ELECTRICAL SERVICE TECHNICIAN EXAMINATION 12 September 2015 QUESTION AND ANSWER BOOKLET

Transcription

1 EST 2066 Candidate Code No. For Board Use Only Result Date Int Result Date Int ELECTRICAL SERVICE TECHNICIAN EXAMINATION 12 September 2015 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: 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) AS/NZS 3760:2010 PLEASE HAND THIS PAPER TO THE SUPERVISOR BEFORE LEAVING THE ROOM

2 Question 1 (a) You have carried out repairs to a permanently connected motor in a threephase electrical installation. Refer to the Electricity (Safety) Regulations 2010 and state TWO testing requirements for the repairs that were carried out. (1) (2) Ref:. (b) (i) State TWO typical applications for a universal motor. (1) (2) (ii) State TWO typical applications for a capacitor-start induction motor. (1) (2) EST SEPTEMBER

3 Question 1 continued (c) AS/NZS 3000 details the connection methods for the conductors of cables. Refer to AS/NZS 3000 and state TWO common requirements for the connection of the conductors of cables. (1) (2) Ref:. (d) Refer to the Electricity (Safety) Regulations, and define the term low risk prescribed electrical work. EST SEPTEMBER

4 Question 1 continued (e) A switch controls two heating elements of the same resistance on an electric blanket. The switch has three positions low, medium and high. The blanket is faulty because it draws the same current only in the high and medium positions. The switch is not faulty. (i) What situation would cause this fault? (ii) State ONE reason why the blanket draws the same current only in the high and medium positions. (f) A single-phase Class I electrical appliance requires a new flexible cord. Criteria used in selecting the cord include; the number of cores; length, colour of the conductors and current rating. State TWO other criteria that should be considered when selecting the flexible cord for the Class I electrical appliance. (1) (2) EST SEPTEMBER

5 Question 1 continued (g) When electrical appliances are used outdoors, portable residual current devices (PRCDs) can provide additional protection from electric shock. Refer to AS/NZS 3760 and state the test current value and maximum tripping time for a Type II RCD. Ref:. (h) Refer to the Electricity (Safety) Regulations 2010 and state the obligation of a landlord of residential premises in respect to fittings and appliances. Ref:. EST SEPTEMBER

6 Question 1 continued (i) A 230V handheld electrical appliance used by a person who is partially immersed in a conductive substance must be used in conjunction with a method of protection. Refer to the Electricity (Safety) Regulations 2010 and state ONE method of protection for the 230V handheld electrical appliance. Ref:. (j) An HRC fuse has blown on a three-phase switchboard. The single-phase final subcircuit protected by the fuse supplied a socket outlet. The electrical appliance that was plugged into that socket outlet has been disconnected and tagged with an out-of-service tag. When the fuse was replaced, the fuse blew again. (i) What is the cause of the fault? (ii) What action needs to be taken to rectify the fault? EST SEPTEMBER

7 Question 2 Introduction A Class I, plug-in, electric heater rated at 230V/240V has two elements of 31.4Ω and 40.2Ω. When the heater selector switch is on high, the elements are connected in parallel. When the heater selector switch is on low, the elements are connected in series. Use the information in the introduction to this question to answer parts 2(a), 2(b), 2(c) and 2(d) (a) Calculate the minimum current drawn by the heater when operating at 230V (b) Calculate the maximum power dissipated by the heater when operating at 230V (4 marks) EST SEPTEMBER

8 Question 2 continued (c) There is an HRC fuse incorporated in the heater. (i) Calculate the full load current of the heater when operating at 230V (ii) Which of the following HRC fuses is the minimum fuse size for the heater based on the calculation in (c)(i)? 4A 10A 20A 6A 16A 25A (d) State ONE reason why the power the heater dissipates will increase if the supply voltage was 240V. EST SEPTEMBER

9 Question 3 (a) You are using a clip-on (clamp) ammeter to measure the current drawn by a live electrical appliance. (i) Describe how the clip-on (clamp) ammeter should be used to measure the current. (ii) State the reason why the clip-on (clamp) ammeter is not clamped around the flexible cord of the appliance. (b) Two multi-function test instruments are being used to measure voltage and current values of a live 230V electrical appliance. State TWO precautions relating to the test instruments that will ensure personal safety. Note: 1. All the necessary safety equipment (overalls, rubber mats etc.) is available. 2. Set procedures are available. 3. All conductive items (e.g., rings) have been removed. (1) (2) EST SEPTEMBER

10 Question 3 continued (c) For a faulty permanently-connected electrical appliance: (i) What does switching off the appliance mean? (ii) What does isolating the appliance mean? (d) A 230V, permanently-connected electrical appliance has been isolated to allow repairs to be carried out. Describe TWO methods of preventing the electrical appliance being reconnected to the supply while repairs are being carried out. (1) (2) (e) The prove-test-prove method using a voltmeter is being used to find out whether 400V, three-phase permanently-connected electrical appliance has been isolated. State ONE reason why only testing between phases is not a safe test in this situation. EST SEPTEMBER

11 Question 4 (a) The mineral insulated metal sheathed (MIMS) element in a Class I, portable oven has been replaced. The oven must comply with the inspection and testing requirements of a Standard before it is returned to service. Refer to that Standard and complete the following table in relation to the tests required to be carried out using test instruments. (3 marks) Test Test instrument Test result value that permits the oven to be returned to service Ref:. (b) A polarity test should be carried out on a Class I, electrical appliance after a replacement flexible cord has been fitted. The appliance is controlled by a single-pole switch. Detail the THREE important points this polarity test will confirm? (3 marks) (1) (2) (3) EST SEPTEMBER

