SKAA 2032/ SAB 2023/ SAM 3012 1
FINAL EXAMINATION
SEMESTER II, SESSION 2011/2012
COURSE CODE
:
SKAA 2032/ SAB 2032/ SAM 3012
COURSE
:
MECHANICAL AND ELECTRICAL SYSTEM
PROGRAMME
:
SKAW/ SAW
DURATION
:
2 HOURS
DATE
:
JUNE, 2012
INSTRUCTION TO CANDIDATES:
1.
SECTION A AND B MUST BE WRITTEN IN SEPARATE BOOKS.
SECTION A : ELECTRICAL
1.
ANSWER TWO (2) QUESTIONS FROM SECTION A.
2.
ALL CALCULATIONS MUST BE CLEARLY SHOWN.
SECTION B : MECHANICAL
1.
ANSWER TWO (2) QUESTIONS FROM SECTION B.
2.
ALL VALUES ARE AS USED IN LECTURE NOTES
WARNING!
Students caught copying/cheating during the examination will be liable for
disciplinary actions and the faculty may recommend the student to be expelled from
the study.
This examination question consists of (7) printed pages only. SKAA 2032/ SAB 2023/ SAM 3012 2
SECTION A: ELECTRICAL
Q1.
(a)
An electrical circuit consists of a 120 V source and two lamps which
have resistances 10 Ω and 40 Ω respectively (Figure 1). Using Ohm’s
law find:
i)
Current total (IT) which flows the circuit
(7 marks)
ii)
Total power (P) dissipated in the circuit
(7 marks)
120 V
+ L1
10 Ω
L2
40 Ω
FIGURE 1
(b)
(c)
Differentiate between AC and DC current. Sketch the two quantities.
(5 marks)
Explain briefly and give examples for the electrical terms below:
i)
Electrical energy source
(2 marks)
ii)
Electrical load
(2 marks)
iii)
Electrical power transmission
(2 marks)
(25 marks)
SKAA 2032/ SAB 2023/ SAM 3012 3
Q2.
(a)
Describe the principle operation of a transformer
(5 marks)
(b)
What are the advantages and disadvantages of induction motor
compared to other AC motors?
(5 marks)
(c)
A 250 KVA, 11000 V/415 V, 50 Hz single-phase transformer has 80
turns on the secondary, Calculate:
i)
The approximate value of the primary and secondary currents;
(5 marks)
ii)
The approximate number of primary turns
(5 marks)
iii)
The maximum value of the flux
(5 marks)
(25 marks)
Q3.
(a)
Sketch the main circuit of a single dwelling’s wiring from the 240 Volt
supply from TNB with proper labeling of the components involved.
Further explain the function of each component in ensuring the safety
of the building and its occupants.
(12 marks)
(b)
Explain the need of grounding (earthing) for electrical installations of a
building and list down at least 3 items in a typical building that can be
used as connection to earth.
(8 marks)
(c)
Wiring codes stresses on the installation of electric cables to meet
certain conductor sizes. Explain the dangers of undersized wire usage
and describe that process. What other components in a wiring system
help prevent that danger?
(5 marks)
(25 marks)
SKAA 2032/ SAB 2023/ SAM 3012 4
Q4.
(a)
State the reasons for the wide application of induction motors.
(8 marks)
(b)
List the applications of single, two and three phase induction motors.
(7 marks)
(c)
A single phase induction motor is wound for 6 poles and is supplied
from a 50 Hz supply system. Determine:
i)
The synchronous speed
(2 marks)
ii)
The slip (in percentage), if the rotor speed is 700 r.p.m.
(revolutions per minute)
(2 marks)
iii)
If the supply frequency is changed to 60 Hz, recalculate the slip
(4 marks)
iv)
If the rotor is suddenly stopped by a heavy load, how much is
the slip at that moment?
(2 marks)
(25 marks)
SKAA 2032/ SAB 2023/ SAM 3012 5
SECTION B: MECHANICAL
Q5.
Q6.
(a)
What are the factors that could contribute to head loss in the service
pipe of a domestic water supply system. Illustrate your answer with
suitable sketches.
(10 marks)
(b)
Figure Q5 shows the plumbing diagram for a shelter for senior citizens.
Estimate the flow rate for this shelter .If the combined physical length
of the service pipe and communication pipe is 8 m what is the total
head loss from the mains to the cistern which is located 3.5 m above
the mains .If the service pipe diameter is increased to 40 mm, what
is the reduction in total head loss.
(15 marks)
(25 marks)
(a)
State the main differences between the functions of lifts and escalators.
(5 marks)
(b)
Sketch a typical escalator installation and explain why is the floor
reaction at the upper level greater than the floor reaction at the lower
level.
(5 marks)
(c)
An escalator could transport a maximum of 80 persons at any instant.
It is inclined at 30o to the horizontal. The distribution of passengers on
the escalator are 8 each on the lower flat and upper flat sections and the
remaining passengers stand on the slope. If the escalator moves at 0.5
m/s and the friction between the escalator and it guide rails is
negligible, estimate the power require by the escalator.
You may assumed that each step accommodate two passengers and
the combine weight per step is 140 Kg.
(15 marks)
(25 marks)
SKAA 2032/ SAB 2023/ SAM 3012 6
Q7.
Q8.
(a)
The Indoor Air Quality Act refers to the quality of air inside a building.
What are the factors that could cause indoor air quality to deteriorate.
(5 marks)
(b)
A room measures 12 m by 12m and 4 m high. It is required that the air
in the room be changed every 15 minutes. Suggest suitable exhaust
fan(s) for the room. If the pressure drop across an exhaust fan is 8 cm
of water, estimate the power required by the fan(s). Take density of air
as 1.2 kg/m3
(10 marks)
(c)
Sketch a typical layout and explain the operationof a Split Type AirConditioning System.
(10 marks)
(25 marks)
(a)
Describe six (6) classifications of fire and the appropriate fire fighting
media (agent) to extinguish each class of fire.
(10 marks)
(b)
An electrical substation is provided with total flooding carbon dioxide
(CO2) fire fighting system. The dimension of the room is 10 m x 15 m
x 4 m.
(i)
(ii)
(iii)
Sketch the installation of the room and indicate all fittings for
CO2 fire fighting system.
Estimate the quantity of CO2 in kg for this room.
If 1 cylinder contains 45 kg of CO2 gas, estimate the number of
CO2 cylinder.
(15 marks)
(25 marks)
SKAA 2032/ SAB 2023/ SAM 3012 7
FIGURE Q5