et26

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Author:
gregw
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104115
Filename:
et26
Updated:
2011-09-25 04:30:33
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electrical theory
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electrical theory
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  1. Calculate the line current of a three phase 15kW deep fryer. The supply is 400 V, 50Hz.
    • W = root3 x VL x IL
    • IL = 15000/1.732 x 400
    • = 21.65A
  2. (i) What test voltage is required when carrying out an insulation resistance test on a three-phase, 400 V, MEN supply cable?
    (ii) What is the minimum value of the permitted test result for the insulation resistance test on the cable?
    • (i) 500 V dc
    • (ii) 1Mohm
  3. Briefly explain the purpose of the interlocks in a star/delta starter.
    To prevent the star and delta contactors closing simultaneously Causing a short-circuit.
  4. State TWO advantages that HRC fuses have - other than cost - when compared to MCBs
    • Good discrimination
    • Good back-up protection
    • Fast operation
    • Range of utilisation categories (fusing factor)
    • Higher rupturing capacity
  5. State TWO disadvantages that MCBs have - other than cost - when compared 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
  6. 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 a rotating magnetic field.
    • Increase the start-up torque
  7. For what reason would you connect a resistor across a circuit after the electricity supply to that circuit has been isolated.
    To discharge a large capacitor
  8. 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.
  9. What is the synchronous speed of a three-phase, four-pole, 50 Hz motor?
    • 1500 rpm
    • The speed of the rotating field
  10. State the main reason why it is important to consider prospective short-circuit current when choosing a protection device.
    A protective device must be selected that can safely handle the maximum short-circuit current under fault conditions..
  11. What type of fuse would you install where the prospective short-circuit current is calculated to be 20,000 A?
    An HRC fuse
  12. A residual current device (RCD) has operated. The RCD protects a circuit and a fixed-wired Class I electrical appliance. What has the RCD detected that would cause it to operate?
    • Earth leakage current in the circuit.
    • 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
  13. Draw and label a diagram to represent the supply arrangements for an MEN distribution system that includes:
    A delta-star-connected 11 kV/400 V supply transformer showing the neutral and earthing arrangements
    The conductors supplying a single-phase consumer installation and a three phase consumer installation
    The single-phase consumer switchboard that shows the main switch,neutral and earthing arrangements
    The three-phase consumer switchboard that shows the main switch,neutral and earthing arrangements Fuse protection for each installation
  14. State TWO reasons why the neutral conductor is earthed in an MEN system.
    • The neutral ensures a low impedance fault loop on an earth fault.
    • Limits the voltage to the nominal phase voltage.
    • Mass of earth provides an alternative return path for the current if neutral is lost.
    • To attempt to maintain the neutral at earth potential
  15. State TWO factors that limit the prospective short-circuit current in an electrical installation.
    • Capacity of the supply source
    • Transformer impedance.
    • Circuit impedance from the transformer to the switchboard (length of cable run).
  16. Describe FOUR electrical hazards that may occur in a low voltage installation if the impedance of the main neutral is higher than the impedance of main earth.
    • A touch voltage hazard between conductive parts and the mass of earth.
    • Protective devices may not operate under fault conditions.
    • Fire hazard at any high resistance joint in the main neutral.
    • Fire hazard because the main earthing conductor is carrying a higher current than it can safely carry continuously.
    • Over or under voltage could damage equipment – particularly on three phase installations.
    • Damage could occur on neighbouring electrical installation due to load current passing through the earth.
  17. Describe the method by which a single-phase supply may be used to simulate a rotating magnetic field effect for a split-phase single phase induction motor.
    • The two stator windings are spaced 90 electrical degrees apart and connected in parallel to a single-phase supply.
    • The start winding has a higher resistance and lower reactance than the run winding.
    • The current through the start winding is closer in phase with the applied voltage.Therefore, the current through the start winding has a phase displacement to the current in the run winding. This produces the simulated rotating magnetic field.
  18. If an external starting relay is to be used to for a single-phase, split-phase induction motor, which winding will it switch?
    The start (or auxiliary) winding.
  19. In a single-phase, capacitor start induction motor, what would be the effect on the performance of the motor if the run winding was open-circuited:

    (i) And the motor was switched on whilst on load?
    (i) The motor would not start.
  20. In a single-phase, capacitor start induction motor, what would be the effect on the performance of the motor if the run winding was open-circuited:

    (ii) While the motor was running whilst on load?
    • (ii) The motor would slow down until the centrifugal switch closed.
    • And For light loads- The motor will speed up again as the start winding is energised.
    • The centrifugal switch will open at around 75% full-load speed so the motor will cycle as the switch opens and closes.
    • Or For heavy load- The motor will stall and eventually burn out.
  21. The following graph shows the time/current characteristics of six HRC fuses

