MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)

SUMMER-2024 EXAMINATION
Model Answer – Only for the Use of RAC Assessors

Subject Name: Maintenance of Electrical Equipment Subject Code: 22625

Important Instructions to examiners: XXXXX
1) The answers should be examined by key words and not as word-to-word as given in the model answer
scheme.
2) The model answer and the answer written by candidate may vary but the examiner may try to assess the
understanding level of the candidate.
3) The language errors such as grammatical, spelling errors should not be given more Importance (Not
applicable for subject English and Communication Skills.
4) While assessing figures, examiner may give credit for principal components indicated in the figure. The
figures drawn by candidate and model answer may vary. The examiner may give credit for any equivalent
figure drawn.
5) Credits may be given step wise for numerical problems. In some cases, the assumed constant values
may vary and there may be some difference in the candidate’s answers and model answer.
6) In case of some questions credit may be given by judgement on part of examiner of relevant answer
based on candidate’s understanding.
7) For programming language papers, credit may be given to any other program based on equivalent
concept.
8) As per the policy decision of Maharashtra State Government, teaching in English/Marathi and Bilingual
(English + Marathi) medium is introduced at first year of AICTE diploma Programme from academic year
2021-2022. Hence if the students write answers in Marathi or bilingual language (English +Marathi), the
Examiner shall consider the same and assess the answer based on matching of concepts with model
answer.

Q. Sub Answer Marking

No. Q. Scheme
N.
1. Attempt any FIVE of the following: 10 Marks
a) State the reasons for electrical accidents may happen. ½ Mark for
Ans a) Unsafe Work Practices: Ignoring safety procedures, such as working on live circuits each of any
. without proper training or equipment, can lead to accidents. four Points
b) Faulty Equipment Safety tools & Accessories.: Defective or poorly maintained = 2 Marks
electrical equipment, such as frayed wires, damaged outlets, or malfunctioning
appliances, safety tools & accessories can cause accidents.
c) Overloading Circuits: Plugging too many devices into a single outlet or circuit can
overload it, leading to overheating, sparks, or even fires.
d) Damaged Insulation: Worn or damaged insulation on electrical wires can expose
live wires, increasing the risk of electric shock or short circuits.
e) Water Contact: Water is a conductor of electricity, so contact between electrical
components and water can lead to short circuits, electrical shocks, and even
electrocution.
f) Improper Installation: Incorrect installation of electrical systems, such as incorrect
wiring or insufficient grounding, can create hazards.
g) Lack of Ground Fault Circuit Interrupters or protective devices or earthing
System: GFCIs are designed to quickly interrupt the flow of electricity when a
ground fault is detected, preventing electric shocks. Absence ground fault circuit
protection, especially in wet or damp environments like bathrooms or kitchens,
increases the risk of accidents.
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(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)

h) Inadequate Training: Insufficient training or lack of awareness about electrical

safety practices among workers can lead to accidents.
i) Natural Causes: Lightning strikes, power surges, or other natural phenomena can
cause electrical accidents, especially if proper precautions are not in place.
(OR Equivalent Answer / Points )
b) State the meaning of type test. Give one example of type test. 1 Mark for
Ans These tests are carried out on 2 or 3 randomly selected machines from the lot / batch of Meaning of
. the manufactured machines of same design and specification. The test results of the few test & 1
tested machines are treated same, for the complete lot of the machine. mark for
For example as below example
1. Temperature-rise type test. = 2 Marks
2. Dielectric type tests
3. Momentary overload test
4. Full load Test
5. Impulse Voltage Test
6. Induced voltage test of transformer or DVDF (Double voltage double frequency)
(OR Equivalent Answer / Points)
c) State any four advantages of predictive maintenance. ½ Mark for
Ans 1. Longer lifespan of equipment: Data-driven predictive maintenance reduces wear and each of any
. tear, minimizes malfunctions, and allows maintenance teams to act quickly when four Points
abnormality occurs. = 2 Marks
2. Lower maintenance costs: predictive maintenance helps organizations avoid costly
repairs, emergency maintenance expenses, and unplanned downtime.
3. Less downtime: Predictive maintenance empowers engineering teams to avoid
unplanned downtime while also reducing planned downtime.
4. Enhanced routine maintenance activities: It does this by providing data-driven fault
detection and diagnostics capabilities with an advanced and increasingly accurate
assessment of equipment performance & enhance routine maintenance activities
5. Improved Efficiency: Predictive maintenance optimizes maintenance activities by
focusing resources and efforts on critical assets that require attention based on their
actual condition
6. Increased Equipment Uptime: Predictive maintenance allows for the early detection
of potential equipment failures by analyzing data collected from sensors and monitoring
systems.
7. Safety and Risk Mitigation: Proactively identifying and addressing potential
equipment failures through predictive maintenance helps mitigate safety risks associated
with equipment malfunctions or breakdowns.
8. Increased Equipment Reliability and Availability: This enables them to take
proactive measures to address issues before they lead to equipment failures, thereby
improving overall reliability and availability of critical assets.
(OR Equivalent Answer / Points)
d) List out factors affecting on life of insulating material. ½ Mark for
Ans 1. Water: If insulation is near water for the long period than its life reduces. each of any
. 2. Moisture: If insulation contains moisture for the long period than its life reduces. four Points
3. High Temperature: Due to over loading insulation gets heated than its life reduces = 2 Marks
4. Mechanical Stress: Any mechanical stress on insulation for the long period that its
life reduces
5. High voltage stress: If insulation is used other than designed for voltage than there
will be high voltage stresses it may reduces life of insulation.
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(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)

