2008

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mike9111
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159161
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2008
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2012-06-18 01:01:00
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  1. State TWO reasons why the neutral conductor is earthed in an MENsystem
    The neutral ensures a low impedance fault loop on an earth fault.E Limits the voltage to the nominal phase voltage.E Mass of earth provides an alternative return path for the current ifneutral is lost.E To attempt to maintain the neutral at earth potential
  2. State TWO factors that limit the prospective short-circuit current in anelectrical installation
    Capacity of the supply sourceE Transformer impedance.E Circuit impedance from the transformer to the switchboard (length ofcable run).
  3. Describe FOUR electrical hazards that may occur in a low voltageinstallation if the impedance of the main neutral is higher than theimpedance of main earth
    A touch voltage hazard between conductive parts and the mass ofearth.E Protective devices may not operate under fault conditions.E Fire hazard at any high resistance joint in the main neutral.E Fire hazard because the main earthing conductor is carrying a highercurrent than it can safely carry continuously.E Over or under voltage could damage equipment – particularly on threephase installations.E Damage could occur on neighbouring electrical installation due to loadcurrent passing through the earth
  4. Describe the method by which a single-phase supply may be used tosimulate a rotating magnetic field effect for a split-phase single phaseinduction motor
    E The two stator windings are spaced 90 electrical degrees apart andconnected in parallel to a single-phase supply.(1 mark)E The start winding has a higher resistance and lower reactance thanthe run winding.(1 mark)E The current through the start winding is closer in phase with theapplied voltage.(1 mark)E Therefore, the current through the start winding has a phasedisplacement to the current in the run winding.(1 mark)E This produces the simulated rotating magnetic field.
  5. If an external starting relay is to be used to for a single-phase, split-phaseinduction motor, which winding will it switch?
    The start (or auxiliary) winding
  6. In a single-phase, capacitor start induction motor, what would be the effecton the performance of the motor if the run winding was open-circuited:(i) And the motor was switched on whilst on load?(1 mark)________________________________________________________________________________________________________________________________________________________________________(ii) While the motor was running whilst on load?
    (i) The motor would not start.(1 mark)(ii) The motor would slow down until the centrifugal switch closed.(1 mark)And For light loads- The motor will speed up again as the start winding isenergised.(1 mark)- The centrifugal switch will open at around 75% full-loadspeed so the motor will cycle as the switch opens andcloses.(1 mark)Or For heavy load- The motor will stall and eventually burn out
  7. 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.
  8. Under what circumstances must back-up protection be installed in a motorcircuit?
    To provide short-circuit protection(1 mark)Because the existing protective devices have inadequate breaking capacity.
  9. What is meant by the term “current rating” when applied to fuses
    This is the maximum continuous current that the fuse can carry withoutdeteriorating
  10. The circuit supplying a 230V, single-phase wash-down pump has both RCCBand MCB protection. The pump isolating switch has been replaced.(i) What would be the effect if the phase and neutral were accidentallyinterchanged at the supply side of the isolating switch?
    The protection will still be effective under fault conditions.E The pump will operate normallyE The neutral would be switchedE The pump circuitry would be live when the isolating switch is inthe “off” position.
  11. What would be the effect if the neutral and earth were accidentallyinterchanged at the supply side of the isolating switch?
    The RCCB will trip as the PEC would carry the return current
  12. State ONE test that would detect the interchange of the phase and neutralconductors before the electricity supply is livened
    Any ONE of:E Circuit connections test (polarity)E Visual check
  13. You need to take a three-phase, 10 kW pump motor away for repair. Youhave isolated the motor and have disconnected the motor cables. Themotor cables are still connected to the DOL starter.State TWO precautions which must be taken to ensure the safety ofpersons and property after you have completed the isolation anddisconnection
    Each conductor must be insulated and made electrically safe.E Protect the cables to prevent damageE Take steps to prevent access to the cables
  14. How is “isolating” a three-phase machine different from just “switching off”the machine?
    “Isolating” means that the motor has been deliberately disconnectedfrom the electricity supply and precautions taken to preventreconnection(2 marks) “Switched off” means that the electricity ceases to be supplied to themotor
