ELECTRICIAN S THEORY EXAMINATION 19 June 2010 QUESTION AND ANSWER BOOKLET

Transcription

1 Candidate Code No. ET34 For Board Use Only Result Date Int Result Date Int ELECTRICIAN S THEORY EXAMINATION 19 June 2010 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. Non-programmable calculators may be used. 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) In an MEN system the neutral is earthed at the star point of a transformer and at the main switchboard of an installation. State TWO technical reasons why the neutral is earthed in an MEN system (1) (2) (b) 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. (c) The tests on a star-connected squirrel cage motor show the following readings: L 1 L 2 L 1 L 3 L 2 L 3 20Ω open circuit open circuit State the most likely fault indicated by these readings. 2

3 Question 1 continued (d) AS/NZS 3760 states that testing the integrity of the insulation of a Class I electrical appliance can be carried out either by: An insulation resistance test An earth leakage test. (i) State the type of test you would carry out before testing the integrity of the insulation of a Class I electrical appliance (½ mark) (ii) Describe how you would carry out an earth leakage test on a Class I electrical appliance. Note: A PAT tester is not to be used (1½ marks) (e) State TWO reasons why it is preferable to operate an electrical installation at near unity power factor rather than at 0.85 power factor. Note: Cost is not a reason. (1) (2) 3

4 Question 1 continued (f) State the reason why the voltage on the secondary terminals of a transformer is less at full load than it is with no load. (g) Calculate the copper losses of a 100kVA distribution transformer operating at 75% full load when the full-load copper losses are 1200W. (h) Most appliances sold in New Zealand are designed to operate at 230V or 240V. An electric heater is rated at 2kW when operated at 230V. Calculate the power output when supplied at 240V turn over) 4

5 Question 1 continued (i) An earth fault loop impedance tester was used at the main switchboard in a domestic electrical installation. The following information was recorded: 233V 0.175Ω From this information, calculate the prospective short circuit current at the switchboard. (j) (i) What is the minimum value of the permitted test result for the insulation resistance test of a three-phase, 400V, mains cable? (ii) What is the minimum value of the permitted test result for the insulation resistance test of a 230V, Class I, plug-in electrical appliance? 5

6 Question 2 Wiring systems must be selected so that they: Protect against physical contact with live parts by durable insulation materials. Satisfy the current carrying capacity, voltage drop and other minimum size requirements for the conductors. Protect against mechanical damage, environmental and other external influences. (a) Cables carrying large currents are installed in horizontal cable trays. State why is it necessary to securely fasten the cables in the cable trays? (b) A 400 metre cable supplies a sawmill. Apart from length, state TWO other factors that will determine the voltage drop of the cable. (1) (2) (c) TPS flat cable is required to have maximum temperature rating not exceeding 75ºC when operated on full load. Explain why it is dangerous to continuously overload the cable so that it exceeds its maximum temperature rating? 6

7 Question 2 continued (d) A 25mm² single-core XLPE insulated copper conductor has a current rating of 165 amps. A 25mm² single core PVC insulated copper cable has a current rating of 143 amps. Explain the reason why the XLPE insulated conductor has a higher current rating than the PVC insulated conductor. (e) A 35 mm² cable is run 600 metres to a pump station. The insulation resistance of the cable is 140MΩ. Calculate the change in the insulation resistance value if the cable was shortened to 450m when the pump station was relocated. 7

8 Question 3 (a) When carrying out testing on a low voltage electrical installation, which document details the tests and checks required to be carried out? (b) The two figures below represent single-phase socket outlets that comply with AS/NZS 3112 and IEC respectively. Both socket outlets are connected to a standard low voltage supply. Label the polarity of the socket outlet connections on the figures. Note: You must show the polarity completely correct to gain marks. (3 marks) 8

9 Question 3 continued (c) You have isolated a single-phase 230V domestic electrical installation so alterations can be carried out at the switchboard. After completing the alterations you relivened the installation. You are using a voltage tester on the front of a switched socket outlet to ensure power is present. Note: The voltage tester is operating correctly. (3 marks) (i) What voltage you would expect to find with the switch OFF: Between Active Neutral Between Active Earth Between Neutral Earth (ii) What voltage you would expect to find with the switch ON: Between Active Neutral Between Active Earth Between Neutral Earth 9