6. Dirt & Dust Particles: If dirt & dust particles accumulated on insulation than it will
absorb moisture in the air which will reduces the insulation resistance s its may cause the
failure of insulation
7. Improper Handling: If it is handle roughly than it may damage.
8. Ageing: After a long period it„s dielectric strength reduces.
9. Effect of oxygen & humidity:- Some organic or inorganic material decomposes
presence in moisture & oxygen rubber oxidized and cracks when exposed to light reduce
life of insulation.
10. Chemical action:- In the soil due to chemical action, it causes corrosion of
insulation. It will deteriorates insulation material reduces life of insulation.
(OR Equivalent Answer / Points)
e) List the different methods of drying of insulation. ½ Mark for
A) By external heat methods: each of any
1) Circulating hot oil with high vacuum filter machine. four Points
2) Circulating hot air = 2 Marks
3) Hot oil spray under vacuum.
4) Heating by induction coil.( Induction heating method)
(a) Baking in the oven : It is conveniently carried out by placing the machine, having
winding. The winding of machine with varnish is dried in baking oven at constant
temperature. The temperature is kept constant with the help of thermostatic control.
(b) By using boiler : The steam from the boiler heats up the top. of machine. The heat
raises the temperature of the windings and dries the windings. The temperature of the
windings is measured by thermometers attached to the coils should not exceed 90°C.
(c) By electric lamps, heaters : Electric heaters are kept surrounding the machine to
dry the insulation. Sometimes the heat of 200 watt, 500 watt lamp is used to dry the
insulation. The drying takes place through radiation of heat. Hot air furnace can be used
for bring the insulation.
B) By Internal heat methods:
1. In this method a correct amount of D.C current is being circulated through
Stator or armature or field winding whichever is available. This circulated Dc
current generate heat due to I2R effect. This generated heat is being utilized
for drying the insulation.
2. By short circuiting the LV winding & applying reduced voltage to the HV
winding. This process is continued till moisture in oil gets
removed/evaporated
(OR Equivalent Answer / Points)
f) State any four objectives of preventive maintenance. ½ Mark for
Ans 1. To keep the plant in good working condition at the lowest possible cost. each of any
. 2. To determine the need for major & minor repairs. four Points
3. To avoid unnecessary production loss = 2 Marks
4. To reduce loss in production time.
5. To provide greater safety & protection to the workers.
6. To increase life of machine/equipment.
7. To prevent premature failure.
8. To maintain the accuracy of the plant equipment.
9. To avoid direct loss of profit.
10. To avoid need for over-time.
11. To avoid rescheduling of production.
12. There will be energy saving if equipment or machine is well maintained
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(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)

13. To use less standby equipment

14. To avoid major breakdown or fault.
(OR Equivalent Answer / Points)
g) List the different methods of testing of electrical equipment with its one example. 1 Mark for
Ans 1) Direct loading test: Break load test on induction motor or direct loading on each of any
. transformer or Alternator. two points
2) Indirect loading test: Blocked rotor & No-load test on induction motor or OC & SC = 2 Marks
test on Transformer or alternator.
3) Regenerative test: Hopkinson's test or back to back.
(OR Equivalent Answer / Points)
Q. 2 Attempt any THREE of the following: 12 Marks
a) List out internal and external causes of failure of transformer. ½ Mark for
Ans  Internal Faults Causes : each of any
. 1. Insulation break down between winding & earth four points
2. Insulation breaks down between different phases. = 2 Marks
3. Insulation breaks down between adjacent turns i.e. inter-turn fault.
4. Open circuit (either in H.V or L.V)
5. Short circuit (between in H.V and L.V)
6. Ground fault (between L.V. and core)
7. Ground fault (between L.V / H.V.or and supporting structure)
8. Shorted turns (either in H.V or L.V)
9. Presence of moisture in transformer oil.
10. Failure of magnetic circuit
11. Transformer core fault.
 Externals Faults Causes:-
1. External short circuit, the short circuit may occurs in two or three phases of
electrical power system.
2. High voltage disturbance
3. Sustained Power frequency over voltage
4. Lighting Surges
5. Switching Surges (There may be always a chance of system over voltage due to
sudden disconnected of large load.)
6. Arcing Grounds
7. Travelling Waves
8. Sudden Changes in system condition
9. Resonance
10. Under frequency effect in power transformer: If frequency reduces in a system
the flux in the core increases (ɸ α v/f) it causes similar effect that of the over voltage.
(OR Equivalent Answer / Points)
b) Prepare the trouble shooting chart (Probable reasons & remedies) for single phase 1 Mark for
Ans induction motor. each of any
. Trouble shooting Chart for 1Φ Induction Motor: four trouble
Sr. Type of fault / trouble Causes Remedies with cause
No. abnormal conditions and remedy
1 Motor fails to start 1. Blowing of fuse/tripping of MCB. Rectify & = 4 Marks
or not accelerate Faults in 2. Terminal voltage is too low. repair the
starting supply circuit/ 3. Defective starting mechanism. cause
Motor switch ON but does 4. Tripping of protection device.

