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2012-06-09 00:27:38

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  1. Calculate the line current of a three phase 15kW deep fryer. The supply is400 V, 50Hz.
    W = 83 x VL x IL(½ mark)IL = 150001.732 x 400(½ mark)= 21.65A
  2. What test voltage is required when carrying out an insulationresistance test on a three-phase, 400 V, MEN supply cable?
    500 V dc
  3. What is the minimum value of the permitted test result for theinsulation resistance test on the cable?
  4. Briefly explain the purpose of the interlocks in a star/delta starter.
     To prevent the star and delta contactors closing simultaneously(1 mark) Causing a short-circuit.
  5. State TWO advantages that HRC fuses have - other than cost - whencompared to MCBs
    Good discrimination Good back-up protection Fast operation Range of utilisation categories (fusing factor) Higher rupturing capacity
  6. State TWO disadvantages that MCBs have - other than cost - whencompared to HRC fuses.
    Not very fast operating speeds Must wait to reset for thermal operation Fixed fusing factor Nuisance tripping in older models Mechanical mechanism could fail Lower breaking capacity
  7. What is the purpose of the capacitor in starting a single-phase a.c.capacitor-start induction motor?
    To increase the phase displacement in the start winding to produce arotating magnetic field. Increase the start-up torque
  8. For what reason would you connect a resistor across a circuit after theelectricity supply to that circuit has been isolated.
    To discharge a large capacitor
  9. What is meant by the term “pullout torque” as it applies to an a.c.induction motor?
    The maximum value of torque the motor can produce.
  10. What is the synchronous speed of a three-phase, four-pole, 50 Hzmotor?
    Any ONE of: 1500 rpm The speed of the rotating field
  11. State the main reason why it is important to consider prospectiveshort-circuit current when choosing a protection device.
    A protective device must be selected that can safely handle themaximum short-circuit current under fault conditions
  12. What type of fuse would you install where the prospective short-circuitcurrent is calculated to be 20,000 A?
    An HRC fuse
  13. A residual current device (RCD) has operated. The RCD protects a circuitand a fixed-wired Class I electrical appliance. What has the RCD detectedthat would cause it to operate?
    Earth leakage current in the circuit.E Earth leakage current in the appliance. An imbalance in current between phase and neutral A neutral to earth connection has occurred Transposition of neutral and earth in the circuit
  14. Describe the characteristics of a thermistor
    A thermistor is a device which changes resistance with temperatureNote: They can either be n.t.c. or p.t.c
  15. Where in a motor circuit would a thermistor be located?
    It would be located in the windings of the motor
  16. Some fittings in an electrical installation have a fault duty lower than theprospective short circuit rating of the installation. State TWO hazards thatcould occur when a high prospective short-circuit fault current occurs in theelectrical installation.
    9 Fire hazard due to fittings or cables overheating9 Flash over on switchboards due to inadequate rating of protectivedevices9 Personal injury if people are in the vicinity of failing fittings or cables..
  17. When taking measurements in a single phase a.c. inductive circuit, thefollowing readings were obtained6 235V6 12A6 2115WCalculate the power factor of the circuit.
    235 x 12 = 2820VA(½ mark)= 2115 W2820 VA(½ mark)= 0.75
  18. An RCD is used for personal protection. What is the maximum timepermitted for the RCD to trip at 5 times its residual current rating?
    40 ms
  19. What device is used to provide starting torque in a shaded pole motor?
    The copper shading ring
  20. Describe how a shaded pole motor can be reversed
    Turn the motor or rotor around
  21. Define the term “slip speed” as it applies to a.c. induction motor
    The difference between the synchronous speed and the rotor speed
  22. What is meant by the term “current rating” when applied to an HRCfuse?
    This is the maximum current that the fuse is designed to carrycontinuously
  23. What is meant by the term “breaking (or rupturing) capacity” whenapplied to an HRC fuse?
    The maximum fault current the fuse can safely interrupt
  24. State TWO reasons why a run capacitor would be fitted to a single-phasesplit-phase induction motor.
    Any TWO of: Improved power factor Quieter running Better running torque Less current drawn
  25. State TWO advantages electronic starters have when compared toelectromechanical type starters, such as auto-transformer types
    More efficient - less power loss.9 Smoother starting - less wear and tear on the motor.9 Easier to adjust for overcurrent and other parameters.9 Power factor very high because of d.c. bus9 Visual feed-back through display panels.9 Physical size smaller than auto-transformer.9 More economical to maintain – less equipment involved.9 Reduces mechanical shock to equipment9 Constant current flow9 Constant torque9 Torque is very constant at start9 Cheaper in the larger sizes9 Built in protection and monitoring
  26. A heater draws 2000W at 230V. What power will the heater draw if thevoltage drops 5%?
    About 1800W(2 marks)Or(218.5 ÷ 230)2 x 2000= 1805WOr(0.95)2 x 2000= 1805W

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