will probably cure the fault.
37. What is the rating of a fuse ?
Fuse rating is the current fuse will carry' continuously and it is important that rating is correct for the normal current flowing in the circuit it protects.
38. What are the reasons for installing fault protection in electrical distribution system?
1. To disconnect and isolate faulty equipment in order to maintain the power supply to the remaining healthy circuits in the system.
2. To prevent damage to equipment from thermal and magnetic forces that occur during short circuit and overload faults.
3. To protect personnel from electric shock.
39. How is the strength of circuit breaker or fuse is specified ?
By its breaking capacity, which is the maximum fault current it can safely interrupt.
40. Why HRC fuses are normally preferred for Electrical installations on board ship?
• Time/current fusing characteristics are very consistent.
• HRC fuses are reliable and non deteriorating in service
• Fuses have very high breaking capacity.
• They also have very high speed of operation at high short circuit fault, faster than a circuit breaker.
41. 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 were 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. For motor, fuses are typically rated at 2-3 times mot full load.
42. How the overload protection device is tested and set?
Overload relays and circuit breakers should be tested periodically. Bimetal elements in thermal overload relays become insensitive to heat from metal fatigue and fail to operate under overload condition. These relays should be checked for corrosion, accumulation of dirt and other foreign matter that block free movement of tripping element.
Over current relays and circuit breakers may be tested by passing a specified over current through the heater or overload coil and observing the time required for it to trip. The actual tripping time should then be compared with the manufacturer’s current time characteristics.
Test an overload rely, it should be disconnected from the power line and connected to low voltage, high current power source like variable autotransformer. Current is adjusted to test value by connecting to one of the bimetal heaters via heavy test leads. A stop watch is used to determine tripping time.
43- What is under voltage protection for Generator and Motor?
An under voltage release is fitted to all generator's circuit breaker. Its main function is to trip the circuit breaker when severe voltage dip occurs. The under voltage release on a generator circuit breaker also prevents it being closed when the generator voltage is very low or absent. The under voltage relay prevents the closure of the circuit breaker of the dead generator.
Under voltage protection is also required for motor starters. The starter contactor normally provides this protection as it drops out when the supply voltage is lost or is drastically reduced. The starter circuit will not normally allow the motor to restart when the voltage supply is restored, except when special automatic restarting facilities are provided. This ensures that when the supply returns after a black out, all the motors do not start together, there by tripping the generator circuit breaker again..
Essential motors may be arranged to start automatically by sequential start with staggered time delays.
44. 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 : 1. 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.
45. How does a current flow in the shaft of an generator ?
Machines with a very strong magnetic field often generates a voltage in the shaft that causes a current flow through bearings.
46. What are the effects of shaft current in a generator?
Flow of electric current through the bearings causes electrolytic action, which will cause bearing failure in due course and possible sludging of lubricating oil.
47. How the shaft currents are prevented ?
To eliminate shaft currents either (a) outboard bearing shell is insulated in the housing, or (b) outboard bearing pedestal is insulated from the foundation.
All lub oil pipe connections that make connection with the shaft or the particular bearing must also be insulated.
48. Why both bearings are not insulated?
Insulating both bearings would permit the build up of electrical charges (static electricity). The resulting high voltage will be dangerous to operating personnel.
49. How the insulation of pedestal bearing checked?
1. The pedestal insulation of a dismantled generator may be tested with a megger. Insulation of 20 кVolt and above is considered satisfactory
2. Condition of insulation can also be checked while machine is running by measuring milli-volts 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).
50. How is the insulation resistance of alternator 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 minimum value is lMO, but a lower value may be acceptable to surveyor based on lkO/volt, e.g. 450 кП for a 450 volt generator.
51. What is the purpose of shading coil in a contactor?
To reduce vibration, chattering and noise.
52. What is the purpose of ICCP? (impressed current cathodic protection)
To prevent corrosion of the hull, propeller, rudder and line shafting
53. What is the supply given to shipboard general alarm?
24V DC.
54. When a component is to be removed from a PCB, what precaution will be taken?
Before desoldering the component a tightly knit metal braid wire will be used.
55- Armature coils are tested with a low voltage source. What will be the indication for short circuited coils?
For short circuited coils, the voltmeter reading will show low or zero, other coils will have higher readings.
56. What do you understand by the term ‘varnished cambric’?
This is an insulating material made of cotton cloth dipped (or coated) with insulating varnish.
57. What is understood by ‘dead front’ switchboard?
Dead front switchboard is one with insulated switches and no exposed terminals.
58. What is the unity power factor?
• The voltage and current developed in an AC circuit reach their peak values at the same time.
• The voltage and current are said to be in phase when power factor is unity.
59. Under what circumstances would semi-conductor (or high speed fuses) be used on a soft starter application? What purpose do they serve?
Semiconductor fuses are an energy limiting fuse using specially shaped silver elements in a silicon sand environment. Because of their energy limiting characteristics, they are able to protect semiconductor devices from excess energy let through under fault current conditions.
A soft starter uses reverse parallel connected SCRs or thyristors in series with the supply to the motor. If a short circuit occurs between the starter and the motor, or in the motor itself, there is no limiting impedance to restrict the current flow and so the current is essentially the short circuit current of the supply. In most cases, this is enough to damage or destroy the SCRs. It is not practical to turn an SCR off once current has begun to flow, so electronic protection is not practical. Current will stop at the next zero crossing which could be half a cycle later. The semiconductor fuse is able to interupt the current flow in less than half a cycle, and the rupture time is dependant on the total energy flow.
SCRs have a maximum short term energy rating that is usually called I squared t (Current squared times time) and provided the maximum let through l2t (or total clearing l2t) of the fuse at the operating voltage of the supply, is less than the l2t of the SCRs, then the fuse should fail before the SCRs.
Semiconductor fuses can be a bit of a problem at times because their pre arcing l2t is well below their total clearing l2t and they can be stressed by the normal operation of the starter and fail prematurely. It is important to compare the overload time current curve of the fuse with the operation of the starter.
60. What is slip?
The difference between the speed of the rotating magnetic field (which is always synchronous) and the rotor in a non- svnchronous induction motor is known as slip and is expressed as a percentage of a synchronous speed. Slip generally increase with an increase in torque.