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(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)

not start. 5. The motor controller is not operating.

6. Open circuit in supply cable.
7. Overloaded.
8. Loose connections of motor leads.
9. Motor rotor or driven load is locked.
10. Bearing is seized.
11. Open circuit in auxiliary winding.
12. Open circuit in main winding.
13. Opening of capacitor.
14. Shorting of capacitor
2 Motor stalls 1. Terminal voltage is toolow. Rectify &
2. Overload repair the
3. Wrong application. cause
3 Motor does not 1. Starting load is too high Rectify &
pick 2. Low supply voltage. repair the
Up rated speed 3. Low supply frequency. cause
4. Broken rotor bars or loose rotor.
4 Runs slow(Motor starts 1. Low supply voltage. Rectify &
sluggishly) 2. Low supply frequency. repair the
3. Overload. cause
4. Shorted stator coils.
5 Motor run shot/ Motor 1. Over/Under voltage. Rectify &
overheats / Winding 2. Unbalanced voltage repair the
overheating 3. Over/Under frequency cause
4. High ambient temperature
5. Ventilating fan is not working
6. Poor motor ventilation /Airflow
obstructed or inadequate
ventilation.
7. Overload.
8. Worn bearings.
6 Motor vibrates 1. Loose fitting of iron core. Rectify &
2. Dynamic unbalance of the rotor. repair the
3. Misalignment of rotor. cause
4. Bent shaft.
5. Run out of shaft.
6. Warn out bearings.
7. In correct leveling.
8. Loose foundation bolts.
7 Noisy operation of motor 1. Misalignment of rotor Rectify &
2. Bent shaft. repair the
3. Run out of shaft. cause
4. Improper fitting of bearing.
5. Warn out bearings.
6. Incorrect leveling.
7. Loose foundation bolts.
8. Cooling fan is touching with
stationary part.
9. Non-uniform air gap or rotor
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(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)

rubbing on stator.
10. Unbalancing of rotor.
11. Faulty gears and gear trains.
8 Bearing failure 1. Misalignment of rotor. Rectify &
2. Due to bent shaft/Run out shaft repair the
3. Bearings not properly fitted. cause
4. Warn out bearings.
5. Overloaded bearing.
6. Use of poor quality of
grease/lubricating oil
7. No greasing/lubricating oiling.
c) List out the different fire extinguisher & state the advantages of CO2 fire 2 Mark for
extinguisher. Types & 2
When it comes to the types of fire extinguishers, there are five main types including Marks for
1. wet chemical, 4
2. CO2 advantages
3. dry powder, = 4 Marks
4. foam
5. water.
 The advantages of CO2 fire extinguishers include:
1) Effective on flammable liquids and electrical equipment.
2) Leaves no residue, making it suitable for sensitive equipment and environments.
3) Non-conductive and non-corrosive, minimizing damage to electrical equipment.
4) Can be used in enclosed spaces without displacing oxygen.
5) Fast extinguishing action and minimal cleanup required after use.
6) CO2 is a natural gas and does not contribute to ozone depletion or leave harmful
residues.
d) Draw neat diagram and explain the polarity test on single phase transformer. 2 Marks
Ans Polarity test is must for transformers when parallel operation is done. Because while For
. doing parallel operation, if you connect terminals of opposite polarity, it will result in a Diagram &
dead short - circuit. So, to connect the same polarity windings together both in primary 2 marks for
and secondary, polarity test is done. explanation
Polarity test: It is essential to know the relative polarity of primary and secondary of test = 4
terminals, at any instant for making correct connections, when the two transformers are Marks
to be connected in parallel to share the load on the system.
Both .A.C. and D.C methods can be used for detecting the polarities of the induced
EME‟s. The dot method is used to indicate the polarities.
1) A.C. method
Circuit Diagram:

Fig: Polarity test on transformer by AC Voltage Method.

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(ISO/IEC - 27001 - 2013 Certified)

The rated voltage (V1) is applied to the primary and its two terminals are marked as A1
and A2, respectively, as shown in figure.
• The secondary winding terminals are also marked as a1 and a2, as shown in figure and
the voltage measured across it will be V2.
• Now a voltmeter V3 is connected across A2 and a2. if it measures the difference of V1
and V2, Then the A2 and a2 are of the
same polarity.
• If a voltmeter V3 measures the addition of V1 and V2 then the A2 and a2 are of
opposite polarity.
(OR Equivalent Answer / Points)
2) D.C method
Circuit Diagram:

Fig: Polarity test on transformer by DC Voltage Method.

