ELECTRICAL TECHNOLOGY
GRADE 12 
NSC PAST PAPERS AND MEMOS
SEPTEMBER 2017

MEMORANDUM 

QUESTION 1: OCCUPATIONAL HEALTH AND SAFETY 
1.1 No person may enter or remain in a workplace under the influence of drugs as  he may place himself ✔ and other persons in danger while operating  machinery. ✔ (2) 
1.2

• Earth-leakage system ✔
• Overload circuit breakers ✔
• No-volt coil prevents automatic restarting after power interruption.
• Fuses
• Emergency stop switch
  (ANY TWO) (2) 

1.3 If Aids is not brought under control: 

• it can affect productivity in that employees with skills may become ill or  die ✔
• person may not function well, may take time off work, ✔
• Colleagues may be unwilling to work with a person having HIV due to the  stigma associated with HIV.
  (ANY TWO RELEVANT ANSWERS) (2) 

1.4

• Safety is the responsibility of any person who enters or works in an  electrical technology workshop. ✔
• Each person has a responsibility to himself and others around him. ✔
• It only takes one person to ignore safety procedures to cause serious  problems for all in the electrical technology workshop. ✔
  (ANY SOUND MOTIVATED RESPONSE MUST BE CONSIDERED) (3) 

1.5

• Wet floors ✔
• Wet work areas 
• Bare conductors
  (ANY OTHER RELEVANT ANSWERS) (1)

[10]

QUESTION 2: THREE-PHASE AC GENERATION 
2.1
2.1.1

(3)
2.1.2 

(3)
2.2 

 (3) 
2.3

• For high power generation the three-phase system is functional and  efficient. ✔ 
• The voltages between all phases (i.e. line voltages) are the same
• The direction of rotation of three-phase machines can be easily changed
• Transmission and distribution are fairly simple
  (ANY ONE) (1) 

2.4 A generator has a lagging power factor ✔ as it consists of coils which are  inductive. ✔ Current through an inductor connected to an AC supply lags the  applied voltage. ✔ (3) 
2.5 The function of a kWh meter is to measure the amount of power consumed by  a consumer over a period of time (energy). ✔ (1) 
2.6 120 degrees (1)
2.7 (4) 

2.8 Effective value or rms value ✔ (1)

[20] 

QUESTION 3: THREE-PHASE TRANSFORMERS 
3.1

• Copper losses ✔ losses due to the resistance of the copper wires used ✔
• Stray losses ✔ occurs when some of the magnetic field does not cut the  secondary winding ✔ 
• Iron losses ✔ heat losses due to hysteresis of the core ✔
• Dielectric losses ✔ occurs when insulation of windings is damaged,  causing leakage currents ✔ (4) 

3.2
3.2.1 

• S =    P      ✔
       cos θ
  = 10000
       0,8
  = 12, 5kW ✔  (3)

3.2.2 

• I_L(S) =           P              ✔ 
               √3_L(S) cosθ       
  =         10000            ✔ 
       √3 × 400  × 0,8
  = 18,04A  ✔    (3)

3.2.3 

• I_Ph(S) = I_L(S) ✔
  = 18,04 A   ✔   (2) 

3.3 To create a three-phase four-wire system ✔ so that a transformer can supply  both single-phase ✔ and three-phase. (To distribute power to both domestic  and industrial installations.) ✔ (3) 
3.4

• Losses and current flow in transformers causes heat build-up ✔
• Overloading ✔
• Poor Cooling ✔
• Lack of Ventilation 
• Poor Connections / Hot connections
• Excessive vibration due to poor mechanical construction
  (ANY RELEVANT ANSWERS) (3) 

3.5 The primary phase current will also be doubled ✔ as it is directly proportional to  the load. ✔ (2)

[20] 

QUESTION 4: THREE-PHASE MOTORS AND STARTERS 
4.1 The purpose of using a starter to start a three-phase motor is to safely control  the motor, ✔ protect electrical equipment and the user of the motor. ✔ (2) 
4.2

• Stator ✔
• Rotor ✔
• End plates ✔
• Fan 
• Terminal box 
• Bearings
  (ANY THREE) (3) 

4.3 To reduce the voltage at start-up. ✔ This in turn reduces the starting current.▪✔ Reduced starting current leads to fewer nuisance tripping problems at start or  to less heat build-up and decreases the chance of burn-out of the motor. ✔ (3) 
4.4
4.4.1 

• I_L =       P         
         √3 × V_Lcosθ✔
  =        15000       
      √3 × 380 × 0,9 ✔
  = 25,32 A ✔ (3)

4.4.2 

• Pap_p   =        P   ✔
                   cosθ
  =   15000
         0,9 ✔
  = 25
     √3
  = 16,67 kVA ✔ (3)

4.4.3

• I_L =  √3 × I_PH✔
  I_PH = I_L 
  √3
  = 25
    √3 ✔
  = 14,43A ✔ (3)

4.5
4.5.1

• S =   P     
        cos θ 
  =90 000
       0,85 
  = 105,88 kVA (2) 

4.5.2

• I_L=      P          
      √3V_L cos θ 
  =    90 000       
  √3 x 400 x 0,85 
  = 152,83 A (3) 

