2009

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mike9111
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157891
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2009
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2012-06-08 22:43:02
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  1. A CT metering circuit is measuring the consumption of an installation atnear to full load.(i) State the precaution that must be taken before disconnecting themeters from the CT.(1 mark)________________________________________________________________________________________________________________(ii) State ONE reason why this precaution is taken.
    (i) The CT secondary must be short-circuited.(1 mark)(ii) Any ONE of: To prevent a high voltage developing in the circuit. To prevent damage to the CT accuracy through core saturation
  2. State the TWO technical factors that will determine the size of a cable usedto supply a three-phase induction motor.
    Current (load)• Length of run, or similar
  3. State ONE precaution that could be taken when carrying out aninsulation resistance test on equipment containing semi-conductordevices?
    Any ONE of: Disconnect semi-conductor devices Bridge out between phase and neutral Apply a 250V d.c. test voltage
  4. AS/NZS 3000 requires that a test voltage of 500V d.c. is used for aninsulation resistance test of a low voltage electrical installation.(i) Briefly explain why a voltage of 500V is applied
    To stress the insulation at a voltage above 325V a.c. (or peaka.c. voltage.= To stress the insulation at a voltage of twice the r.m.s voltage of250V.
  5. Briefly explain why the voltage is a d.c. voltage.
    To ensure capacitive or inductive reactance does not influencethe test result. A constant maximum voltage is a more stringent test than themomentary peaks of an a.c. waveform.
  6. Calculate the power factor of a small three-phase printing factory withmainly motor loading. Metering instruments have recorded the followinginformation:; Watts = 30000W; Volts = 406V; Amps = 57A per phase
    pf = WattsVolt Amps(½ mark)= 30000@3 x 406 x 57(1 mark)= 0.748
  7. 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.
    Any TWO of:= Fire hazard due to fittings or cables overheating= Flash over on switchboards due to inadequate kA rating of protectivedevices= Personal injury if people are in the vicinity of failing fittings or cables
  8. A three-phase cable was run between a distribution pillar and a buildingand connected only in the pillar. Three months after the cable wasconnected to the pillar an electrician received an electric shock from thecable while attempting to terminate the cable at the main switchboard ofthe building.No other person worked on the cable in the time between it beingconnected to the pillar and the electrician receiving the shockThe electrician had tested the end of the cable with a multimeter and foundno voltage present. The meter was proved to be working before and afterthe test. She then proceeded to strip the cable and received the shock.(i) State the reason why the electrician received a shock.(1 mark)________________________________________________________(ii) State the action the electrician failed to take when testing.
    Any ONE of: One phase was still live. All phases were live at the same voltage (that is, paralled)(1 mark)(ii) The electrician did not test between each phase and remote earth.
  9. State the reason why you would carry out an earth fault loop impedancetest at the switchboard of a low voltage electrical installation.
    Any ONE of:To ensure that in the event of a fault: The fault rating of the protection devices is adequate to safelyinterrupt a circuit.= The impedance of the fault loop is low enough to operate protectivedevices in 0.4s.
  10. 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
  11. 100 metres of three-phase mains cable gave an insulation test result of100MW. Calculate the insulation resistance of 500m of the same type ofcable.
    R2 = R1 x L1L2(½ mark)= 100 x 100500(½ mark)= 20MW
  12. State the main reason why the low voltage windings in large powertransformers are placed closest to the iron core?
    They are easier to insulate from theiron core than the high voltagewindings. Lower voltage potential to the core
  13. State TWO reasons, other than cost, for balancing the load over threephases.
    Any TWO of: Reduce system losses Reduce voltage drop Prevent premature failure of highestloaded phase Minimise neutral current Lower rated protective devices can beused Smaller cable sizes can be used
  14. State TWO reasons why an industrial electrical installation is likely to havea larger prospective short circuit current than a domestic electricalinstallation.
    There is lower source impedance (1 mark) As the transformer is either close to theinstallation or on-site
  15. List TWO types of single-phase induction motors, other than a capacitorstart induction motor.
    Any TWO of: Capacitor-start, capacitor-run Shaded pole Split phase (or resistance start) Permanently split
  16. AS/NZS 3760 permits two methods of verifying that the insulation of aClass 1 electrical appliance is in sound condition. One method is aninsulation resistance test using an insulation resistance tester. For theother test state:(i) The name of the test.(1 mark)________________________________________________________(ii) The maximum permitted value of that test.
    (i) Earth leakage test (1 mark)(ii) 5 mA
  17. 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
  18. A heater draws 2000W at 230V. What power will the heater draw if thevoltage drops 5%?
    About 1800WOr(218.5 ÷ 230)2 x 2000

    = 1805WOr(0.95)2 x 2000= 1805W
  19. State the reason why you would carry out an earth fault loop impedancetest at the switchboard of a low voltage electrical installation
    The fault rating of the protectiondevices is adequate to safely interrupta circuit.; To find the prospective short circuitcurrent level of the installations To determine the impedance at theswitchboard under fault conditions.
  20. A CT metering circuit is measuring the consumption of an installation atnear to full load.(i) State the precaution that must be taken before disconnecting themeters from the CT.(1 mark)________________________________________________________________________________________________________________(ii) State ONE reason why this precaution is taken.
    (i) The CT secondary must be shortcircuited.(1 mark)(ii) Any ONE of: To prevent a high voltagedeveloping in the circuit. To prevent damage to the CTaccuracy through core saturation
  21. A three-phase motor has the term “230V/400V” on its nameplate. Whatmethod of connection would you use to connect the motor to a three-phasesupply?
    It means that the motor must be connectedin star.

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