1. Fig. shows the D.C method of testing the polarity.

2. When the switch S is closed if the pointer of voltmeter shows momentary deflection
(kick) towards positive reading the assumed polarity is correct.
3. If the pointer shows a kick towards negative side (below zero), the assumed polarity is
wrong.
3 Attempt any THREE of the following: 12 Marks
a) Describe the various factors affecting preventive maintenance schedule. 1 mark for
 Following factors affects the preventive maintenance schedule:- each (4
marks)
1. Non availability of spares & raw material required for preventive maintenance (PM)
2. Non availability of tools, trackless, jacks, fixture required for PM.
3. Non availability of trained & skilled technician.
4. If the machine is continuously in working condition due production requirement it
affects PMschedule.
5. Ageing of machine/equipment (If the breakdown takes place, the cost of the repair will
be morethan the cost of the machine.)
6. Cost of the maintenance affects PM schedule.
7. Machine and its Importance.
8. Due to accident, fires, worker strike the work is held up for certain period. This is also
a cause ofdisturbing a PM schedule.
Explain the need & steps to conduct the reduced voltage running up test on the
three phase induction motor.
 Need of reduced voltage running up test on the three phase induction motor.
1. To determine the ability of motor to run equal and nearly equal to rated speed of the
motor even at reduced voltage in the both directions – forward & reverse.
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2. To see whether there is any tendency of cogging & crawling presents in the motor.
3. This test is conducted to check the noisy running of motor.
4. To see whether, if noise level is more than tolerance limit which may be because of
damaged bearings, also presence of loose bars & wrong connection of stator winding.
 Steps to conduct reduced voltage running up test on the three phase
induction motor.
1. The motor up to 37 kW shall be supplied with reduced voltage 1 / √3 of rated value for
each direction of rotation with the help of auto transformer.
2. For motors above 37 kW, the voltage shall be 1/ √3 of rated value but motor shall be
run only in the specified direction of rotation with the help of auto transformer.
3. The speed in this case is also recorded. In both the cases, the speed should be equal and
nearly equal to rated speed of the motor.

OR Equivalent Answer / Points

c) Lists out Do’s & Don’ts to achieve safety for electrical supervisor while working in 1 Mark
Ans substation (any four points) each
.  Dos for Electrical Supervisors:

1. Work on low & medium voltage mains and apparatus should be carried only
by authorized person(s) and all mains and apparatus to be worked upon shall be
isolated from all sources of supply before starting the work.
2. Warning boards shall be attached to or kept adjacent to the line apparatus and
the limit of the zone, in which work may be carried out, should be specifically
indicated.
3. Ensure that all the safety apparatus such as rubber mats, stool, platforms or
other safety devices tobe used, should be in good condition.
4. When any live mains are to be earthed, the procedure prescribed should be
scrupulously followed.
5. Maintenance on H.T. Breaker should be attempted only when it is fully
isolated and withdrawn.
6. No work should be done on the transformers unless, it is disconnected from
all external electricalcircuits and all windings have been solidly earthed.
7. De - energize incoming power before removing top and side access or cover
plates of any bus cover. Lock out the incoming power source. Earth the main
horizontal bus before working on the bus.
8. Check voltage, if any, by multi meter / test lamp so as to be sure that circuit
breaker / switches areopen or that the bus is de - energized.
9. Isolate all remote-control voltage sources when working on the board.

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10. Pad - lock the breakers with isolated position and test before working on a
branch circuit.
11. Follow proper Electrical Isolation procedures to avoid in - advertent
activation of any Electricalequipment i.e., put "Men at Work" tally on the isolated
circuit. Remove the fuses etc. before working.
12. Obey warnings to stay away from electrical circuits and locked - out
equipment.
13. Wear rubber gloves and any other assigned protective clothing and safety
equipment whileworking.
14. Inspect electrical tools before each use.
15. Keep all electrical circuit contact points enclosed.
16. Do use a quality surge suppresser with enough sockets for every component.
17. Do look out for overhead power lines every time you use an insulated ladder
or pole. Stay at least10 feet away from any electrical lines.
18. When in doubt, seek help from experts and advice to the juniors.
19. Keep away any chemicals which are compatible from electrical panels etc.
20. Follow manufacturers‟ recommendations and requirements while working
on that machine /equipment.
21. Eliminate all potential tripping hazards in the work area.
22. If any device / machine emits an unusual odor, turn it off and unplug it
immediately. Do not usethe device / machine until it is repaired.
23. Wear safety glasses while working.
24. If you see that cables of electrical device are worn or frayed, make sure you
replace it as soon aspossible.
25. Ensure every rotating / static machine has been installed properly and is stable.
26. Employers should control any remaining risk by providing the worker /
operator with the necessary information, instruction, training, supervision and
appropriate safety equipment.
27. Ensure control switches are clearly marked to show what they do.
28. Have emergency stop controls where necessary.
29. If machines are controlled by programmable electronic systems, changes Made in
any programme should be carried out by a competent person to all the concern
persons.
30. Know the work content and work sequentially.
31. Place sign “Men are working” or other warning boards on the Main Switch before
working.
32. Cultivate the habit of turning your face away whenever an electric arc or flash
may occur.
33. Guard against arcs as well as high voltages, remember that burns from arcs may
be very serious.
34. Take extreme care when breaking an inductive circuit as dangerously high
voltage is likely toresult.
35. Thoroughly discharge to earth all the cables before working on their cores.
36. Always treat the circuit alive until you have proved them to be dead.
37. Test insulating rubber glows periodically.
38. Place rubber mats in front of electrical switching panels.

 Don‟ts for Electrical Supervisors:

1. Do not touch a person who is in direct contact with live electrical conductors. By this,
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(Autonomous)
(ISO/IEC - 27001 - 2013 Certified)

you can receive serious shock too.

