EST 2063 Candidate Code No. For Board Use Only Result Date Result Date Int ELECTRICAL SERVICE TECHNICIAN EXAMINATION 22 November 2014 QUESTION AND ANSWER BOOKLET Time Allowed: Two Hours INSTRUCTIONS READ CAREFULLY You have 10 minutes to read this paper but do not start writing until you are told to do so by the supervisor. Write your Candidate Code Number in the box provided above. Your name must NOT appear anywhere in this paper. Answer all questions. The pass mark for this examination is 60 marks. Use a pen for written answers. Do not use pencils or red pens. Drawing instruments and pencils may be used when diagrams are required. Marks are allocated on the basis of correctness. Do not use correcting fluid or correcting tape. It is recommended that the reference source for your answers be included in the space provided if a question can be answered from the Act, Regulations, Standard or Code of Practice. However, just stating a reference only will earn no marks. For calculation questions all workings, including formulae, must be shown to gain full marks. Warning You could get 0 marks for any question, or part of a question, if you show anything hazardous or dangerous in your answer. You will need to use the following documents in this examination: Int Electricity (Safety) Regulations 2010 reprint dated 1 February 2014 or the reprint dated 1 August 2014 AS/NZS 3000:2007 (incorporating amendments 1 and 2) PLEASE HAND THIS PAPER TO THE SUPERVISOR BEFORE LEAVING THE ROOM
Question 1 (a) Refer to the Electricity (Safety) Regulations 2010 and define the term electrically unsafe as it applies to an electrical installation. Ref:. (b) The use of basic insulation for protection against electric shock is common to both Class I electrical equipment and Class II electrical equipment. Refer to AS/NZS 3000 and state how Class I electrical equipment is different from Class II electrical equipment. Ref: EST 2063 22 NOVEMBER 2014 2
Question 1 continued (c) A 230V, 2kW electrical appliance produces full power output when plugged into a socket outlet. (i) State the effect on the resistance of the elements of the electrical appliance if it was supplied by a 10m extension cord. (ii) State the effect on the power output of the electrical appliance if it was supplied by a 10m extension cord. (d) State the circumstances where an isolating switch would carry two safety warning tags. (e) A three-phase fixed-wired electrical appliance has been operating normally. The only supply to the appliance is through one isolating switch. When that isolating switch was turned OFF and a test for isolation carried out at the appliance terminals, one terminal was live. State a reason why one terminal of the appliance is live. EST 2063 22 NOVEMBER 2014 3
Question 1 continued (f) A switch controls two heating elements of the same resistance of an electric blanket. The switch has three positions low, medium and high. (i) Identify the fault that would cause the electric blanket to draw current only on low and high. The switch is not faulty. (ii) Identify the fault that would cause the electric blanket to draw current only on low. The switch is not faulty. (g) An electrical service technician intends to isolate a faulty fixed-wired threephase electric motor, and intends to withdraw the fuses for that motor on the factory switchboard. State TWO dangerous conditions that could occur if the electrical service technician withdrew the wrong fuses. (1) (2) EST 2063 22 NOVEMBER 2014 4
Question 1 continued (h) An electrical fitting is labelled IP56. Refer to AS/NZS 3000 and state: (i) The level of protection specified by the number 5. Ref:. (ii) The level of protection specified by the number 6. Ref:. (i) A three-phase induction motor is protected by thermal overloads in a DOL starter. State the reason why HRC fuses are installed at the origin of the final subcircuit supplying the motor. EST 2063 22 NOVEMBER 2014 5
Question 1 continued (j) HRC fuses protect the final subcircuit supplying a three-phase motor. Describe how you would carry out a test, using an ohmmeter, to find out whether the protective earthing conductor of the three-phase motor is connected the earthing system (that is, the protection will operate if a fault occurs). EST 2063 22 NOVEMBER 2014 6
