ELECTRICAL SERVICE TECHNICIAN EXAMINATION 16 September 2017 QUESTION AND ANSWER BOOKLET

Transcription

1 Candidate Code No. EST 2074 For Board Use Only Result Date Int Result Date Int ELECTRICAL SERVICE TECHNICIAN EXAMINATION 16 September 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 pencils or red pens. 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. For calculation questions all workings, including formulae, must be shown to gain full marks. You will need to use the following documents in this examination: Electricity (Safety) Regulations 2010 reprint dated 1 August 2014 or the Electricity (Safety) Regulations reprint dated 4 April AS/NZS 3000:2007 (incorporating amendments 1 and 2). PLEASE HAND THIS PAPER TO THE SUPERVISOR BEFORE LEAVING THE ROOM

2 Question 1 (a) (i) A three-phase motor has been stopped after one of the incoming supply phases was lost. What type of protective device has operated to stop the motor? (ii) A new three-phase supply has been installed to an existing threephase motor and starter. The motor would not start when turned on. What type of protective device is preventing the motor from starting. (b) When replacing an HRC fuse which has blown, the replacement must have four electrical characteristics that are like the original fuse. State TWO of those characteristics. (1) (2) (c) The flexible cable connecting a three-phase motor to a DOL starter incorporates three phase conductors and an earth conductor. Refer to AS/NZS 3000 and state the colours for the conductors of the flexible cable. Ref: EST SEPTEMBER

3 Question 1 continued (d) 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 10 metre extension cord. (ii) State the effect on the power output of the electrical appliance if it was supplied by a 10 metre extension cord. (e) Refer to the Electricity (Safety) Regulations 2010 and define the term electrically unsafe as it applies to an electrical installation. Ref:. EST SEPTEMBER

4 Question 1 continued (f) 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:. (g) Describe how the direction of rotation can be reversed in a universal motor. EST SEPTEMBER

5 Question 1 continued (h) A three-phase electric motor with a flexible cord and plug attached is to be tested before being returned to service. An insulation resistance test of the motor shows a value of 0.75 MΩ. Does this test result allow the motor to be returned to service? State a reason to support your answer. (i) A single-phase capacitor-start motor is being tested under no-load on a workbench. When the motor was switched on, the motor failed to start. Tests showed both the start and run windings to be continuous. State TWO types of fault that would cause this to happen. (1) (2) (j) An electrical service technician notices that an electrical fitting on electrical equipment is damaged and if used, could endanger personnel. The equipment is controlled by a lockable isolator. State the action should the electrical service technician must take to ensure the safety of personal. EST SEPTEMBER

6 Question 2 (a) Test instruments to measure voltage and current values are being used to test live 400V electrical equipment. State FOUR precautions relating to the test instruments that will ensure that testing can safely be carried out. (4 marks) (1) (2) (3) (4) (b) An electrical service technician used a voltmeter to test for isolation using the prove-test-prove method on the supply side of a three-phase isolator. Testing between each phase showed no reading on the meter. The electrical service technician dismantled the isolator and received an electric shock. Explain why the testing was ineffective. EST SEPTEMBER

7 Question 2 continued (c) Explain how the prove-test-prove method of testing for isolation with a voltmeter is carried out. (3 marks) (d) State ONE method of ensuring the continued isolation of a fixed wired electrical appliance after it has been isolated and a danger tag attached. turn over) EST SEPTEMBER

8 Question 3 (a) Define the term current rating as it applies to an HRC fuse. (b) A 230V socket outlet final subcircuit is protected by a MCB. The MCB has operated before the final subcircuit has become fully loaded. State ONE reason why the MCB has operated. (c) An HRC fuse on a switchboard protects a final sub-circuit. State ONE reason why the circuit has been damaged under fault conditions. EST SEPTEMBER

9 Question 3 continued (d) A MCB used as back-up protection for a motor final subcircuit has the markings 6 ka and C16 (16A). (4 marks) (i) Explain the meaning of the term 6 ka. (ii) Explain the meaning of the term C16 (16A) (e) Describe how the internal mechanism of a thermal type circuit breaker operates when an overload occurs. EST SEPTEMBER

10 Question 4 (a) The diagram below shows the low voltage control circuit of a three-phase direct-on-line (DOL) starter (i) State the name of the numbered parts of the circuit. (4 marks) (ii) State the operating voltage of the circuit EST SEPTEMBER