2. In hazardous area, avoid direct contact between explosive mixture and means
of ignition.
3. Do not inadvertently activate any electrical equipment. Follow standard
electrical isolating procedure to avoid inadvertent activation on any electrical
equipment.
4. Do not panic in case of power failure. Emergency lighting have been provided
in all working areas in company through Diesel Generating Sets which start
automatically within 10 - 15 seconds of the power failure. Stay calm until then
5. Do not work alone in main switch or feeder panel enclosure.
6. Never leave electrical equipment without properly earthed.
7. Do not put in use any piece of electrical equipment that gives a tingling
sensation when touched. This is defective. Inform about it to your supervisor
immediately.
8. No live part should be within unsafe distance of a person working on live low
and medium voltage mains, so that he does not come in contact with it unless he
is properly protected.
9. Do not touch or temper with any electrical gear or conductor, unless you have
made sure that it is dead and earthed. High voltage apparatus may give leakage
shock or flash over even without touching.
10. Do not disconnect earthing connections.
11. Do not expose your eyes to an electric arc. Painful injury may result even
with short exposure.
12. Do not close or open a switch or fuse slowly or hesitatingly, do it quickly
and positively.
13. Do not use metal case flash light around apparatus which is energized.
14. Do not get closer than 10 feet to a power line (if you‟re an unqualified
employee).
15. Do not reach blindly into a space that may contain energized equipment.
16. Do not use a power tool that smokes, sparks, smells, or shocks.
17. Do not store liquids of any sort near electrical equipment.
18. Do not assume the black coating on wires is insulation – it could be just
plastic weatherproofingthat provides no protection from contact injuries.
19. Do not use damaged or brittle electrical cords, even if bare wires aren‟t
visible.
20. Do not allow untrained, unqualified staff to handle key maintenance or
inspection tasks.
21. Do not undertake maintenance tasks in an arbitrary order. You need a way to
rank electrical components in order of how critical they are and then follow that
order logically.
22. Do not take a run - to - failure attitude to electrical parts. You should replace
them before they become inefficient and potentially dangerous.
23. Do not renew a blown fuse until you are satisfied as to the cause and you
have rectified the irregularity.
24. Do not close any Switch / GOS / Breaker unless you are familiar with the
circuit, which it controlsand know the reason for its being kept open.
25.Do not work on energized circuits without taking extra precautions as use of
rubber gloves and gauntlets.
26. Do not touch or tamper with any electrical equipment or conductor, unless
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you have made surethat it is dead and earthed.

27. Do not work on the live circuit without the specific orders of the supervisor
and make certain thatall safety precautions have been taken.
28. Do not disconnect earthing connection or render ineffective safety gadgets
installed on mains.
29. DO not use fire extinguisher on electrical equipment unless it is clearly
marked for that purpose.
30. DO not throw water on live electrical equipment in case of fire.
31. DO not remove danger notice plates or other signs or interface with safety
barriers or go beyondthem.
32. DO not allow visitors and un-authorized persons to touch or handle electrical
apparatus or comewithin the danger zone of H.V. apparatus.
d) What are the factors to be considered in designing of the machine foundation.
 Factors to be considered in designing of the machine foundation
Foundations help in static or dynamic load of the machines to the ground. For
designing the foundation for the machine the factors like weight of the machine 1 marks
, its H.P capacity, speed of rotation, type of drive, soil conditions, etc. must be for each
taken into consideration . Machine foundations are not based on any definite type
calculations, but they are dependent on practical , industrial experience and the (4 marks)
data available. The machine foundation should satisfy the following
requirements.
1. The foundation should be horizontal in level.
2. The foundation must be rigid.
3. It should be free from vibrations.
4. It should transmit both static and dynamic weight of the machine to ground
5. The foundation should be free from defects like cavities, pits, surface cracks etc.
6. The foundation should be made from concrete mix i.e. cement, sand water in
definiteproportion.
7. The design of foundation should ensure against sinking of foundation by distributing
theload over as much as soil area, in relation with soil bearing capacity.
8. The foundation should not slide over the soil due to horizontal thrust of running
machine. This is achieved by sufficient frictional resistance between foundation
błock and soil.
9. The foundation should be so designed that the resultant force posses within the
block toprevent turning of the machine.
4 Attempt ant THREE of the following: 12 marks
a) State & Explain the properties of transformer oil (any four) 1 marks
Ans 1. Dielectric strength or breakdown voltage (BDV):The transformer oil should for each
. have high dielectric strength not less than 40kV (rms) in drums and 30kV type
(rms) in the tank for gap of 4mm of electrodes. (4 marks)
2. Water Content: Moisture or water content in transformer oil is highly
undesirable as it affects the dielectric properties of the oil adversely. Water
content is expressed as Particles Per Million (PPM) and its permissible value
is 50PPM.
3. Acidity Content: The acidity content value should be equal to or less than
0.4mg of KOH / gm. The oil should be reconditioned if the value of acidity is
0.5 to 1 mg of KOH / gm.
4. Flash Point: It is desirable to have high flash point of transformer oil. In
general, it is more than 160 c.
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5. Fire Point: The temperature at which an oil will ignite and continues for
burning is the fire point. This should be about 25% above the flash point so it
should be at least of 200 c.
6. Chemical Stability: The oil should be chemically stable i. e. should not be
affected by chemicals.
7. Purity: The oil must not contain impurities such as sulphur and its compounds.
Sulphur whenpresent, causes corrosion of metal parts.
8. Viscosity: Good transformer oil should have a low viscosity so that it offers
less resistance tothe conventional flow of oil thereby not affecting the cooling
of a transformer.
9. Sludging: The remedy is use oil which will not contain Sulphur & which
remains without sludge formation for long period.
10. Density: This indicates the mass of substance per unit volume. As per IS, this
should be 0.89gm/cm3 maximum.
11. Appearance: The oil should be perfectly clear and has pale clear yellow
colour, transparentand free from suspended matter of sediments.
Pour Point: It is the lowest temperature expressed on a multiple of 3o at which the oil
isobserved to flow when cooled. The oil should have high pour point at least 9c.
b) With the help of neat diagram explain the phasing out test on transformer. Diagram 2
Marks and
Explanatio
n 2 Marks
(04 Marks)

