ELECTRICAL SERVICE TECHNICIAN EXAMINATION 25 November 2017 QUESTION AND ANSWER BOOKLET

Transcription

1 Candidate Code No. EST 2075 For Board Use Only Result Date Int Result Date Int ELECTRICAL SERVICE TECHNICIAN EXAMINATION 25 November 2017 QUESTION AND ANSWER BOOKLET INSTRUCTIONS READ CAREFULLY Time Allowed: Two Hours 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 a pencil or a red pen. Drawing instruments and pencils may be used when diagrams are required. Marks are allocated based on correctness. Do not use correcting fluid or correcting tape. If a question can be answered from the Act, Regulations, Standard or Code of Practice it is recommended that the relevant clause or regulation number be stated in the reference space provided. You will need to use the following documents in this examination: The Electricity (Safety) Regulations reprint dated 4 April AS/NZS 3760:2010. PLEASE HAND THIS PAPER TO THE SUPERVISOR BEFORE LEAVING THE ROOM

2 Question 1 (a) Notifying WorkSafe of danger is required if damaged electrical equipment presents an immediate danger to life or property. Refer to the Electricity (Safety) Regulations 2010 and state ONE aspect that must be included in the notification to WorkSafe. Ref: (b) A 230V handheld electrical appliance used on a building or structure under construction must be used with an appropriate safety system. Refer to the Electricity (Safety) Regulations 2010 and state ONE safety system. Ref:. EST NOVEMBER

3 Question 1 continued (c) Use AS/NZS 3760 to answer parts (i) and (ii). (i) State the permitted test result for an insulation resistance test on a 230V, Class I electrical appliance. Ref: (ii) State the permitted test result for an earth leakage test on a 230V, Class I electrical appliance. Ref: (d) Explain how a phase-reversal relay protects a three-phase, 400V induction motor. (e) When a three-phase induction motor, controlled by a DOL starter is restarted using the start button, the contactor closes, but as soon as the start button is released the contactor drops out of the circuit. State TWO causes for this type of fault. (1) (2) EST NOVEMBER

4 Question 1 continued (f) (i) Describe how to reverse the direction of rotation of a single-phase capacitor-start induction motor. (ii) Describe how to reverse the direction of rotation of a single-phase universal motor. (g) A replacement HRC fuse must have electrical characteristics similar to one it replaces. Two of those characteristics are voltage rating and current rating. State TWO other electrical characteristics. (1) (2) (h) A 240V/230V plug-in electric heater draws 10 A when plugged into a 240 V supply: (i) Will the current increase or decrease if the heater is plugged into a 230 V supply? (ii) Will the power dissipated increase or decrease if the heater is plugged into a 230 V supply? EST NOVEMBER

5 Question 1 continued (i) An isolating switch has been opened to enable the isolation of a fixed-wired electrical appliance. When the appliance terminals were tested they were still live. State TWO circumstances that may cause such a situation to occur. (1) (2) (j) Which TWO of the following factors will not affect the voltage drop in a two-core flexible cord? Insulation resistance Resistance Fusing factor Cross-sectional area (1) (2) EST NOVEMBER

6 Question 2 (a) A test instrument is being used to measure voltage and current values of a live 230V, 2 kw electrical appliance. State FOUR precautions relating to the test instruments that will ensure testing can be carried out safely. (4 marks) (1) (2) (3) (4) (b) Testing for isolation on the supply side of a three-phase isolator is being carried out using the prove-test-prove method. The testing between phases showed no reading on the voltmeter. The electrical service technician dismantled the isolator and received an electric shock. Explain why the testing was ineffective. turn over) EST NOVEMBER

7 Question 2 continued (c) Explain how the prove-test-prove procedure is carried out when using a voltmeter to test for isolation. (3 marks) (d) A MCB protects a 230V final subcircuit that supplies a fixed wired, 230V electrical appliance controlled by a lockable isolator. The electrical appliance has been isolated. Describe ONE method of ensuring that the fixed-wired electrical appliance remains isolated while the work site is left unattended. turn over) EST NOVEMBER

8 Question 3 (a) (i) Define the term current rating as it applies to an HRC fuse. (ii) Define the term voltage rating as it applies to an HRC fuse. (iii) Define the term category of duty as it applies to an HRC fuse. (b) Define the term inverse time-current characteristic as it applies to protective devices. EST NOVEMBER

9 Question 3 continued (c) Explain how gm HRC fuses provide back-up protection for the thermal overloads in a DOL starter supplying a three-phase induction motor. (d) State TWO reasons why it is important to thread the fuse wire from terminal to terminal through the tortuous path in the fuse carrier, when reloading a rewireable fuse. (1) (2) EST NOVEMBER

