ELECTRICAL INSTALLER THEORY EXAMINATION 21 June 2014 QUESTION AND ANSWER BOOKLET

Transcription

1 Candidate Code No. EIN 08 For Board Use Only Result Date Int Result Date Int ELECTRICAL INSTALLER THEORY EXAMINATION 21 June 2014 QUESTION AND ANSWER BOOKLET INSTRUCTIONS READ CAREFULLY Time Allowed: Three 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 on 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 on the basis of correctness. Do not use correcting fluid or correcting tape. 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. Candidates are not permitted to use any Act, Regulation, Standard, Code of Practice, Handbook or other reference text in this examination. PLEASE HAND THIS PAPER TO THE SUPERVISOR BEFORE LEAVING THE ROOM.

2 Question 1 (a) (i) An insulation resistance test is being carried out on a 400V, Class 1, three-phase appliance. (A) State the minimum acceptable insulation resistance for the appliance if it does not incorporate MIMS elements. (½ mark) (B) State the minimum acceptable insulation resistance for the appliance if it incorporates MIMS elements. (½ mark) (ii) Testing is being carried out to test the integrity of the insulation of a 230V, Class 1, single-phase appliance that does not incorporate MIMS elements. (A) State the minimum acceptable insulation resistance test for the appliance. (½ mark) (B) State the maximum acceptable earth leakage test result for the appliance. (½ mark) (b) State TWO reasons why reduced-voltage starting of a large three-phase induction motor may be required. (1) (2) 2

3 Question 1 continued (c) With reference to a three-phase system of supply, explain the term phase sequence. (d) State the main reason why it is important to consider the category of duty when selecting a protective device. (e) A Residual Current Circuit Breaker (RCCB) is installed in a single-phase final subcircuit. State TWO other protective devices, either of which is needed to provide electrical protection for the final subcircuit. (1) (2) (f) Define the term inverse time-current characteristic as it applies to HRC fuses and MCBs 3

4 Question 1 continued (g) State the TWO types of operation employed by most MCBs. (1) (2) (h) State the TWO types of iron losses in the magnetic core of a power transformer. (1) (2) (i) A three-phase induction motor normally runs on full-load. It has been serviced and you are re-commissioning it. When you load test the motor you find that it is now required to run lightly loaded. State why it is not desirable to allow a motor to run lightly loaded when it normally runs on full load. (j) The tests on a three-phase, star-connected squirrel cage motor show the following readings: L 1 L 2 20Ω L 1 L 3 open circuit L 2 L 3 open circuit State the most likely fault indicated by these readings. 4

5 Question 2 (a) (i) State the main reason for carrying out an earth fault loop impedance test on a single-phase final subcircuit supplying a socket outlet. (ii) Describe how the earth fault loop impedance tester is connected. (½ mark) (b) (i) State the main reason for carrying out an RCD test on a single-phase final subcircuit supplying a socket outlet. (ii) Describe how the RCD tester is connected. (½ mark) 5

6 Question 2 continued (c) Explain why Portable Residual Current Devices (PRCDs) used in New Zealand are required to be voltage dependent. (d) (i) Describe how the prove test prove method of testing for isolation is carried out (3 marks) (ii) Why is it important to use the prove test prove method before working on electrical equipment? (2 mark) 6

7 Question 3 (a) Use the information given on this name plate to answer questions 3(a)(i), 3(a)(ii) and 3(a)(iii). Type MOT 112M No Year kw 4.0 kw H.P Hz 50 R.P.M 1440 Ph. 3 - Ins. F Amb. Temp Cos θ 0.82 η %. Duty S1 Protection. IP 53 V 415 A 8.52 V A (i) Calculate the motor efficiency (4 marks) 7

8 Question 3 continued (ii) Calculate the percentage slip at full load (iii) The motor is rated IP53. The letters IP stand for International (Ingress) Protection. Explain what is meant by the term IP. (b) The motor is to be started direct-on-line and protected by a thermal overload unit. The motor final subcircuit is protected by HRC fuses. Explain why both of the protection devices are required. (i) Thermal overload (ii) HRC fuses 8

9 Question 4 (a) A 400V, three-phase, four-wire installation has the following resistive loads on each phase: Red 60kW White 40kW Blue 82kW Determine the neutral current in amps. A scaled diagram may be used. (4 marks) 9

10 Question 4 continued (b) State TWO technical reasons why it is important to balance single-phase loads on a star-connected three-phase installation. (1) (2) (c) State the formula used to determine the power consumed by: (i) An inductive three-phase load. (ii) A resistive single-phase load. (d) A 230V, plug-in heater has an output of 2kW. The supply voltage to the heater fluctuates between 230V to 240V. Describe the effect this voltage fluctuation has on the power output of the heater elements. 10

11 Question 5 Introduction You have been called to a fixed-wired concrete-making vibrating plant where a staff member received an electric shock off the frame of the plant. The vibrating plant is driven by a 400V, three-phase, 1.5kW induction motor controlled by a DOL starter. The motor and starter are controlled by an adjacent lockable isolator. Use the information in the introduction to this question to answer parts 5(a), 5(b) and 5(c). (a) Describe how you will isolate the vibrating plant so testing can be carried out to locate the fault. (3½ marks) 11

