Applied Electricity Paper 2 , May/June 2012
 Questions: 1 2 3 4 5 6 Main
Weakness/Remedies
Strength

Question 1

A moving-coil instrument has a coil resistance of 50 Ω and gives a full-scale deflection (fsd) for a current of 250 mA.

1.  Sketch the circuit diagram.
2.  Calculate the:
3. shunt resistance required for the meter to read an fsd of 5 A;
4. resistance required for the meter to read 25 V.
_____________________________________________________________________________________________________
Observation

The question was on Measuring Instruments. The candidates who attempted this

question were reported to do better in part (b) than in part (a)

Question 2

1. Sketch the phasor diagram of the RL circuit shown in Fig. 7

1.  A current flowing through a circuit is represented by the expression

i = 10 sin 628t Amperes
Calculate the:

1. r.m.s. value;
2. average value.

(a)
VS or Z                            VL or XL

VR or R                           I
(b)  (i)         Ir.m.s =   = 0.707 x maximum value
= 0.707 x 10 = 7.07 A
(ii) average value = 0.637 x maximum value
= 0.637 x 10 = 6.37 A
The question was on Alternating Current Circuit Theory. The question was                  unpopular with the candidates.
Many candidates who attempted the question could not draw the phasor diagram. They did not know that the r.m.s. and average values should be computed from the peak value of the instantaneous current.

1.  A current flowing through a circuit is represented by the expression

i = 10 sin 628t Amperes
Calculate the:

1. r.m.s. value;
2. average value.

)  (i)   Ir.m.s =   = 0.707 x maximum value
= 0.707 x 10 = 7.07 A
(ii) average value = 0.637 x maximum value
= 0.637 x 10 = 6.37 A
The question was on Alternating Current Circuit Theory. The question was unpopular with the candidates.
Many candidates who attempted the question could not draw the phasor diagram. They did not know that the r.m.s. and average values should be computed from the peak value of the instantaneous current.

Question 3

1. Describe briefly with the aid of a diagram, the principle of operation of a shunt-wound d.c. motor.
2.  State the effect of increasing the value of the resistance in series with the field winding.

Principle of operation

A shunt motor converts electrical energy to mechanical energy. When the field winding is self-excited and the armature conductors are supplied from the d.c. source, field and armature fluxes are produced respectively. The fluxes produced interact to produce a force which causes a motion of the armature (i.e. rotor). The direction of motion is determined by Fleming’s Left Hand Rule with amplitude expressed as
F = BLI (N)

(b)     The speed of the motor will increase since the speed is inversely proportional to the flux.
The question was on Direct Current Machines. It was unpopular with the candidates. The few candidates who attempted this question did not answer correctly.

Question 4

1.  Explain the layout diagram with respect to electrical wiring systems.
2.  A layout diagram has a scale of 1:100. If the length of the cable on the diagram is 65 mm, calculate the actual length of the cable.
3. A ring main circuit has a 32 A fuse 240 V supply. Calculate the maximum power available at any time.

(a) Layout diagram is one that details the positioning of accessories in electrical                     wiring.
(b)  Scale =  =
size of object = 100 x size of image = 100 x 65 mm
= 6,500 mm
(c)  Power,P = IV
= 32 x 240
= 7.68 kW or KVA

The question was on Electrical Wiring. It was very popular with the candidates. Many candidates who attempted this question could not answer the parts (a) and (b) correctly but got part (c).

Question 4

1.  Explain the layout diagram with respect to electrical wiring systems.
2.  A layout diagram has a scale of 1:100. If the length of the cable on the diagram is 65 mm, calculate the actual length of the cable.
3. A ring main circuit has a 32 A fuse 240 V supply. Calculate the maximum power available at any time.

(a) Layout diagram is one that details the positioning of accessories in electrical                     wiring.
(b)  Scale =  =
size of object = 100 x size of image = 100 x 65 mm
= 6,500 mm
(c)  Power,P = IV
= 32 x 240
= 7.68 kW or KVA

The question was on Electrical Wiring. It was very popular with the candidates. Many candidates who attempted this question could not answer the parts (a) and (b) correctly but got part (c).

Question 4

1.  Explain the layout diagram with respect to electrical wiring systems.
2.  A layout diagram has a scale of 1:100. If the length of the cable on the diagram is 65 mm, calculate the actual length of the cable.
3. A ring main circuit has a 32 A fuse 240 V supply. Calculate the maximum power available at any time.

(a) Layout diagram is one that details the positioning of accessories in electrical wiring.
(b)  Scale =  =
size of object = 100 x size of image = 100 x 65 mm

= 6,500 mm

(c)Power,P = IV
= 32 x 240
= 7.68 kW or KVA

The question was on Electrical Wiring. It was very popular with the candidates. Many candidates who attempted this question could not answer the parts (a) and (b) correctly but got part (c).

Question 5

1.  Define a battery.
2.  State the two types of cell.

Fig. 9 is a schematic diagram of a Leclanche cell

1. Identify:
2. part A;
3. part B;
4. part C;
5. part D.

(a)  A battery is a group of cells connected together.
(Accept any other reasonable definition)

(b)         Primary cell and Secondary cell
(c)         (i)      Carbon rod or positive terminal (Part A)
(ii)      Depolarizer/Manganese dioxide (Part B)
(iii)    Electrolyte/Ammonium chloride (Part C)
(iv)    Cathode material/Zinc (Part D)
The question was on Electrical Energy Supply. The question was fairly popular with the candidate. Many candidates who attempted this question could not define battery. Majority got the part (b) correctly while less than half of those who attempted the part (c) were able to identify correctly the labeled parts.
Question 6

1.  Explain the following terms:
2. final subcircuit;
3. ring circuit.
4. State two causes of fire outbreak in an electrical installation.
5. List two factors that are considered in the selection of the type of wiring.

(a)  Final sub circuit is an outgoing circuit connected to a distribution board and           intended to supply electrical energy to the circuit accessories.
Ring circuit is a power distribution wiring method which provides the same phase        voltage to number of outlets and eventually comes back to the same source (i.e.         fuse, circuit breaker or bus bar).
(b) Short circuit
Insulation breakdown
Earth fault
Over voltage
Wrong connection
(c)   The nature of the location
The nature of the structure supporting the wiring
Cost
Aesthetics

The question was on Electrical Wiring. The question was unpopular with the candidates. The performance was reported poor.
The candidates who attempted this could not explain final sub-circuit and ring circuit with the remaining parts answered poorly as well.