1. Short primary & secondary winding ofother phases expect the one
under test.
2. Connect voltmeter to secondarywinding.
3. A small DC current is circulated through the primary winding through
switch.
4. Now with the help of switch interrupt the DC supply instantly &
repeatedly. If voltmeter indicator deflects than it indicates the two
windings concerned belong to the same phase. If not deflect then two
windingsare not belong to same phase.
5. Repeat the procedure by connecting voltmeter to secondary side to next
secondary windingtill voltmeter gives deflection. In this way we can search the
phasing out.
OR Equivalent Answer / Points)
c) State the function of following maintenance tools. 1 Marks for
i. Bearing Puller each (04
ii. Dial Tester Marks)
iii. Growler
iv. Filler Guage
 Function of maintenance tools -
i. Bearing Puller - For taking out and refitting of bearings.
ii. Dial Tester - Checking shaped, alignment of shaft
iii. Growler - For finding the shorted turns of coils.
iv. Filler Gauge - For checking the air-gap between two parts.
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d) What are the causes of fire due to electric faults? And what precautions to be taken 2 marks
to avoid fire due to electric reasons. for causes
 Following are the Causes of electrical Fires:- & 2 marks
1. Overloading on cables/wires/machine for long period, increases for
temperature, causes possibilityof fire. precaution
2. The majority of fires are caused due to selection of incorrect rating of the s (04
fuses or use of incorrect rating MCB/RCCB/MCCB or incorrect setting of Marks)
safety switch.3
3. Poor joints in wiring/cables may cause overheating & lead to fire.
4. Due to loose connection in the electrical installation may produces spark
causes fire.
4. Stored highly flammable liquids near electric oven/furnace. 6. Kept electric
heaters nearcurtains/furniture.
5. If HV/equipment clearances are not maintain as per voltage level, there is
possibility of sparkingleads to fire.
6. If insulation damage/deterioration, a short circuit may occur causing fire.
7. Electrical faults inside appliances are a common cause of electric fire.
8. Faulty electrical installation may cause fire.
9. Due to old wiring and unsafe appliances may cause fire.
10. Electrical installation & equipment's used in hazards area are not used as per the
specification/type of protection.
 Following are the precautions to be taken to avoid fire due to electric reasons.

1. Use correct rating fuse and MCB.

2. Follow IE Rules while working with every electrical installation.
3. Use ISI Mark material / equipment.
4. Provide sound and proper Earthing.
5. Use correct rating wires, cables, auxiliaries etc.
6. Carry out regular and proper maintenance.
7. Keep heat - producing appliances unplugged when not in use.
8. Don‟t ignore signs of trouble like as burn marks/discoloration around a
socket, burning smell when an item is plugged in or in use, electrical sparks each
time you plug in adevice, one (or all) of your breakers frequently trips or your
fuses regularly blow etc.
9. Keep always ready and use proper fire fighting equipment.
10. Avoid poor joints and loose connections.
11. Provide proper protective gears/system.
12. Replace or repair loose or frayed cords on all electrical devices.
13. Avoid running extension cords across doorways or under carpets.
14. Follow manufacturers instructions for operating every electrical machine /
device .
15. Consider having additional circuits or outlets added by a qualified
electrician so you do nothave to use extension cords.
Avoid overloading outlets. Plug only one high-wattage appliance into each receptacle
outlet at a time.
e) Distinguish between O. C. test & S. C. test performed on transformer. (any four 1 Mark for
points) each (04
O. C. test S. C. test Marks)
The secondary side is OPEN The secondary side is shorted,
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Rated Voltage is applied Small Voltage is applied

Very small current is flowing Full load current is flowing
It will give iron losses It will give Copper losses
performed to find out the shunt or no load
performed to find series branch
branch parameters of equivalent circuit of
parameters of an equivalent circuit such
a transformer. as equivalent impedance (Zo1 or Zo2),
total winding resistance (Ro1 or Ro2) and
total leakage reactance (Xo1 or Xo2).
HV winding is open or Voltmeter is LV winding is shorted by a thick wire
connected across it.
Supply is given LV side (as primary). Supply is given HV side (as primary).