12 Question 5 continued (b) The vibrating plant has been isolated. Describe how you will carry out testing to locate the fault. Include in the description: The type of test instrument used The test voltage Where on the vibrating plant the testing takes place How the testing is carried out The expected test result when the fault is located. (4 marks) 12

13 Question 5 continued (c) The fault on vibrating plant has been repaired and the plant reconnected to the supply. Describe the testing you need to carry out to ensure that in the event of a fault developing on the motor, the motor electrical protection will operate. Include in your description: The type of instrument used How the test is carried out A test result that will permit the motor to be livened (2½ marks) 13

14 Question 6 Introduction The figure below represents 3 fluorescent lights supplied from a 230V MEN supply. Switchboard Active (Phase) RCD 16A MCB Light switch Earth 1A 1A 1A Neutral Fitting A The fuses in the light fittings are HRC fuses. Fitting B Fitting C Use the information in the introduction to this question to answer parts 6(a), 6(b), 6(c), 6(d) and 6 (e) (a) (i) Which parts of the circuit are protected by the RCD. (ii) What type of fault does the RCD provide protection for? 14

15 Question 6 continued (b) (i) Which parts of the circuit are protected by the MCB. (ii) What types of faults does the MCB provide protection for? (c) (i) Which parts of the circuit are protected by the HRC fuse in Fitting A? (ii) What types of fault does this fuse provide protection for? (d) How is discrimination achieved in this circuit? (e) State TWO reasons why the fuses in the fittings must not be shorted out or increased in current rating. (1) (2) 15

16 Question 7 (a) When carrying out testing on a 230V final subcircuit, which Standard and section of that Standard details the tests and checks required to be carried out? (b) The two figures below represent the front view of two single-phase socket outlets. Both socket outlets are connected to a standard low voltage supply. (3 marks) Label the polarity of the socket outlet connections on the figures. Note: You must show the polarity completely correct to gain marks. 16

17 Question 7 continued (c) You are using a voltage tester on the front of a 230V switched socket outlet to ensure power is present. Note: The voltage tester is operating correctly. (3 marks) (i) What voltage would you expect to find with the switch OFF: Between Active Neutral Between Active Earth Between Neutral Earth (ii) What voltage would you expect to find with the switch ON: Between Active Neutral Between Active Earth Between Neutral Earth (d) You are using a voltage tester on the front of a 230V switched socket outlet to ensure power is present and get the following results. Test between Socket Terminals Voltages Switch OFF Voltages Switch ON Active - Neutral 0V 230V Active - Earth 0V 0V Neutral - Earth 0V 0V Note: The voltage tester is operating correctly. State TWO reasons why you would get these readings? (2 mark) (1) (2) 17

18 Question 7 continued (e) You are using a voltage tester on the front of a 230V switched socket outlet to ensure power is present and get the following results: Test between Socket Terminals Voltages Switch OFF Voltages Switch ON Active - Neutral 0V 230V Active - Earth 0V 0V Neutral - Earth 230V 230V Note: The voltage tester is operating correctly. State the reason why you would get these readings. 18

19 Question 8 (a) The figure below represents the components of a DOL starter with a 400V control circuit and a remote stop/start station: L1, L2, and L3 represent the three-phases connected to the main contacts. U, V and W represent the three conductors from the thermal overloads to the motor (i) Draw the conductors on the figure to complete a working 400V control circuit. L1 L2 L3 C4 U V W (ii) State the purpose of the component labelled C4. 19

20 Question 8 continued (b) Electronic starters are often used with three-phase a.c. induction motors. Explain how the operation of an a.c. variable speed controller is different to the operation of an electronic soft starter. (c) Explain the term ramp time as it relates to an a.c. variable speed controller. (d) A 400V, three-phase motor is started with a DOL starter. When started, the motor draws 90A and produces 200% full-load torque. Calculate the starting current if the same motor is started with a star/delta starter. 20

21 Question 8 continued (e) A 400V, three-phase motor is started with a DOL starter. When started, the motor draws 90A and produces 200% full-load torque. Calculate the percentage (%) of starting torque if the same motor is started with an auto-transformer starter with an 80% tap. 21

22 Question 9 (a) (i) Sketch a circuit diagram of a single phase, capacitor-start induction motor. (3 marks) (ii) State ONE advantage that a single phase, capacitor-start induction motor has when compared to other single-phase induction motors. (iii) Describe how to change the direction of rotation of a single phase, capacitor-start induction motor. (½ mark) 22

23 Question 9 continued (b) (i) Sketch a circuit diagram of a single phase permanently-split capacitor motor. (3½ marks) (ii) State ONE advantage that a single phase permanently-split capacitor motor has when compared to other single-phase induction motors. (iii) Describe how to change the direction of rotation of a single phase permanently-split capacitor motor. 23

24 For Candidate s Use For Examiner s Use Only Questions Answered Marks 1 In the box, write the number of EXTRA sheets you have used. Write NIL if you have not used any TOTAL 24