  15. Describe THREE different methods of safely ensuring the continuedisolation of a fixed-wired, three-phase machine.
    Attach a safety warning tagE Lock open the isolating switch.E Move the fuse carriers of the fuses controlling the circuit supplying themotor to a safe location.E Use an access permit or “hold card” system.E Disconnect the circuit supplying the motor at source
  16. State the TWO reasons why the prove-test-prove method of testing forisolation is used
    To ensure that the test meter/instrument operates correctly during theprocedure.E To ensure that the circuit has been isolated before it is worked on
  17. State THREE hazards that will occur if any one of the three phases and theneutral are transposed at a point between the distribution line and theswitchboard
    Any metal not normally live could belive.; If the earth fault path is of highimpedance the main fuse may not blow; Fire hazard could occur through themain earth conductor carryingexcessive current; The earth bar/neutral bar/MEN linkcould be live at up to 230V.; The neutral is being switched at themain switchboard.; 400V could be present at any socketoutlet; Damage could occur to any appliance /motor; Lighting circuits could have 400Vpresent at the light socket
  18. You have been asked to carry out a polarity test on this installation as theremay be a mains transposition between one phase and the neutral.The equipment you have to carry out the test is: A voltmeter A trailing (or flying) lead A temporary earthDescribe how you would carry out a polarity test on the installation usingthis equipment. In your description state:D The connection arrangement for the equipmentD The points on the installation being testedD The expected test result at each point if a transposition has takenplace.D The expected test result at each point if no transposition has takenplace.
    Insert temporary earth stake remotefrom main earth(½ mark) HazardousRemoval of MEN link; Test in turn with voltmeter and trailinglead:(½ mark)* between each phase and remoteearth(½ mark)* between the neutral/earth bar andremote earth(½ mark); If a transposition has taken place:* There will be 230V between two ofthe phases and the remote earth(1 mark)* There will be 230V between theneutral/earth bar and the remoteearth(1 mark)* There will be 0V between one ofthe phases and the remote earth(1 mark); If no transposition has taken place:* There will be 230V between allphases and the remote earth(1 mark)* There will be 0V between theneutral/earth bar and the remoteearth
  19. A three-phase squirrel cage induction motor is running on full-load. It iscontrolled by a DOL starter.(i) A small mechanical overload over a long period of time has occurred.What effect does this fault have on:(A) The motor(½ mark)________________________________________________________________________________________________________(B) The motor protection
    (i) (A) Any ONE of:; The current increases; The motor slows; The motor temperature rises(½ mark)(B) Any ONE of:; The thermal overload willoperate.; The protection will operate
  20. A mechanical overload has occurred that caused the motor to exceedit pull-out torque (break-down torque). What effect does this faulthave on:(A) The motor(½ mark)________________________________________________________________________________________________________(B) The motor protection.
    Any ONE of:; The motor will stall(½ mark)(B) Any ONE of:; The thermal overload willoperate.; The protection will operate
  21. A three-phase squirrel cage induction motor is running on full-load. It iscontrolled by a DOL starter.(i) Under-voltage has occurred on the supply to the motor and starter.What effect does this fault have on:(A) The motor(½ mark)________________________________________________________________________________________________________(B) The motor protection.
    Any ONE of:; The motor will slow; Excess current will flow; Motor torque will reduce(½ mark)(B) Any ONE of:; The thermal overload willoperate.; The protection will operate; The no-volt protection willoperate
  22. An open circuit has occurred in one winding of the motor. What effectdoes this fault have on:(A) The motor(½ mark)________________________________________________________________________________________________________(B) The motor protection.
    (A) Any ONE of:; The motor will stall; A high current will flow(½ mark)(B) Any ONE of:; The thermal overload willoperate.; The protection will operate; The differential overload willoperate.
  23. Describe how the reduction in speed of a three-phase induction motorcaused by an increase in mechanical load causes an increase in line current.
    Motor slowing creates more slip (1 mark); Increase in rotor frequency(½ mark); Rotor voltage and current increase.(1 mark); Creates more rotor flux.(½ mark); This will reduce the back-emf in thestator windings¬†So the effective voltage applied to thestator windings increases(1 mark); So the stator current increases
  24. Give a brief description of circumstances when a Danger Tag is used
    Where there is a possibility of personaldanger through the supply beingrestored.