Any other appropriate point

5 Attempt any TWO of the following: 12 Marks
a) Compare the difference between routine, preventive and breakdown maintenance. 1 Mark for
Parameter Routine Preventive Breakdown any six
Maintenance Maintenance Maintenance points
Definition Maintenance Maintenance carried Maintenance
carried daily for not out as a precaution carried after
to have any before the occurrence breakdown / fault
breakdown of of the fault. intended to repair
equipment. equipment.
When to Almost daily Before fault After fault /
Execute? breakdown.
Objective Includes visual Regular inspection, To bring back the
inspection, checking, cleaning faulty machine into
cleaning, minor etc service at the
repairs in initial earliest.
stage.
Nature of It is a partial planed It is fully planned It is not a planned
Schedule schedule. schedule. schedule. Carried
after breakdown.
Scope for Delay There is scope for No scope for delay This cannot be
delay sometimes / delayed at any cost
delay of one / two
days
Staff Involved This work is carried Expert / skilled staff This work is carried
by operator of is required by experts / trained
machine or by the maintenance staff
maintenance staff. only.
Down Time No down time Down time occurs Cannot predict
occurs as the sometimes about downtime as
activities are simple it depends on the
and can perform / nature of fault, but
check on line also. definitely downtime
occurs.
Example / Cleaning / checking Inspection, checking, Rewinding of burnt
Activities / daily records etc. minor repair winding,

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(ISO/IEC - 27001 - 2013 Certified)

replacement of
blown fuses and the
work carried out
after fault.
Schedule There is a fix There is a fix There is no fix
maintenance maintenance schedule maintenance
schedule / / programme as per schedule /
programme as per service manual. programme.
service manual.
b) Draw and explain the circuit diagram to perform no load and blocked rotor test on
three phase induction motor.
No Load Test on Three Phase Induction Motor:

1 ½ Marks

1 ½ Marks
1. Determine the meters and their ratings based on the name plate readings of the three
phase induction motor under test.
2. Connect the circuit as shown in circuit diagram.
3. Set / check the three-phase autotransformer to be at zero output.
4. First switch on the 3-phase supply and close the TPST. 1 ½ Marks
5. In this test the stator winding is connected to supply through auto transformer.
6. Gradually increase the voltage applied to the machine to the rated voltage. Motor runs
at a speed quite close to its synchronous speed.
7. Take the corresponding readings of voltmeter (as input voltage Vo), ammeter (as input
current Io), wattmeter (as input power Wo) & speed. 1 ½ Marks

Block Rotor Test on Three Phase Induction Motor:

1. Determine the meters and their ratings based on the name plate readings of the three
phase induction motor under test.
2. Connect the circuit as shown in circuit diagram.
3. Set / check the three-phase autotransformer to be at zero output.
4. Block the rotor mechanically in such a way that it does not revolve.

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5. Now switch ON the 3-phase supply and close the TPST.

6. Gradually increase the voltage applied to the motor until up to full load current will
Circulate in the stator winding.
7. Take the corresponding readings of voltmeter (as input voltage Vsc), ammeter (as
input current Isc), wattmeter (as input power Wsc).
c) Explain trouble shooting chart of three phase transformer (any six troubles)
Cause Trouble shooting steps
Primary side fuses blown out Replace the blown out fuses by rectifying reason 1 Mark for
behind it. any six
Tripping of incoming circuit Check and rectify causes of circuit breaker trippng / points
breaker / MCCB MCCB tripping
Failure of primary winding Repair if possible or rewind primary winding
No contact of tap changer Properly connect of tap changer tappings.
Opening of the bushings Make bushing connections proper and tight
connections
Failure of primary winding Repair if possible or rewind primary winding
Failure of secondary winding Repair if possible or rewind secondary winding
Failure / shot circuit / opening Repair if possible or errect new supply cable
of supply cable
No incoming supply to primary Check and rectify causes of incoming supply failure
winding
Tripping off incoming supply Check and rectify causes of wrong operation of
because mal-functioning of one Protective relays.
of the protective relays
Loose contact of tap changer Properly connect of tap changer tappings
Loose connections at bushings Make bushing connections proper and tight
Failure of primary winding Repair if possible or rewind primary winding
Failure of secondary winding Repair if possible or rewind secondary winding
Faulty OLTC Repair OLTC
Heavily overloading / Load properly
Unbalance loading
Incorrect use of turns ratio Make use of correct turns ratio
Shorted turns in the transformer Check and rectify causes of shorted turns in the
transformer.
Low primary voltage Check and rectify causes of low primary voltage
Any internal fault such as short- Replace the transformer and core to be lifted for
circuited core, core bolts/ thoroughly checking. Take corrective action
clamps insulation failure etc. according to observations and oil test report.
Low oil level in conservator Check the oil level in conservator and top up if
required
Slugged oil Carry out purification of oil to remove sludge
Heavy overloading Load the transformer as per its rating
/Unbalanced loading
Faulty / inoperative cooling Repair / use effective cooling arrangements
system
Short circuit in winding / core Check and rectify causes of short circuit in winding /
of transformer core in the transformer.
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(ISO/IEC - 27001 - 2013 Certified)