10 Question 3 continued (d) You are using a voltage tester on the front of a 230V switched socket outlet to ensure power is present and you 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 State TWO reasons why you would get these readings? Note: The voltage tester is operating correctly. (2 mark) (1) (2) (e) You are using a voltage tester on the front of a 230V switched socket outlet to ensure power is present and you 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 State the reason why you would get these readings. Note: The voltage tester is operating correctly. 10

11 Question 4 A new three-phase water heater has been installed in an industrial installation to replace an existing water heater. It has three elements rated at 400V, each with a resistance of 30Ω. The elements can be connected either in star or delta. No changes will be made to the circuit supplying the new water heater. (a) (i) Calculate the line current drawn when the elements are connected in star. (ii) Calculate the line current drawn when the elements are connected in delta. (3½ marks) (iii) The circuit is protected by 10A rewireable fuses. In relation to the electrical protection, state which connection arrangement is most suitable for this installation? (½ mark) 11

12 Question 4 continued (b) Calculate the difference in the power consumed by the water heater between when the elements are connected in star and when the elements are connected in delta (4 marks) 12

13 Question 5 A small commercial factory is connected to a three-phase 400V MEN power supply and has the following loads: 10kW of electric heating W metal halide light fittings with a power factor of 0.8 lag. Several lightly loaded machines with a total load of 20kW and an overall power factor of 0.5 lag Use the information in the preamble above to calculate the total power in kw and kva of the installation and the total line current in the mains if all loads are operating together with no diversity. (a) Calculate the total power of the installation in kw (b) Calculate the total power of the installation in kva (6 marks) 13

14 Question 5 continued (c) Calculate the total line current in the mains if all loads are operating together with no diversity. 14

15 Question 6 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 (direct-on-line) starter. The motor and starter are controlled by an adjacent isolator. You have isolated the vibrating plant, by turning off the isolator and proving the isolation on the load side of the switch with the prove-test-prove method. The only test instruments you have are a multimeter and clamp type ammeter. (a) (i) What is the first electrical test you would carry out? (ii) What instrument would you use to carry out the test you have stated in (a)(i). (½ mark) (iii) Describe how you would carry out the test you have stated in (a)(i) (iv) What is the minimum or maximum test result value would you consider acceptable for the test you have stated in (a)(i)? (b) (i) What is the second electrical test you would carry out? (ii) What instrument would you use to carry out the test you have stated in (b)(i). (½mark) 15

16 Question 6 continued (iii) Describe how you would carry out the test you have stated in (b)(i) (iv) Given that the motor circuit is faulty, what test result value would you expect for the test you have stated in (b)(i)? (c) Explain why you would carry out the test stated in (a)(i) before the test you have stated in (b)(i). (d) In addition to a staff member receiving an electric shock off the frame of the vibrating plant, you were also told that in fact the motor had been troublesome for some time. Sometimes the motor would run and at other times it would hum then trip the starter overload unit. State TWO possible reasons why the motor would run and, at other times, it would hum then trip the starter overload unit. (1) (2) 16

17 Question 7 (a) In a squirrel cage induction motor the rotor bars are skewed at an angle across the face of the rotor. State TWO reasons why the rotor bars are skewed rather than being straight. (1) (2) (b) List THREE types of reduced-voltage starters (other than electronic) that can be used with a three-phase induction motor. (3 marks) (1) (2) (3) 17

18 Question 7 continued (c) Sketch the speed/torque characteristic graph of a squirrel cage induction motor, and show the location of: The starting torque The pull-out or break-down torque. Full-load torque Running torque Rotor speed N R Synchronous speed N S. (5 marks) 18

19 Question 8 (a) The figures below are outlines of a three-phase RCD and a 5 pin socket. Draw and label: The internal circuit diagram of the three-phase RCD used for personal protection that protects the 5-pin socket outlet. The supply to the socket outlet The earthing arrangements (5 marks) Three-phase RCD Three-phase socket outlet 19

20 Question 8 continued (b) Describe the operation of an RCD when there is an active (phase) to earth fault that occurs on the circuit the RCD protects. (4 marks) (c) State TWO types of electrical protection provided by a RCBO. (1) (2) 20

21 Question 9 (a) A 230V/100V step-down transformer with 2000 turns on the primary supplies 15A on the secondary when fully loaded. (i) Calculate the number of secondary turns. (ii) Calculate the primary current when fully loaded. (iii) What would be the most common form of cooling for this transformer? 21

22 Question 9 continued (b) For a double-wound transformer, explain why an increase in the secondary current causes an increase in the primary current. (5 marks) 22

23 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 23