NATIONAL CERTIFICATE (VOCATIONAL) ELECTRICAL SYSTEMS AND CONSTRUCTION NQF LEVEL 4 NOVEMBER 2009

Transcription

1 NATIONAL CERTIFICATE (VOCATIONAL) ELECTRICAL SYSTEMS AND CONSTRUCTION NQF LEVEL 4 NOVEMBER 2009 ( ) 12 November (Y-Paper) 13:00 16:00 This question paper consists of 6 pages.

2 ( ) -2- NC730(E)(N12)V TIME: 3 HOURS MARKS: 150 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- NC730(E)(N12)V QUESTION 1: ELECTRICAL INFRASTRUCTURE 1.1 Alternators sometimes have to be removed from the grid for maintenance purposes and then brought back What is this process called? What conditions must be met in this process? (4 x 1) (1) (4) 1.2 Name FIVE components of a coal-fired power station. (5) 1.3 Name FIVE components / parts / pieces of equipment required for the installation of medium-voltage overhead networks and explain the function of each. (5 x 3) (15) 1.4 Explain, with the aid of a diagram, what happens in a radial system when one of the feeders fails. (5 x 2) (10) [35] QUESTION 2: DESIGN AND CONSTRUCT A THREE-PHASE CIRCUIT 2.1 Explain the operational procedure of the direct-on-line starter circuit diagrams in FIGURE 1 and FIGURE 2 below. (6 + 3) (9) L1 L2 Fuse V Emergency stop Stop Overload relay Start Coil F Control circuit FIGURE 1

4 ( ) -4- NC730(E)(N12)V three- Three-phase phase or c/b isolator or isolator Contactor A Overload relay MOTOR THREE-PHASE MOTOR MAIN\POWER CIRCUIT FIGURE Explain the operational principle of a circuit breaker under short circuit conditions. (4) 2.3 Choose the correct word(s) from those given in brackets. Write only the word(s) next to the question number ( ) in the ANSWER BOOK A timer can be set on (on-delay/off-delay/either of the two). (2) Limit switches are used to (initiate a control sequence/limit power). (2) An overload relay protects the (supply/load) circuit. (2) The contacts of a contactor are used on the (auxiliary/power) circuit. (2) A switchgear is installed (next to the motor/on the supply side). (2) [23] QUESTION 3: CONSTRUCT A THREE-PHASE MEDIUM-VOLTAGE OVERHEAD SUPPLY TO DOMESTIC HOUSES 3.1 When work has to be done on a three-phase medium-voltage overhead supply, certain worksite procedures should be adhered to. Name SIX such procedures. (6 x 2) (12) 3.2 A three-phase medium-voltage overhead system is to be connected to the grid. Briefly explain what you will inspect on the new installation. (4 x 1) (4)

5 ( ) -5- NC730(E)(N12)V 3.3 Complete the following sentences/paragraphs by filling the missing word(s). Write only the word(s) next to the question number ( ) in the ANSWER BOOK Strain insulators are used on poles to take up the strain. (1) Once the poles and stay wires are completed, can be clamped onto the poles. (1) comprises of the low-voltage networks from which service connections are tapped off to individual consumers. (1) Longer pole size is for lines. (1) poles are set an angle away from the pull of conductors. (1) A is used to tension the conductors. (1) Conductors connected between poles must be given a certain amount of. (1) To enable single poles to carry more weight, they can be built or arranged in formation. (1) At dead-ends, and changes in direction-line of the pole line, to support the pole at the bottom, a/an is driven into the ground. (1) To increase the life span of a pole, it is impregnated with. (1) [26] QUESTION 4: TEST AND INSPECT A THREE-PHASE INDUSTRIAL/COMMERCIAL INSTALLATION 4.1 What is a phase rotation meter used for? (2) 4.2 After completing the wiring of a factory, inspections must be done. Name FIVE inspections that you will perform and record on the certificate of compliance. (10) 4.3 The continuity test is one of the tests that is done on a new installation. Complete the table below. NAME OF TEST Continuity of conductors HOW THE TEST IS DONE (POINTS WHERE THE TEST LEAD MUST BE CLAMPED) INSTRUMENT USED ACCEPTABLE READING (6)

6 ( ) -6- NC730(E)(N12)V 4.4 Make a neat, labelled sketch of a three-phase geyser sub-circuit. Your subcircuit must indicate the components of a distribution board. (18) [36] QUESTION 5: FAULT FIND, REPAIR AND MAINTAIN THREE PHASE VOLTAGE ELECTRIC CIRCUITS 5.1 The motor starts running as the start push-button is pressed, but stops as soon as the start push-button is released. Briefly explain why the motor stops when the push-button is released. (5 x 2) (10) 5.2 An induction motor can experience a variety of faulty conditions. State the outcome/result for each of the following faulty conditions: Polarity not correct on the supply side (2) Short circuit condition (2) Normally open contact wired instead of normally closed (2) One phase to the motor not having current (2) An open circuit in the control circuit (2) 5.3 You are called out to repair a fault on an electrical installation. How would you plan and prepare to repair the circuit. State the steps in planning and preparation to repair. (5 x 2) (10) [30] TOTAL: 150