NATIONAL CERTIFICATE (VOCATIONAL) ELECTRICAL SYSTEMS AND CONSTRUCTION NQF LEVEL 2 SUPPLEMENTARY EXAMINATION 2010

Transcription

1 NATIONAL CERTIFICATE (VOCATIONAL) ELECTRICAL SYSTEMS AND CONSTRUCTION NQF LEVEL 2 SUPPLEMENTARY EXAMINATION 2010 ( ) 4 March (X-Paper) 09:00 12:00 This question paper consists of 6 pages.

2 ( ) -2- NC710(E)(M4)V TIME: 3 HOURS MARKS: 100 INSTRUCTIONS AND INFORMATION Answer ALL the questions. Read ALL the questions carefully. Number the answers according to the numbering system used in this question paper. Write neatly and legibly.

3 ( ) -3- NC710(E)(M4)V QUESTION 1: BASIC ELECTRICAL CIRCUITS AND SYSTEMS 1.1 You are required to wire a domestic installation comprising of the following: x plug circuit 1x light circuit Draw a neatly labelled circuit diagram, to show the wiring of a distribution board for the above circuits indicating ALL necessary components and protective devices. Indicate the size of the circuit breakers and the size of the wires you are going to use for the circuits. (10) 1.2 Compare a geyser and stove sub- circuit. How do they differ with respect to: The size of the circuit breakers used The size of the conductors used Their application (2) (2) (2) 1.3 Explain how you would secure the following: Surface mounted PVC conduit Surface mounted twin flex cable (2) 1.4 Sketch the IEC symbols of the following components A double-wound transformer A coil A fuse A single-pole switch A two-way switch (5) 1.5 After the wiring of an installation is completed, certain tests have to be completed to ensure it corresponds with the requirements laid down by SANS A certificate of compliance will then be issued. List FIVE compliance test that are compulsory. (5) 1.6 What is the purpose of the thermostat as used in a geyser? (2) [30]

4 ( ) -4- NC710(E)(M4)V QUESTION 2: WIRE WAYS AND LOW VOLTAGE CABLES 2.1 The cable from the supplier's meter box must be connected to the distribution board in your home How would you secure the PVC wire armoured cable to the metal distribution board? What tools would you use to secure the cable to the distribution board? 2.2 A newly built house needs to be wired. What types of wire ways would you use to wire this house? (2) 2.3 Choose a statement from COLUMN B that matches a statement in COLUMN A. Write only the letter (A E) next to the question number ( ) in the ANSWER BOOK. COLUMN A COLUMN B Line taps Soldered joint Mechanical connector Scotch cast joint Wire way A B C D E joining kit for armoured and unarmoured cables flexible conduit join overhead lines Britannia joint ferrule (5 1) (5) 2.4 A three-core cord was fitted to a three-pin plug. It was done in the following way: the blue conductor was connected to the load/live pin; the brown conductor was connected to the neutral pin and the green and yellow conductor was connected to the earth pin. Was the three-core cord correctly connected to the three-pin plug? If not how would you change the wiring on the three-pin plug. 2.5 When a PVC conduit is used in wire ways, wires need to be pulled into the conduit What tool would you use to pull the wires into the conduit? Explain how you would use this tool to get the wires into the conduit. (1) [20]

5 ( ) -5- NC710(E)(M4)V QUESTION 3: ELECTRIC MACHINES 3.1 List THREE types of direct current machines in general use. 3.2 Compare the operation of a DC Motor with that of a DC Generator. (Focus should be on energy conversion.) (4) 3.3 What key information is found on the nameplate of a motor? List THREE points? 3.4 Indicate whether the following statements are TRUE or FALSE. Write only 'true' or 'false' next to the question number ( ) in the ANSWER BOOK. Checklist for mechanical inspection: Make sure all covers and guards are in place. Check the bearings for wear. Inspect the brushes regularly for wear. Check if all mounting brackets are secure. Check the nameplate to see if all given data is clearly legible. Check the stator frame and shields. Make sure that the brush holders are accurately set. (7 1) (7) 3.5 A DC motor fails to start. What checks will you perform to establish the fault? [20] QUESTION 4: FAULT-FINDING AND TESTING 4.1 List TWO testing instruments you would use to test or to do fault-finding on components. (2) 4.2 Compare the operation of a fuse and a circuit-breaker. (4) 4.3 A fuse needs to be tested to see whether it is functional or not, after it has been removed from the circuit What testing instrument can be used to perform this test? How is the test done? (1) [10]

6 ( ) -6- NC710(E)(M4)V QUESTION 5: PROTECTION AND MEASURING INSTRUMENTS List FIVE meters that require replacement on a regular basis. List THREE types of displays that are used in measuring instruments. Compare a pre-paid meter with a kilowatt-hour meter. A meter could be mounted in different ways. List THREE ways meters could be mounted on walls or panels. A faulty metering unit needs to be replaced. Explain how you would replace the metering unit. (5) (4) (6) [20] TOTAL: 100