2007

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Author:
mike9111
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157899
Filename:
2007
Updated:
2012-06-09 00:49:59
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  1. One end of a main earthing conductor must be connected to the mainearthing terminal in the main switchboard. State ONE point at which theother end of the main earthing conductor can be connected.
    An earth electrode.8 An earthing conductor or terminal, provided by the electricitydistributor8 The electricity distributor neutral bar or link within the electricalinstallation (e.g. at meter panel), if required by the electricitydistributor.
  2. If three-phase motor has six terminals and windings rated at 230V howshould it be connected to a 400V three-phase supply?
    In Star
  3. What is the purpose of a residual current device (RCD)?
    For protecting people against electric shock when using portable equipment
  4. What is the purpose of an on-load tap-changer in a transformer?
    To raise or lower the voltage to maintain a constant voltage under full load
  5. What is the minimum acceptable insulation resistance of an entire lowvoltage electrical installation:(i) That does not contain a heating appliance?(1 mark)________________________________________________________(ii) That contains a heating appliance?
    (i) 1 M=(ii) 10, 000=
  6. Describe TWO different ways of safely ensuring the continued isolation of afixed-wired electrical appliance
    8 Attach a safety warning tag8 Lock open the isolating switch.8 Lock open the circuit breaker controlling the circuit supplying theappliance.8 Move the fuse carriers of the fuses controlling the circuit supplying theappliance to a safe location.8 Use an access permit or “hold card” system.8 Disconnect the circuit supplying the appliance at source
  7. Most appliances sold in New Zealand are designed to operate at 230V or240V. An electric heater with a 26.45; element is rated at 2 kW whenoperated at 230V. Calculate the power output when supplied at 240V
    P = V2R(½ mark)= 240 x 24026.45(½ mark)= 2180W
  8. Why is a centrifugal switch used in a single phase split-phase (resistancestart)motor?
    It disconnects the start winding when the motor approaches full speed
  9. A two core Mineral-Insulated Metal-Sheathed (MIMS) permanently installedsupply cable has been carefully disconnected from a single phase fuel pumpmotor on an oil-fired furnace.State TWO important mechanical precautions that must be observedwhen reconnecting this particular cable to the motor.
    Avoid damage to metal sheath• Maintain loop or "S" bend in cable.• Do not disturb pot seal• Do not flex the solid conductors• Ensure all conductor connections tight• Ensure gland is clean and tight
  10. A miniature circuit breaker has both thermal and electromagnetic currentsensorfunctions. Briefly explain the internal operation of the circuitbreaker when ONE of these functions detects a fault.
    Any ONE of:Thermal A bi-metal strip bends when heated and trips thecircuit breaker out the heating of the strip is caused bythe current flowing through it – or through a heatercoil.Electromagnetic As the current increases, the magnetic field increases,and at a predetermined point, it attracts an armaturethat trips out the circuit breaker.
  11. Describe what happens in an electrical installation when a fault occurs andthere is “discrimination” between protective devices.
    The protective device nearest the fault operates, before any otherprotective device
  12. Under what circumstances must back-up protection be installed in a circuit?
    To provide short-circuit protection in a circuit(½ mark)when another protective device has inadequate breaking capacity
  13. What is meant by the term inverse time-current characteristic in relation tofuses and circuit breakers?
    As current through a protective device increases, the time taken to operatedecreases.
  14. A three-phase induction motor hums on starting but fails to rotate. ListTWO possible causes for this fault.
    Internal connections of one winding reversed8 Mechanical overload8 Supply voltage insufficient8 Loss of one phase8 Mechanical faults (bent rotor, excessive bearing wear)
  15. A three-phase induction motor overheats while running on load. List ONEpossible cause for this fault.
    Mechanical overload8 Winding fault8 Worn bearings causing contact between rotor and stator8 Fan not operational8 Ventilation holes blocked (restricted air flow)8 Wrong motor for application (ambient temperature too high) Reduced line voltage
  16. State ONE other type of protective device that could be used in place of athermal overload
    A thermistor
  17. State THREE hazards that will occur if the phase and neutral aretransposed
    Any metal not normally live could be live at up to 230V.8 If the earth fault path is of high impedance the main fuse maynot blow8 Shock hazard could occur8 Fire hazard could occur8 Parts of an electrical appliance could be live with the controlswitch in the “OFF” position.8 Parts of the installation could be live with the main switch in the“OFF” position.8 The earth bar/neutral bar/MEN link could be live at up to 230V.8 The neutral is being switched.
  18. An existing low voltage single phase electrical installation has beenoperating satisfactorily for the 10 years since it was new. No electricalwork has been carried out on the installation or on the surroundingdistribution network since the house was built.The occupants have begun to receive electric shocks off their washingmachine you have been called to investigate and make repairs for someNo transposition has occurred to the mains of the house.State TWO likely causes of the problem.
    The main neutral in the house is open-circuited (or high resistance)8 The main distribution neutral is open-circuited (or high resistance) The earth at the distribution transformer is open-circuited (or highresistance)8 There is a voltage rise in the installation earthing system The earth pin on the socket outlet is spread and there is an earth faulton the machine8 There in a broken or high resistance protective earthing conductor andthere is an earth fault on the machine.
  19. Give a brief description of circumstances when a Danger Tag is used
    Where is a possibility of personal danger through someone turning onthe electricity a Danger Tag must be fastened to the relevant isolatingswitch.
  20. Give a brief description of circumstances when an Out-of-Service Tagis used.
    Out-of-Service TagsWhere equipment is faulty or damaged and using that equipmentwould cause damage or injury an Out-of-Service Tag must be fastenedto it.
  21. List THREE precautions to be taken when attaching a danger tag to anisolating switch.
    Make sure the correct isolating switch is tagged.8 Make sure the switch is in the “OFF” position before it is tagged8 Fasten the Danger Tag securely so that it will not come off.8 Ensure your tag is fastened even though others may also beattached Test to ensure isolation has taken place.
  22. Describe how the prove-test-prove method of testing is carried out
    The test instrument is checked to be operating correctly on a knownlive source.8 The equipment is tested to confirm (or otherwise) that it is isolated.8 The test instrument is again checked on a known live source to ensureis still operates correctly.

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