KCSE 2015 Electricity Paper 1 with Marking Scheme

SECTION A (48 marks)
Answer all the questions in this section in the spaces provided.

1.    
   1. State the name of the institution into which each of the following was upgraded:
      1. Kenya Polytechnic;(1 mark)
      2. Mombasa Polytechnic. (1 mark)
   2. State the main duty of class A2 Electrical License holder. (1 mark)
2.    
   1. Name the materials used to make the parts of an AC machine in table 1 and in each case state one reason why the material is used.
      Table 1
      

      PART	MATERIAL	REASON
      Brushes
      Slip rings

      (3 marks)
   2. State the three factors to consider when choosing a motor for a particular installation. (3 marks)
3. Draw a ring final circuit diagram with four socket outlets and a spur. (5 marks)
4.    
   1. State two factors that determine the resistance of a conductor. (2 marks)
   2. Determine the colour code of a 1500 2+5% carbon resistor. (3 marks)
5.    
   1.    
      1. Figure 1 shows a pair of current - carrying conductors. Draw the magnetic flux around the conductors. (2 marks)
         
      2. Name the type of force that exists between the conductors in a(i) above.
   2. With the aid of a labelled diagram show how each of the following permanent magnets are stored to preserve their magnetism: (2 marks)
      1. horse shoe;
      2. bar.
6.    
   1. Name four faults that occur in electric circuits. (2 marks)
   2. State two ways of increasing the magnetic field strength of a moving coil instrument. (2 marks)
7. Figure 2 shows a resistive circuit.
   
   Calculate:
   1. total resistance of the circuit; (3½ marks)
   2. current through R6. (2½ marks)
8.    
   1. State two safety precautions to be observed when using each of the following:
      1. arc welding machine; 
      2. microwave oven. (4 marks)
   2. Name four non-ferrous metals used in manufacturing electrical components. (2 marks)
9.    
   1. State four qualities of successful entrepreneurs. (2 marks)
   2. Sketch the correct symbol for each of the following electronic components: (3 marks)
      1. cored inductor;
      2. LED;
      3. electrolytic capacitor.
10. A 47 kΩ carbon resistor has a power rating ¼ of W. Determine the:
    1. maximum current which can pass through it. (2½ marks)
    2. maximum voltage which can be applied across it. (2½ marks)
       

SECTION B (52 marks)
Answer any four questions from this section in the spaces provided.

11.    
    1. Figure 3 shows an R-C circuit.
       
       Calculate the:
       1. impedance; (2 marks)
       2. current l;(2 marks)
       3. phase angle. (2 marks) 
    2. Draw a labelled diagram of a continuous ringing bell. (7 marks)
12. Figure 4 shows a block drawn in isometric projection.
    
    Using first angle projection draw the following views full size:
    1. front elevation in the direction of arrow F;
    2. end elevation in the direction of arrow E;
    3. plan. (13 marks)
       (Use A, paper provided)
13.    
    1. State three reasons why autotransformers are more efficient than double wound transformers of the same rating. (3 marks)
    2. Outline three functions of oil in a transformer. (3 marks)
    3. Figure 5 shows a step-up transformer supplying a load of 75 kW at unity power factor.
       
       
       1. Neglecting the transformer losses, determine the:
          1. load current; (2½ marks)
          2. input current; (1½ marks)
          3. current between points A and B. (2 marks)
       2. State what happens if there is an opening between points A and B. (1 mark)
14.      
    1. Outline two duties of an electrical contractor. (2 marks)
    2. State two reasons for carrying out installation resistance test in an installation. (2 marks)
    3.      
       1. Draw and label a filament lamp. (6 marks)
       2. A fluorescent lamp flickers in an effort to start. State three possible causes for this action. (3 marks)
15.    
    1. Figure 6 shows a NAND gate in a circuit.
       
       
       1. Names the parts labelled P, Q, R and S. (4 marks)
       2. State the functions of components A, B, C and D. (4 marks)
    2. Figure 7 shows a capacitor discharge circuit.
       
       
       1. Sketch the output voltage - time waveform when switch S is closed. (2 marks)
       2. Determine the duration taken to fully discharge the capacitor. (3 marks)

MARKING SCHEME

SECTION A: SOLUTIONS

1.    
   1.      
      1. Technical University of Kenya
      2. Technical University of Mombasa
         =1 mark
   2. To carry out specialized electrical installation work.
      = 1 mark
2.    
   1. 
      

      MATERIAL	REASON
      Carbon	Self lubricating, good conductor, withstands high temperature.
      Brass	Self lubricating, good conductor, mechanical strength.

      = (3 marks)
   2. Three factors:
      1. type of supply
      2. nature of the load
      3. the type of environment
         (Any suitable 3 x 1 = 3 marks)
3. 
   