4.6 By reversing the connections of any two of the three supply lines to the  stator. ✔ (1) 
4.7 To protect electrical equipment from damage ✔ during faulty operating  conditions and protecting the operator of the equipment. ✔ (2) 
4.8 The motor will keep on operating, but to maintain the same output power ✔ the  current on the other two phases will increase. ✔ If the protection is set  correctly, it will engage, protecting the motor from permanent damage. ✔ (CONSIDER RELEVANT MOTIVATED ANSWERS) (3) 
4.9

4.9.1 The function of a star-delta starter is to reduce the starting current of a  motor at start as a motor draws 3 to 4 times full-load current at start.✔ (1) 
4.9.2 The motor is connected in star at start. ✔ This reduces the voltage  across the motor windings which in turn reduces the current in the  windings.✔ Once the starting current has reduced, the motor windings  are changed over to delta, restoring full-line voltage across the  windings, therefore full current. ✔ (3) 

4.10 The overload relay is designed to protect the motor ✔ and motor wiring against  current fault conditions. ✔ It will open and cut power to the motor. ✔ (4) 
4.11 

(4) 

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QUESTION 5: RCL CIRCUITS 
5.1
5.1.1

• Z = √R²  +  (X_L - K_C)²
  = √500² + (300 - 250)²
  = 502, 49Ω  (3)
  

5.1.2

• cosθ = R   ✔ 
              Z
  =    500    
     502, 49 ✔ 
  = 0,995✔  Lagging ✔  (4)

5.2 

5.2.1 

• R = V² ✔
         P
  = 1102
        60✔
  = 201, 67 Ω ✔ (3) 

5.2.2 

• I =   P  
       V_R  ✔
  =   60  
      110  ✔
  = 0,55 A ✔ (3) 

5.2.3 

• Z = V
         I   ✔
  = 220
     0,55  ✔
   = 400 A ✔ (3) 

5.2.4 

• X_L = √Z² - R² 
  2πfL = √Z² - R² 
  L = √Z² - R²
             2πf ✔
  = √400² - 201, 67²
          2.π.50                 ✔
  = 1,1 H ✔  (4)

[20] 

QUESTION 6: LOGIC 
6.1 A series of instructions ✔ written in a language ✔ that a PLC can recognise and  interpret into an output. ✔ (3) 
6.2 Programmable Logic Controller ✔ (1) 
6.3

• User interface (On the PLC Unit – Screen & Buttons).✔ 
• Computer or laptop with interface cable.✔ 
• Handheld programming device.✔ (3)

6.4 

6.5

• Ladder logic (LL). ✔ 
• Logic block diagram (LBD). ✔
• Function Block Diagram (FBD)
• Structured Text Sequential Flow / Function Chart
  (ANY THREE) (3) 

6.6 (5) 
6.7 Fewer components such as contactors are subject to wear because less of  these items are used. ✔Additionally units have built-in diagnostic functions. ✔ (2)
6.8 

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QUESTION 7: AMPLIFIERS 
7.1

• Linear amplifiers √ 
• Pulse amplifiers √ 
• Buffer circuits Integrating √
• Differentiating 
• Summing amplifiers
  (ANY THREE) (3) 

7.2

• Input draws no current √ 
• The voltage drop between the input terminals is zero √ 
• The open-loop voltage gain is infinite √ 
• Output impedance is zero. Input impedance is infinite
• Frequency Response is infinite
  (ANY THREE) (3) 

7.3 Means that there is no feedback (neither negative nor positive) √ from the  output back to the input. √ The gain of the circuit is at a maximum. √ (3)
7.4
 (3) 
7.5 A portion of the output signal is fed back to the input signal ✔ and is in phase  with the input signal (2) 
7.6 This will reduce the loading effect on the previous circuit. ✔ In so doing, no  current will be drawn ✔ from that circuit. Therefore, the voltage appearing √ at  its output terminals ✔ will be passed on to the op amp with little or no loss. (4) 
7.7

• Audio oscillators ✔ 
• Electronic organ ✔ 
• GPS units ✔ (3)

7.8
7.8.1
 same frequency ✔ amplification ✔ non- inverting ✔ (2) 

7.8.2 Output signal ✔ is fed back to the inverting input ✔ through feedback  resistor R_f .✔ (3) 
7.8.3 If the resistance of R_f is decreased V_rf will decrease ✔ this is feedback  on the inverting input of the op-amp ✔ increasing the overall gain of the  circuit. ✔ (3) 
7.8.4 R_in allows further control ✔ of the op-amp circuit gain. ✔ Setting R_in at a  high value compared to R_f  creates a voltage-follower circuit. ✔ (3)
7.9
 (5) 
7.10 The output of the comparator will be zero. ✔ This is due to a comparator only  amplifying the difference between the input signals. ✔ (2) 
7.11 The frequency of the wave form remains unchanged. ✔ (1)
7.12

The electrical signal that the oscillator produces is called the natural oscillation  frequency. ✔ Natural oscillation diminishes in amplitude and disappears due to  a lack of positive feedback. ✔ (6) 
7.13 When an op-amp is utilised between stages it is used as a buffer amplifier to  adapt/match the impedance between the stages. ✔✔ (2) 
7.14 Any application where a phase shift is not required, ✔ such as: audio  amplifiers. ✔ (2)

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TOTAL: 200