NATIONAL CERTIFICATE (VOCATIONAL) ELECTRICAL SYSTEMS AND CONSTRUCTION NQF LEVEL 3 NOVEMBER 2009

Transcription

1 NATIONAL CERTIFICATE (VOCATIONAL) ELECTRICAL SYSTEMS AND CONSTRUCTION NQF LEVEL 3 NOVEMBER 2009 ( ) 24 November (X-Paper) 09:00 12:00 This question paper consists of 6 pages.

2 ( ) -2- NC720(E)(N24)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- NC720(E)(N24)V QUESTION 1: WIRE AND COMMISSION A SINGLE-PHASE DOMESTIC INSTALLA- TION 1.1 The SANS (10142) Code of Practice has regulations about the installation and wiring of components. What is stated with regard to the following: Colours to be used to identify the live, neutral and earth conductors in a domestic installation (3) You must be able to reach any point of the distribution board for switching purposes. What is the minimum and maximum height from floor level that any point of control in the distribution board can be installed? The minimum current rating of domestic socket outlets The minimum prospective short-circuit rating for switchgear in single-phase domestic installations Name the conductors that are disconnected by the isolating switch (isolator) of a stove or a geyser (2) Is the isolating switch designed to trip on over-current or earth fault? What do you understand by the term point of control? 1.2 The distribution board of a domestic installation, having the following circuits, has to be wired: 1 x 20 A plug circuit 1 x 10 A light circuit 1 x 35 A stove circuit 1 x 20 A geyser circuit Draw a neat, fully labelled wiring diagram of the above-mentioned distribution board and indicate ALL the components and wiring. (10) QUESTION 2: TEST AND INSPECT A SINGLE-PHASE DOMESTIC INSTALLATION 2.1 Name FOUR compulsory electrical tests to be carried out on a domestic installation before a Certificate of Compliance can be issued. (4) 2.2 With the aid of a fully labelled sketch, explain how you would conduct an insulation resistance test between the live and neutral conductors. Indicate the acceptable readings. (10)

4 ( ) -4- NC720(E)(N24)V 2.3 The following readings were obtained after conducting tests on a domestic installation. Analyse the readings and state whether they are ACCEPTABLE or UNACCEPTABLE. Write only the answers in the ANSWER BOOK next to the question numbers ( ) TEST READING ACCEPTABLE/ UNACCEPTABLE Earth and bonding continuity < 0,2 ohms test Insulation resistance test (live to earth conductors ) 10 ohms Polarity test (switch in closed position) 0 ohm Insulation resistance test (between neutral and live conductors) 1 mega-ohm Earth-leakage test (domestic) 100 ma Voltage test at distribution board (no-load) 150 V

5 ( ) -5- NC720(E)(N24)V QUESTION 3: FAULT FINDING ON ALTERNATING-CURRENT (AC) AND DIRECT- CURRENT (DC) SYSTEMS 3.1 The following faults were reported when a stove was tested. Choose a possible cause from COLUMN B that matches a fault in COLUMN A. Write only the letter (A F) next to the question number ( ) in the ANSWER BOOK. FAULT POSSIBLE CAUSE Element works on high only Element trips earth-leakage relay when switched on A B element conductor broken (coil) faulty stove switch V at terminals of element but element does not heat up No voltage at stove switch, but voltage up to fuse box supply Oven temperature cannot be regulated C D E short-circuit between earth and element conductors (coils) faulty thermostat or oven switch blown fuse 3,1,6 No voltage on load side of stove circuit breaker but 220 V on supply side F faulty circuit breaker 3.2 Various instruments may be used when testing circuits Name the measuring instrument used to measure ampère and explain how this instrument is connected in a circuit. (2) Name the measuring instrument used to measure volt and explain how this instrument is connected in a circuit. (2) 3.3 A single-phase transformer is not working properly. Briefly explain the voltage and continuity tests that can be done on this transformer. (4) 3.4 A single-phase AC motor (with capacitor) is not working properly. List the tests you would perform on the motor windings and capacitor and the testing or measuring instruments you would use. (6)

6 ( ) -6- NC720(E)(N24)V QUESTION 4: LOW-VOLTAGE NETWORKS 4.1 What is the maximum supply voltage for a low-voltage network? 4.2 Briefly describe a TN-S earthing system. (3) 4.3 Name THREE types of earthing systems used in low-voltage installations. (3) 4.4 You are required to test whether a network's earthing system is functioning properly. Name TWO tests that can be done before switching on the supply. (2) 4.5 Name FOUR steps that you would follow when you electrically disconnect a circuit from the network (isolation of apparatus). (4) 4.6 A redspot fuse has to be replaced. Explain how you would isolate, remove and activate the new fuse. (7) QUESTION 5: UNDERSTAND AND MAINTAIN LIGHTING SYSTEMS 5.1 All fluorescent lamps need a device to control the flow of current through them. Name this device. 5.2 Name FOUR different types of lamps and where they can be used. (8) 5.3 A fluorescent lamp has broken in your classroom. State the safety precautions you would take when disposing of the lamp. (5) 5.4 When a ripple relay is activated by the supplier of electricity, the supply to the geyser is interrupted. How does the supplier activate the ripple relay? 5.5 Electricity can be saved through the efficient use of energy. Name FIVE practices that can be implemented in a home to save energy. (5) TOTAL: 100