Question 2 (a) The New Zealand single phase 230V supply operates at a frequency of 50 Hz. Define the meaning of the term Hz. (b) The nominal voltage of an electrical installation is 230V. Calculate the peak voltage. (c) State TWO reasons explaining how a protective earthing conductor contributes to the electrical safety of a Class I electrical appliance when a fault occurs. (1) (2) EST 2063 22 NOVEMBER 2014 7
Question 2 continued (d) State TWO reasons why a neutral conductor is required in the cable supplying a three-phase electrical appliance which has heating loads that draw different values of current on each of the phases. (1) (2) (e) In some installations, three-phase loads do not require a neutral conductor. Explain the circumstances under which a three-phase load would not require a neutral conductor. (f) The circuit supplying a 230V, single-phase wash-down pump has both RCCB and MCB protection. The pump isolating switch has been replaced. If the neutral and earth were accidentally interchanged at the supply side of the isolating switch, what would be the effect when the pump was turned on? EST 2063 22 NOVEMBER 2014 8
Question 3 (a) Refer to the Electricity (Safety) Regulations 2010 and define in full the term MEN system. Ref: (b) State TWO reasons why the neutral in the MEN system is multiple-earthed. (1) (2) EST 2063 22 NOVEMBER 2014 9
Question 3 continued (c) For the New Zealand low voltage MEN system state: (i) The nominal voltage between the neutral conductor and earth. (½ mark) (ii) The nominal voltage between any active conductor and the neutral conductor. (½ mark) (iii) The nominal voltage between any active conductor and earth. (½ mark) (iv) The nominal voltage between any two active conductors. (½ mark) EST 2063 22 NOVEMBER 2014 10
Question 3 continued (d) This is a diagram of a 400V/230V MEN distribution system supplying a single-phase MEN switchboard and a three-phase MEN switchboard. Complete the diagram by drawing and labelling the components of the MEN system that are missing. (4 marks) EST 2063 22 NOVEMBER 2014 11
Question 4 (a) Explain why the voltage at the load end of a flexible cord extension set supplying current to an electrical appliance is less than that at the supply end of the cord? (b) State TWO ways in which the effect in (a) above can be reduced. (1) (2) (c) All flexible cords are given a current rating. Define the term current rating. EST 2063 22 NOVEMBER 2014 12
Question 4 continued (d) A 230V, 2 kw, Class I electric heater provides its full power output when plugged directly into a socket outlet. The current drawn on full load is 8.7A The resistance of the elements is 26.45Ω Calculate the drop in the power output of the appliance if it is supplied with a 15 metre, 1.0 mm 2 extension cord. Current Carrying Capacities of Flexible Cords Conductor Size Current Carrying Capacity Voltage drop per ampere per metre - Vd/A.m mm 2 A mv 0.5 3 86 0.75 7.5 57 1 10 43 1.25 13 35 1.5 16 31 2.5 20 18 4 25 11 Use the information in this table to complete your calculations. (6 marks) EST 2063 22 NOVEMBER 2014 13
Question 5 You are required to reconnect a 400V three-phase motor that has been repaired. The seven-core copper-braided cable that is to be connected to the star/delta starter is connected to the motor. TWO tests using test instruments are required to be carried out to ensure the motor is safe to connect to the supply. Note: Assume that the polarity of the windings are correct. (a) State the order in which the tests need to be carried out. (b) State the reason why the tests are carried out in the order stated in (a). EST 2063 22 NOVEMBER 2014 14
Question 5 continued (c) Describe how you will carry out each test. For each test include: The test instrument. The test voltage (if applicable) How the test is carried out. The maximum or minimum permitted acceptable test result for that test (8 marks) Test 1 Test 2 EST 2063 22 NOVEMBER 2014 15
Question 6 (a) The flexible cord and control switch for a 230V, Class I, 2 kw, permanentlyconnected water heater are to be replaced. Provision is to be made for a 3 kw element. (i) Calculate the maximum current that the flexible cord and control switch have to carry. (ii) Refer to the table on page 13 and state the minimum current rating of the flexible cord. (½ mark) (iii) State the minimum current rating of the control switch. (iv) For the flexible cord state the minimum number of cores required and: One set of colour coding for those cores. The polarity for each of those cores. EST 2063 22 NOVEMBER 2014 16