10 Question 4 Introduction HRC fuses on a switchboard protect a final subcircuit that supplies a 400V induction motor. The motor is controlled by a lockable isolating switch and star/delta starter. The motor needs to be repaired. Use the information in the introduction to this question to answer parts 4(a), 4(b) and 4(c). (a) Describe how to isolate the 400V induction motor so it is safe to disconnect the motor cable from the DOL starter. (4 marks) (b) The disconnected motor with cable (6 phase conductors and an earth conductor) attached has been repaired and needs to be returned to service. Describe how to use an ohmmeter to determine which pair of conductors are attached to a winding. EST NOVEMBER

11 Question 4 continued (c) An insulation resistance test needs to be carried out on the motor and cable (6 phase conductors and an earth conductor): State the test instrument used. State the test voltage used. Describe how the test is carried out. State a test result that permits the motor and cable to be returned to service. (4 marks) EST NOVEMBER

12 Question 5 (a) State the voltage that exists between each of the four conductors, and between each conductor and earth in the New Zealand MEN distribution system. (b) Refer to the Electricity (Safety) Regulations 2010 and define the term MEN switchboard Ref: (c) State ONE reason why a neutral conductor is required in a final subcircuit cable supplying a three-phase, 400V load. EST NOVEMBER

13 Question 5 continued (d) State ONE reason why a neutral conductor is not required in a final subcircuit cable supplying a three-phase, 400V load. (e) (i) What is the frequency of the New Zealand low voltage a.c. supply. (ii) If the usual operating voltage of a low voltage domestic electrical installation is 230 V, what is the peak voltage? EST NOVEMBER

14 Question 6 (a) Voltage rating and current rating are two factors that need to be considered when selecting a flexible cord for a single-phase Class I electrical appliance. State FOUR other factors that need to be considered in selecting the flexible cord. (1) (2) (3) (4) (b) A 230V, 3 kw electrical appliance is being supplied by a flexible extension cord. (i) Explain why the voltage at the load end of a flexible extension cord is less than 230V. (ii) State TWO methods of reducing the effect in (b)(i). (1) (2) EST NOVEMBER

15 Question 6 continued (c) Define the term current rating as it applies to flexible cords. (d) A replacement flexible cord is being selected for a single-phase Class I, heater rated at 2 kw. Which main feature of the flexible cord will determine whether the appliance can fully produce 2 kw? (e) Refer to AS/NZS 3760 and state the polarity requirements of a rewireable plug connected to a flexible cord. (3 marks) Ref:. EST NOVEMBER

16 Question 7 AS/NZS 3760 details two instrument tests for testing the insulation of a three-phase, Class I plug-in electrical appliance that has been repaired. The plug has three phase pins and one earth pin. (a) Test 1 Name of test: Ref. Instrument used: How the test is carried out: Test result: Ref. EST NOVEMBER

17 Question 7 continued (b) Test 2 Name of test: Ref. Instrument used: How the test is carried out: Test result: Ref. EST NOVEMBER

18 Question 8 Introduction A 230V, Class I, single-phase, fixed-wired electrical appliance has 4 operating settings and supplied by a flexible cord from a lockable isolator. To find out whether the appliance is operating correctly, the current drawn at each setting must be found. Use the information in the introduction to this question to answer parts 8(a) and 8(b). (a) Describe the procedure used to measure the current drawn at each operational setting using a clip-on ammeter. (4 marks) EST NOVEMBER

19 Question 8 continued (b) The electrical appliance has been isolated and the isolator has been locked open and tagged. Describe the procedure used to find the current drawn. Include the instrument used. (6 marks) EST NOVEMBER

20 Question 9 Introduction A three-phase motor is controlled by DOL starter that contains thermal overloads. The starter control circuit operates at 230V. The motor circuit is protected by HRC fuses. Use the information in the introduction to answer parts 9(a), 9(b) and 9(c). (a) (i) Draw and label the circuit diagram of the 230V DOL starter control circuit. The control circuit incorporates electrical protection. (4 marks) EST NOVEMBER

21 Question 9 continued (ii) The 230V coil in the control circuit has burnt out. The only replacement you have is a 400V coil. State the modifications you would make to the 230V control circuit so the 400V coil can be used. (b) A thermal overload auxiliary contact is replaced with a thermistor control relay. Describe how the thermistor control relay provides protection to a motor (c) When the start button is pushed on the DOL starter the contactor closes, but as soon as the start button is released the contactor drops out. State the reason why this could occur. EST NOVEMBER

22 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 Marks TOTAL EST NOVEMBER