  25. Give a brief description of circumstances when an Out-of-Service Tagis used.
    Where equipment is faulty or damagedand using that equipment would causedamage or injury.
  26. List THREE precautions to be taken when attaching a danger tag to anisolating switch.
    Make sure the correct isolatingswitch is tagged.; Make sure the switch is in the“OFF” position before it is tagged; Fasten the Danger Tag securely sothat it will not come off.; Test to ensure isolation has takenplace.; Appropriate details are entered onthe tag
  27. Explain the difference between: A 10 kW pump motor that has been “isolated”.and A 10 kW pump motor that has been “switched off”.
    “Isolated” means that the motor has been: deliberately disconnected from theelectricity supply and(1 mark) precautions taken to preventreconnection(1 mark); “Switched off” means that theelectricity ceases to be supplied to themotor
  28. Describe how the prove-test-prove method of testing is carried out.
    The test instrument is checked to beoperating correctly on a known livesource.; The equipment is tested to confirm (orotherwise) that it is isolated.; The test instrument is again checkedon a known live source to ensure is stilloperates correctly.
  29. You are using the prove-test prove method at the supply side of a threephasemain switch to see if the switchboard is isolated. What tests wouldyou make to clearly establish that isolation (or otherwise) has taken place
    Test between each phase and earth
  30. You are to carry out an earth fault loop impedance test on a single phasesocket outlet. Commencing at the socket outlet, list the circuit elements –in order - of the electrical installation and distribution system that are beingtested
    ; The socket outlet; The final subcircuit earth conductor; The MEN connection; The consumers main neutral; The distribution neutral; The distribution transformer; The distribution active (phase)conductor; The main switch; The switchboard cables; The protective device; The final subcircuit active conductor
  31. State TWO uses for the information gained from an earth fault loopimpedance test carried out in (a) above
    Determine the prospective short circuitcurrent at the point in an installation Determine that the correct type andfault current rating of circuit protectionis installed The impedance of the protectiveearthing conductor is low enough tooperate the protective device
  32. Why is the earth fault loop path between the earth and neutral bars andMEN link and the star point via the main neutral considered more importantthan the path between the earth and neutral bars and MEN link and the starpoint via the mass of earth
    It is considered to be the path with thelowest impedance
  33. State the main reason why an installation supplied by a 35 mm2 coppermains cable requires a larger main earthing lead than an installationsupplied by a 16 mm2 copper mains cable.
    To ensure the main earthing lead is able tosafely carry the maximum fault current ofthe installation to enable protective devicesto operate.
  34. You have done an earth fault loop impedance test on a domestic installationand, from the test results, have calculated that the fault level is 1.2 kA.On the switchboard there are three rewireable fuses. State TWO reasonsrelating to the fault level why you would replace the rewireable fuses withMCBs.
    Rewireable fuses are not rated for morethan1 kAorRewireable fuses will not safely ruptureunder the stated fault conditions(1 mark); MCBs are rated up to 10kAorMCBs will safely open under the stated
  35. MCBs with both thermal and magnetic functions are installed on aswitchboard in a factory.(i) Briefly describe the internal operation of the MCB when its thermalfunction operates
    A bi-metal strip heats up anddistorts(1 mark) The distortion operates a tripmechanism that opens the breaker
  36. Briefly describe the internal operation of the MCB when its magneticfunction operates
    A strong magnetic field is created inthe coil(1 mark); The field attracts a trip mechanismthat operates and opens thebreaker.
  37. A low voltage, three-phase, five-pin, socket outlet and final subcircuit wiring hasbeen installed in a bakery.(a) State the Standard that specifies the tests and checks that need to becarried out on the socket outlet and final subcircuit wiring.
    AS/NZS 3000
  38. State the document that must be completed once all the testing andchecking has been completed
    A Certificate of Compliance
  39. State the THREE tests, using test instruments you would carry out prior toconnecting the final subcircuit cable at the switchboard to ensure the socketoutlet and final subcircuit wiring is electrically safe.For each test:D Describe how you would carry it out.D State the test instrument used.D State the permitted test result (if applicable
    • Insulation resistance test
    • Protective earthing conductor test
    • Correct circuit connections test (orpolarity)
    • RCD trip-time test¬†

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