   Fuse link - 1
   Ring connection - 1
   4-socket outlets - = 2
   Spur - 1
   Total = (5 marks)
4.    
   1. Two factors
      1. length
      2. cross-sectional area
      3. type of material/resistivity
      4. temperature
         (Any 2 x 1) (2 marks)
   2. Brown, green, red, gold -= 2 marks
      Order - 1 mark
      Total = (3 marks)
5.    
   1.    
      1. 
         
         Correct direction 
         Correct shape 
      2. Repulsion force
   2. 
      
6.    
   1. Four faults:
      • short circuit
      • open circuit
      • overload
      • earth fault
        = (2 marks)
   2. Two ways:
      1. Using a powerful magnet
      2. Introducing iron core in the coil
      3. Increasing the number of turns on the coil
         (= 2 marks)
7.    
   1. Total resistance:
      Parallel branch 1, equivalent R_p1 
      
   2. Current through R6
      
8.    
   1.    
      1. Arc welding machine:
         • Use arc welding shield to protect the eyes and the face.
         • Prevent people from looking at the arc or standing near when arc welding is going on.
           (2 x 1 = 2 marks)
      2. Microwave oven
         • Do not place any part of the body in front of a source of microwave radiation
         • Always close doors of microwave ovens before turning the power on
         • Do not put metals into microwave ovens because they are reflected by microwaves
         • Warn other people of the presence and the danger of microwave radiation
           (Any 2 x 1 = 2 marks)
   2. For non-ferrous metals:
      Copper, aluminium, lead, silver, tin, zinc.
      = (2 marks)
9.    
   1. Four qualities:
      • Risk takers
      • Creative and innovative
      • Self confidence
      • Strong desire to achieve
      • Hardworking/energetic
      • Optimistic
      • Independent
      • Future oriented
      • Opportunity - seeking
      • Initiative
      • Discipline
      • Good leadership skills
        (= 2 marks)
   2. 
      
10. 
    
    (5 marks)

SECTION B (52 marks)

11.     
    1.    
       1. Impedance
          
       2. Current I = V = 240 = 2.26A
                           Z   106.2 
       3. Phase angle, TanØ = X _c
                                         R
          = 106 = 17.7
               6
          Ø = 86.8° (6 marks)
    2. Continuous ringing bell
       
       Labelling (6 x ½) = (3 marks)
       Correct drawing = (1 mark)
       Drawing = (3 marks)
       Total = (7 marks)
12.  
    
13.    
    1.    
       • They use less copper and therefore experience less copper losses
       • They use smaller core and are lighter in weight
       • They have better magnetic linkage between the primary and secondary sections of the winding
       • They are less voluminous
         (Any 3 x 1 = 3 marks)
    2. Functions of oil in a transformer
       • Acts as a cooling medium
       • Insulate the winding from the transformer core and casing
       • Excludes dirt and moisture from coil insulation
         (3 x 1 = 3 marks)
    3.    
       1. Local current 1_L = P 
                                     E_s
          =75000 = 37.5A
             2000
          (2 marks)
       2. For ideal transformer, input power = output power
          = 75 kW
          input current _In = P _in
                                    E_in
          75000 = 30A
          15000 
       3. Current between A and B
          = I_L - I_JN
          = 37.5 A - 50 A = 12.5 A (2 marks)
          What happens if there is an opening between A and B
          Source voltage will appear in the secondary terminals resulting to same voltage
14.    
    1. Duties of an electrical contractor
       • To install and wire the building according to recommended regulations
       • To test and commission a completed installation
       • to issue a completion certificate
         Any 2 x 1 = 2 marks
    2.     
       1. Reason for installation test
          • To ensure there is no possibility of earth leakage current
          • To ensure there is no leakage current between conductors
            2 x 1 = 2 marks
    3.    
       1. 
          
          1 – Inside of glass bulb clear or frosted.
          2 - Support wires with closed loops.
          3 - Coiled tungsten filament.
          4 - Bayonet cap.
          5 - Fuse
          6 - Inert gas.
          7 - Exhaust tube.
          8 - Contacts
          Fig. 7.2: Construction of tungsten lamps.
          Drawing = 2
          Labelling = 4
          6 marks
       2. Reasons for flickering
          • faulty starter switch
          • low voltage
          • faulty lamp
          • old fluorescent tube
            Any 3 x 1 = 3
15.    
    1.    
       1. Name of parts PQRS
          P is the input terminal
          Q is the ground terminal
          R is the output terminal
          S is the positive voltage terminal
          = 4 marks
       2. Function of each components A, B, C, D
          A Switches voltage to input P
          B Limits current to input
          C Indicates state of input
          D Limits current on the output
          = 4 marks
          1. 
             
             axes = 1
             shape = 1
          2. C = 0.01μF
             R= 15 KΩ
             T= RC = 15,000 x 0.01 x 10^-6
             = 15.0 x 10⁺³ x 0.01 x 10^-3
             = 0.15 x 10^-3
             = 1.5 x 10^-6
             = 1.5 x 10^-6 = 1.5μs
             (5 marks)