Question 6 continued (b) A 230V, 2kW, plug-in Class II portable space heater incorporates a threeheat switch that controls two nichrome wire wound elements of equal resistance. The switch settings are low, medium and high. Prior to putting the heater back into service you want to test each element to ensure that each will produce 1,000W of heat (that is, neither are opencircuited nor have a shorted section). The elements cannot be disconnected. (i) What type of instrument would you use for this test? (½ mark) (ii) Describe how you would carry out this test. (iii) Determine by calculation the expected results of your test if both elements were undamaged. (3 marks) EST 2063 22 NOVEMBER 2014 17
Question 7 (a) (i) Draw and label a circuit diagram of a single-phase capacitor-start motor. (3 marks) (ii) State ONE device that can be used in the single-phase capacitor-start motor in place of a centrifugal switch? (iii) Explain how the direction of rotation can be reversed for the capacitorstart induction motor. EST 2063 22 NOVEMBER 2014 18
Question 7 continued (iv) Describe the purpose of the capacitor in a single-phase capacitor-start motor. (b) A small electric motor has a nameplate that reads: Voltage 230 Phases 1 Horsepower 0.75 Speed 1425 r.p.m. Calculate the current drawn by the motor. (3 marks) EST 2063 22 NOVEMBER 2014 19
Question 8 (a) The diagram below shows the low voltage control circuit of a three-phase direct-on-line (DOL) starter. 1 3 4 7 8 5 2 3 6 (i) State the name of the numbered parts of the circuit. (4 marks) 1 2 3 4 5 6 7 8 (ii) State the operating voltage of the circuit EST 2063 22 NOVEMBER 2014 20
Question 8 continued (b) A three-phase induction motor can be protected by a thermistor. Explain how a thermistor protects the motor. (c) A three-phase induction motor is controlled by an isolator and direct-on-line (DOL) starter and is protected by a phase reversal relay. The three-phase final subcircuit between the switchboard and the isolator has been replaced. When the start button on the starter was pushed, the motor did not start. State the reason the motor did not start. (d) Using a direct-on-line (DOL) starter for a three-phase induction motor can accelerate the load up to speed more quickly than a reduced voltage starter. State TWO reasons why a reduced voltage starter would be used with this induction motor. (1) (2) EST 2063 22 NOVEMBER 2014 21
Question 9 (a) A three-phase induction motor, controlled by a direct-on-line (DOL) starter, drives a wood planer. The three-phase motor hums noisily but fails to rotate when started. State TWO possible causes of this type of fault. Note: 1. The voltage level is normal. 2. The protection is correctly rated and is not faulty. 3. The wood planer is not mechanically overloaded or jammed (1) (2) (b) A three-phase induction motor is controlled by a direct-on-line (DOL) starter. The motor overheats while running, but the current in all three phases is the same as the current rating on the motor nameplate. State TWO possible causes of this type of fault. Note: 1. The voltage level is normal. 2. The protection is correctly rated and is not faulty. (1) (2) EST 2063 22 NOVEMBER 2014 22
Question 9 continued (c) A three-phase induction motor is controlled by a direct-on-line (DOL) starter. The motor gets up to speed but runs noisily and overheats when a light load is applied. State TWO possible causes of this type of fault. Note: 1. The voltage level is normal. 2. The protection is correctly rated and is not faulty. (1) (2) (d) A single-phase, split phase motor hums and fails to rotate during bench testing. State the TWO likely causes of the split phase motor failing to rotate. (1) (2) (e) A single-phase capacitor-start motor is being tested under no-load on a workbench. When switched on, the capacitor-start motor failed to start. Tests showed both the start and run windings to be continuous. Which type of fault would cause this to happen? EST 2063 22 NOVEMBER 2014 23
Question 9 continued (f) A single-phase capacitor-start motor is operating on full-load. The motor suddenly slows down and until the centrifugal switch closes. Which type of fault would cause this to happen? EST 2063 22 NOVEMBER 2014 24
For Candidate s Use In the box, write the number of EXTRA sheets you have used. Write NIL if you have not used any For Examiner s Use Only Questions Answered 1 2 3 Marks 4 5 6 7 8 9 TOTAL EST 2063 22 NOVEMBER 2014 25