12 Question 4 continued (c) A single phase, 230V, Class I, plug-in electric heater is rated at 2000W. The heater has been operating normally by producing 2000W of heat. The flexible cord on the heater was replaced due to it being mechanically damaged. When the heater was plugged into the electricity supply it was found to produce only 1800W of heat. State TWO characteristics of the new flexible either of which would prevent the heater producing 2000W of heat. (1) (2) (d) 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? State TWO ways in which the effect of voltage drop can be reduced in this situation. (1) (2) EST SEPTEMBER

13 Question 5 (a) State the primary purpose of HRC fuses or MCBs used for final subcircuits on a switchboard in an electrical installation. (b) State the primary purpose of RCDs used for final subcircuits on a switchboard in an electrical installation. (c) (i) Describe the internal operation of a combined thermal/magnetic type MCB when an overload fault occurs (ii) Describe the internal operation of a combined thermal/magnetic type MCB when a short-circuit fault occurs EST SEPTEMBER

14 Question 5 continued (d) (i) Define the term current rating as it applies to an HRC fuse. (ii) Define the term fusing factor (utilisation category) as it applies to an HRC fuse. (e) A three-phase final subcircuit protected by HRC fuses supplies a threephase induction motor controlled by a direct-on-line (DOL) starter. The starter: Incorporates thermal overloads. Control circuit operates at 230V The motor is operating when one HRC fuse blows. State TWO effects on the operation of the motor because of the blown fuse. (1) (2) EST SEPTEMBER

15 Question 6 (a) A three-phase induction motor, controlled by a direct-on-line (DOL) starter, drives a wood planer. The planer has been operating normally and is switched off at lunchtime. When the planer is restarted the motor hums noisily but fails to rotate. State TWO possible causes of this type of fault. Note: 1. The protection is correctly rated and is not faulty. 2. The wood planer is not mechanically overloaded or jammed 3. The motor is mechanically sound (1) (2) (b) A three-phase induction motor, controlled by a direct-on-line (DOL) starter, drives a wood planer. The planer has been operating normally, but the motor begins to overheat although the current in all three phases is the same as the nameplate rating. 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 SEPTEMBER

16 Question 6 continued (c) A three-phase induction motor, controlled by a direct-on-line (DOL) starter, drives a wood planer. The planer has been operating normally and is switched off at lunchtime. When the planer is restarted the motor 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. 3. The motor is mechanically sound (1) (2) (d) A single-phase, split phase motor hums and fails to rotate while being tested under load. State the TWO likely causes of the split phase motor failing to rotate. (1) (2) EST SEPTEMBER

17 Question 6 continued (e) A single-phase capacitor-start motor is being tested under no-load on a workbench. It appears to operate normally. The motor drives a fan. When the motor is re-installed in the fan it does not drive the fan up to full speed and quickly overheats. State the cause of this fault. EST SEPTEMBER

18 Question 7 Introduction A 400V three-phase final subcircuit supplying a three-phase induction motor is protected by a three-phase motor-rated MCB. The motor is controlled by a star/delta starter and is supplied via a six-core screened flexible cable between the motor and the starter. The star/delta starter is controlled by a lockable isolating switch. The operator has reported that MCB has tripped. When reset, the MCB tripped again. The MCB is not faulty and is correctly rated. The final subcircuit up to and including the star/delta starter is not faulty. Use the information in the introduction to this question to answer parts 7(a) and 7(b). (a) Describe how the motor can be isolated so the fault can be located. (3½ marks) EST SEPTEMBER

19 Question 7 continued (b) Locating the fault. Describe the testing required to locate the fault. Include: The test instrument used. The test voltage used. The expected test result of the fault. (6½ marks) EST SEPTEMBER

20 Question 8 In New Zealand the MEN distribution system is used. (a) State TWO technical reasons why the neutral conductor is earthed in the MEN system. (1) (2) (b) A 230V, Class I, dishwasher is used in an earthed situation. Explain how the effective earthing of the metal frame of the dishwasher provides protection against electric shock. EST SEPTEMBER

21 Question 8 continued (c) An RCCB and MCB protect a final subcircuit supplying a 230V, single-phase wash-down pump. The pump isolating switch has been replaced. (i) State what would occur when the pump was turned on, with the phase and neutral transposed at the supply side of the isolating switch? (ii) State what would occur when the pump was turned on, with the neutral and earth transposed at the supply side of the isolating switch? (iii) State ONE test that would detect the interchange of the phase and neutral conductors before the electricity supply is livened. EST SEPTEMBER

22 Question 8 continued (d) State ONE reason why Class II (double-insulated) electrical equipment does not need to be connected to a protective earthing conductor. EST SEPTEMBER

23 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 this question to answer parts 9(a), 9(b), 9(c) and 9(d). (a) (i) Draw and label the circuit diagram of the 230V DOL starter control circuit. The control circuit incorporates electrical protection. You do not need to show the main contacts or the motor (4 marks) (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. EST SEPTEMBER

24 Question 9 (b) The thermal overload protection is replaced with thermistor protection. Describe how the thermistor would provide protection to the 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 ONE reason why this situation would occur. (d) A new remote stop/start station has been connected to an existing DOL starter controlling a three-phase motor. The stop button in the remote station was connected in parallel with the DOL stop button. State how this affects the operation of the 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