    (i) Why is the graph referred to as an inverse time current characteristic graph?
    (ii) Which fuse would trip when a current of 27A flows with a pre-arcing time of 1.0 second?
    (iii) What is the gG utilisation category (fusing factor) of a 6A rated fuse?
    (iv)What value of current will operate the 6A fuse?
    • (i) It shows that the higher the fault current the shorter the time it takes the fuse to trip.
    • (ii) The fuse curve for 10A.
    • (iii) The gG utilisation category (fusing factor) is in the range between 1.2 and 1.5.
    • (iv) Between 7.2A and 9.6A
  22. Describe what happens in an electrical installation when a fault occurs and there is “discrimination” between protective devices.
    The protective device nearest the fault operates, before any other protective device.
  23. Under what circumstances must back-up protection be installed in a motor circuit?
    • To provide short-circuit protection
    • Because the existing protective devices have inadequate breaking capacity.
  24. What is meant by the term “current rating” when applied to fuses.
    This is the maximum continuous current that the fuse can carry without deteriorating.
  25. The circuit supplying a 230V, single-phase wash-down pump has both RCCB and MCB protection. The pump isolating switch has been replaced.
    (i) What would be the effect if the phase and neutral were accidentally interchanged at the supply side of the isolating switch?
    • Any THREE of:
    • The protection will still be effective under fault conditions.
    • The pump will operate normally
    • The neutral would be switched
    • The pump circuitry would be live when the isolating switch is in the “off” position.
  26. The circuit supplying a 230V, single-phase wash-down pump has both RCCB and MCB protection. The pump isolating switch has been replaced.
    (ii) What would be the effect if the neutral and earth were accidentally interchanged at the supply side of the isolating switch?
    (ii) The RCCB will trip as the PEC would carry the return current.
  27. State ONE test that would detect the interchange of the phase and neutral conductors before the electricity supply is livened.
    • Any ONE of:
    • Circuit connections test (polarity)
    • Visual check
  28. A three-phase, 400V, four-wire distribution board has a load of 32A on the red phase, 12A on the white phase and 40A on the blue phase. Determine the neutral current using a graphical method.
  29. You need to take a three-phase, 10 kW pump motor away for repair. You have isolated the motor and have disconnected the motor cables. The motor cables are still connected to the DOL starter.State TWO precautions which must be taken to ensure the safety of persons and property after you have completed the isolation and disconnection
    • Any TWO of:
    • Each conductor must be insulated and made electrically safe.
    • Protect the cables to prevent damage
    • Take steps to prevent access to the cables.
  30. How is “isolating” a three-phase machine different from just “switching off” the machine?
    • “Isolating” means that the motor has been deliberately disconnected from the electricity supply and precautions taken to prevent reconnection
    • “Switched off” means that the electricity ceases to be supplied to the motor
  31. Describe THREE different methods of safely ensuring the continued isolation of a fixed-wired, three-phase machine.
    • Any THREE of:
    • Attach a safety warning tag
    • Lock open the isolating switch.
    • Move the fuse carriers of the fuses controlling the circuit supplying the motor to a safe location.
    • Use an access permit or “hold card” system.
    • Disconnect the circuit supplying the motor at source.
  32. State the TWO reasons why the prove-test-prove method of testing for isolation is used.
    • To ensure that the test meter/instrument operates correctly during the procedure.
    • To ensure that the circuit has been isolated before it is worked on.
  33. A 400V, three-phase star-connected bakery oven is rated at 25 kW. The circuit is protected by 45A, gG type, HRC fuses with utilisation category (fusing factor) of 1.5.
    (a) A fault of negligible impedance has developed between one line and the oven frame while the oven is operating. The protective earth conductor resistance is 0.75W. Calculate the total current in the faulty line
  34. If a fault was 15ohm impedance and the protective earth conductor resistance is 5ohm.
    (i) Calculate the total current in the faulty line
  35. If a fault was 15ohm impedance and the protective earth conductor resistance is 5ohm.
    (ii) Calculate whether the fuse will operate.
    • (ii) The 45A fuses have a fusing factor (gG Utilisation Category) of 1.5
    • Fusing current = 1.5 x 45 = 67.5 A
    • Total fault current is 47.58A
    • so the fuse will not operate within the minimum disconnection time.
  36. If a fault was 15ohm impedance and the protective earth conductor resistance is 5ohm.
    (iii) Calculate the touch voltage that would appear on the frame of the oven.
  37. A three-phase three-wire 11 000V supply is available for the primary connection of the delta connected transformer to a commercial site.You have taken the current measurements at the three-phase, four-wire secondary side of the transformer and have recorded the following maximum demands:
    Red = 145A
    White = 130A
    Blue = 120A
    The line voltage on the secondary side is 400V.
    (a) Using the information above, determine:
    (i) The load in kVA of the heaviest loaded phase.
  38. A three-phase three-wire 11 000V supply is available for the primary connection of the delta connected transformer to a commercial site.You have taken the current measurements at the three-phase, four-wire
    secondary side of the transformer and have recorded the following maximum demands:
    Red = 145A
    White = 130A
    Blue = 120A
    The line voltage on the secondary side is 400V.
    (a) Using the information above, determine:
    (ii) The minimum sized kVA rating of the three phase distribution transformer required to supply load for this site.
  39. You have been requested to wire a DOL forward and reverse starter that controls a three-phase delta connected motor.
    (a) Draw and label the power circuit for the DOL starter. The drawing must show the contactors and conductors that enables the motor to be operated in the forward and reverse directions.
  40. Draw and label the 230V control circuit for the DOL forward and reverse starter.

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