11 Question 4 continued (b) A three-phase induction motor is protected by a thermistor. Explain how a thermistor protects the motor. (c) A three-phase induction motor is protected by a thermal overload. Explain how a thermal overload protects the motor. (d) Two phases were reversed at the output terminals of a DOL starter supplying a three-phase induction motor when the wiring was reconnected. What will happen when this induction motor is livened? EST SEPTEMBER

12 Question 5 (a) Refer to the Electricity (Safety) Regulations 2010 and define the term MEN system. Ref: (b) State ONE reason why the neutral in the MEN system is multiple-earthed. EST SEPTEMBER

13 Question 5 continued (c) For the New Zealand, low voltage MEN system state: (i) The voltage between the neutral conductor and earth. (½ mark) (ii) The voltage between any phase conductor and the neutral conductor. (½ mark) (iii) The nominal voltage between any phase conductor and earth. (½ mark) (iv) The nominal voltage between any two phase conductors. (½ mark) EST SEPTEMBER

14 Question 5 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 SEPTEMBER

15 Question 6 (a) An earth continuity conductor test on a 230V, Class I, 750 W, single-phase plug-in washing machine showed the protective earthing conductor had a resistance of 9 Ω. There is a phase to earth fault on the washing machine between the machine control panel and the motor. (i) Calculate the total current that would flow in the washing machine when the machine was operated without being repaired. (5½ marks) (ii) The final subcircuit supplying the washing machine outlet is protected by a 20 A MCB that has a 1.5 tripping factor. Determine by calculation whether the MCB would operate. (2½ marks) EST SEPTEMBER

16 Question 6 continued (b) A 230V, Class I, plug-in electrical appliance is controlled by a single pole switch. You have disconnected the appliance and are testing it with an ohmmeter. Describe the TWO situations that could cause the neutral to be switched instead of the phase in the appliance. (1) (2) EST SEPTEMBER

17 Question 7 Introduction The following figure is a block diagram of a three-phase circuit to a three-phase star-connected hot water cylinder in a factory. There are no other connections to the circuit. Switchboard Lockable isolator Contactor Hot water cylinder The cylinder circuit is protected by HRC fuses on a three-phase switchboard and the lockable isolator and contactor are adjacent to the cylinder. The existing hot water cylinder is to be replaced. installed later. The new cylinder will be Use the information in the introduction to this question to answer parts 7(a), 7(b) and 7(c). EST SEPTEMBER

18 Question 7 continued (a) The wiring to the existing cylinder is to be disconnected at the contactor. Describe the procedure to isolate circuit so it is safe to disconnect the hot water cylinder wiring. (4 marks) (b) Sometime later the new hot water cylinder and new wiring between the contactor and cylinder is to be connected. There is no safety tag attached to the isolator. State the first action that needs to be taken before attempting to connect the new hot water cylinder and wiring. (3½ marks) EST SEPTEMBER

19 Question 7 continued (c) Two instrument tests need to be carried out on the new hot water cylinder and wiring to ensure it is safe to connect to the contactor. For ONE of those tests state: The name of the test. The function and range selected on the test instrument How the test is carried out. A test result that will permit the cylinder to be connected. (2½ marks) EST SEPTEMBER

20 Question 8 Introduction The figure below is a 230V Class I oven with three compartments. There is one element in each compartment. Element 1 is Ω Element 2 is Ω Element 3 is Ω P Fuse Control Switch N Use the information in the introduction to this question to answer parts 8(a) and 8(b). EST SEPTEMBER

21 Question 8 continued (a) (i) Calculate the maximum current drawn by the oven when all three elements are operating. (5 marks) (ii) State the minimum size HRC fuse that should be used. EST SEPTEMBER

22 Question 8 continued (b) Calculate the power consumed by the oven when only element 2 and element 3 are operating. (4 marks) EST SEPTEMBER

23 Question 9 (a) (i) Draw and label a circuit diagram of a single-phase split-phase motor. (4 marks) (ii) State ONE device that can be used in a single-phase split-phase motor instead of a centrifugal switch? (iii) Describe how the direction of rotation can be reversed for a singlephase split-phase motor EST SEPTEMBER

24 Question 9 continued (b) (i) Explain how the direction of rotation can be reversed for a capacitorstart induction motor. (ii) Explain how the direction of rotation can be reversed for a universal motor (c) Calculate the power (watts) of 0.75 horsepower single-phase motor. (d) State ONE advantage a single-phase, capacitor-start motor has when compared to a single-phase, split-phase motor. EST SEPTEMBER

25 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 SEPTEMBER