Use of improper quality of Use proper quality transformer oil

transformer oil
Very high ambient temperature Use better cooling system in such environment
High input voltage Supply transformer with proper input voltage
Slugged oil Carry out purification of oil to remove sludge
Faulty / inoperative cooling Repair / use effective cooling arrangements
system
Low oil level in conservator Check the oil level in conservator and top up if
required
Very high ambient temperature Use better cooling system in such environment
Heavily overloading / Load properly
Unbalance loading
Short circuit in winding / core Check and rectify causes of short circuit in winding /
of transformer core in the transformer.
Inoperative temperature sensors Use proper temperature sensors
6. Attempt any TWO of the following 12 Marks
a) List out routine, type test and special test to be carried out on three phase induction
motor as per IS-4029-1967 and IS-325-1961
Routine tests to be carried out on 3ɸ Induction Motor:
1. Measurement of Winding Resistance / D.C. resistance.
2. Measurement of Insulation Resistance.
3. High voltage test. ½ Mark for
4. Reduced voltage running up test. each of any
5. No load test. four tests
6. Open circuit voltage ratio test (Slip ring induction motor). = 2 Marks
7. Locked / Blocked rotor test.
8. Measurement of slip.
Type tests to be carried out on 3ɸ Induction Motor:
1. Measurement of Winding Resistance / D.C. resistance.
2. Measurement of Insulation Resistance. ½ Mark for
3. High voltage test. each of any
4. Reduced voltage running up test. four tests
5. Open circuit voltage ratio test (Slip ring induction motor). = 2 Marks
6. Locked rotor test.
7. Measurement of slip.
8. Temperature rise test.
9. Momentary overload test.
Special tests to be carried out on 3ɸ Induction Motor:
1. Open circuit test.
2. Short circuit test. ½ Mark for
3. Full load test. each of any
4. Determination of noise level. four tests
5. Presence of harmonics. = 2 Marks
6. Determination of zero phase sequence impedance.
7. Vibration test.
8. Oscillographic test.
b) Explain with neat diagram, back to back test on single phase transformer to
determine efficiency and regulation.

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(ISO/IEC - 27001 - 2013 Certified)

2 Mark
Diagram

2 Mark
Explanatio
n
1. T1 and T2 are two identical transformers, T3 is autotransformer, S1 (At the input
side of primaries), S2 and S3 are switches. A-ammeters, V-voltmeters and W-
watt-meters.
2. As shown in figure the primaries of two transformers (T1 andT2) are connected in
parallel across the supply at rated voltage of primary. Their secondaries are
connected in phase opposition or back-to-back fashion. When primaries of two
transformers are energized by switching ON switch S1, the emfs induced in
secondary windings come in phase opposition. Since the two transformers are
identical, there is no circulating current in the local circuit formed by secondary's
even, if primaries energized.
3. To ensure that the secondaries are connected in phase opposition, a voltmeter
(V3) and a switch S3 is connected in parallel as shown. V3 should be of double
range of that of secondary voltage, because, if the polarities are not connected in
phase opposition the voltmeter may receive twice the voltage of secondary. When
voltmeter V3 indicates zero it ensures that secondaries are connected in phase
opposition, then switch S3 is closed. If voltmeter V3 does indicate zero or 1 Mark
indicate more than zero, then secondary connections are interchanged.
4. To circulate the necessary full load current one auto transformer (T3) is used in
the secondary circuit as shown. Voltage is injected by switching ON switch S2
and by varying the voltage with the help of T3, full load current is circulated in
the secondaries. The current corresponding to this circulating current also flows in 1 Mark
closed circuit formed in primaries, however it does not appear in the ammeter and
wattmeter connected in primary side so the current taken from supply side is only
the total no load current of two transformers.
5. The wattmeter reading (W1) connected in the primary side indicates total no load
loss or iron loss of two transformers.
6. The wattmeter connected in secondary side (W2) indicates total copper loss or
load loss of two transformers caused by the circulating current.
7. Since both the losses are known efficiency of the transformer can be easily
determined.
Determination of Efficiency:
The readings of all ammeters, voltmeters and watt-meters are noted in observation table
as

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given bellow:

Since two transformers are identical,

Determination of regulation:
• Note down the no load voltage of transformer as 0V2
• The injected voltage (V2) supplied to secondary side circulates the full load current in
the secondaries of both transformers. The current corresponding to this circulating
current also flows in the primaries (in the local circuit formed). Thus, this injected
voltage (V2) supplies the voltage drops (IZ drop) of both transformers. Hence voltage
drop of one transformer will be V2 / 2, i.e., injected voltage / 2.

c) Explain the classification of insulating material as per IS-1271-1958 with class 1 Mark for
information, insulating material (any three class wise) and its maximum operating any six
temperature. points

Insulation Operating Examples

Class Temperature in
Degree Celsius
Y Class 90 Silk, cotton, certain thermoplastics that
soften above 90 °C.
A Class 105 Reinforced silk, cotton and paper (with
impregnated varnish or insulation oil).
E Class 120 Powder plastics, polyurethane, molded
polyvinyl epoxy resins, polyethylene
terephthalate.

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B Class 130 Inorganic materials such as asbestos, glass

fibers, mica with varnish.
F Class 155 Mica, class B materials that perform well
at higher temperatures.
H Class 180 Fiberglass, mica, asbestos impregnated in
silicone rubber.
C Class More than 180 Polyimide films, quartz, mica, teflon,
polyimide enamel.

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