EXEMPLAR NATIONAL CERTIFICATE (VOCATIONAL) ELECTRICAL PRINCIPLES AND PRACTICE NQF LEVEL 3 ( ) (X-Paper) 09:00 12:00

Size: px

Start display at page:

Download "EXEMPLAR NATIONAL CERTIFICATE (VOCATIONAL) ELECTRICAL PRINCIPLES AND PRACTICE NQF LEVEL 3 ( ) (X-Paper) 09:00 12:00"

Jane Fisher
5 years ago
Views:

1 NATIONAL CERTIFICATE (VOCATIONAL) ELECTRICAL PRINCIPLES AND PRACTICE NQF LEVEL ( ) (X-Paper) 09:00 12:00 EXEMPLAR This question paper consists of 7 pages.

2 EXEMPLAR -2- NC(V) TIME: 3 HOURS MARKS: 100 INSTRUCTIONS AND INFORMATION Answer ALL the questions. Read ALL the questions carefully. Number the answers correctly according to the numbering system used in this question paper. ALL work you do NOT want to be marked must be clearly CROSSED OUT. Start EACH question on a NEW page. Write neatly and legibly.

3 EXEMPLAR -3- NC(V) QUESTION 1: MATERIALS, COMPONENTS AND INTERPRETATION OF DRAWINGS Copper is extensively used as a conductor in the electrical field. Name THREE properties of copper that makes it a suitable conductor. (3 1) Name the type of insulating material that is used for the manufacture of circuit breaker casings. Choose an item from COLUMN B that matches a description in COLUMN A. Write only the letter (A J) next to the question number ( ) in your ANSWER BOOK. COLUMN A COLUMN B Is a metal rod that connects the earthing lead to the general mass of earth The ability of a component to store electrical charge A protection device that disconnects the supply in the event of current flowing to earth A B C D E F G capacitance conduit fuse cord terminator capacitor earth leakage relay Points where cables are joined together Is a type of pipe or tube through which electrical conductors are drawn H I J earth electrode switch gear busbars (5 1) (5)

4 EXEMPLAR -4- NC(V) 1.4 Draw up a components list of protection devices for the distribution board indicated below Describe the operating principle of a fuse. Name TWO electrical protection devices used in the electrical field. (7) QUESTION 2: ELECTRIC AND MAGNETIC THEORY 2.1 Analyse the TWO circuits below and then answer the questions that follow. 6Ω 3 Ω 9Ω 3 Ω 6Ω 9Ω 12 V Circuit 1 12 V Circuit 2

5 EXEMPLAR -5- NC(V) In which circuit is the total resistance larger? In which circuit is the total current drawn larger? Calculate the total power consumed in Circuit An electric heater rated at watts is used for 10 hours per day. Calculate the cost to operate the heater for 30 days, if the cost of electricity is 33 cents per unit. (4) 2.3 Indicate whether the following statements are TRUE or FALSE. Choose the correct answer and write only true or false next to the question number ( ) Magnetic flux refers to the number of magnetic lines of force per unit area. Magnetic flux density refers to the number of magnetic lines of force produced by a magnet. When using Fleming's left hand rule the thumb indicates the direction of magnetic force (N to S) A bar magnet has a flux density of 5 Tesla and a cross-sectional area of 8 m 2. Calculate the flux at the poles. Name ONE device that operates on the principle of electromagnetism. State TWO factors that influence the strength of the magnetic field of a solenoid. QUESTION 3: DC AND AC CIRCUITS A 160 kilowatt three-phase star-connected motor is fed from a 400 V supply. The motor has an efficiency of 85% while the power factor is 0,86 lagging. Calculate the line current of the motor. State TWO factors that influence the resistance of a material. A coil of wire has a resistance of 15 ohm at 0 C. Calculate the resistance of the wire at 80 C. The temperature coefficient of resistance of the conductor is 0,00428 / C. Three capacitors of 160 µf, 200 µf and 400 µf are connected in parallel. Calculate the total capacitance of the circuit. (4)

6 EXEMPLAR -6- NC(V) 3.5 Transformers are extensively used in the electrical practice. Explain the following about transformers: The general construction Basic operation QUESTION 4: PROTECTION AND MEASURING INSTRUMENTS 4.1 Explain, with the aid of fully labelled sketches, how the following measuring instruments can be connected to a circuit Voltmeter Ammeter (2 2) (4) 4.2 An analogue multimeter has the following DC ranges: Voltage: Current: 3 V, 10 V, 30 V, 100 V 500µA, 1 ma, 10 ma, 100 ma, 1 A Which range would be best suited to measure the following: The terminal voltage of a 4,5 V battery. A voltage drop of 0,6 V across a semiconductor diode. A current of 5 ma through a circuit. The base current of 300 µa through a transistor. (4 1) (4) State SIX precautions you would take when using a measuring instrument Explain, with the aid of fully labelled sketches, how the ranges of a voltmeter and an ammeter can be extended. QUESTION 5: CIRCUIT DIAGRAMS AND ELECTRICAL MACHINES What is the purpose of the overload coil in a direct on line starter? Draw a labelled circuit diagram of a capacitor start -induction run single-phase motor. Draw a neat, labelled circuit diagram of a long shunt compound DC motor. (5) (5)

7 EXEMPLAR -7- NC(V) Explain, by means of a sketch, how you would carry out an insulation resistance test to earth on an electrical motor. State the measuring instrument used and expected reading. Name TWO configurations that can be used to connect a three-phase transformer. TOTAL: 100

SECTION 3 BASIC AUTOMATIC CONTROLS UNIT 12 BASIC ELECTRICITY AND MAGNETISM

SECTION 3 BASIC AUTOMATIC CONTROLS UNIT 12 BASIC ELECTRICITY AND MAGNETISM Unit Objectives Describe the structure of an atom. Identify atoms with a positive charge and atoms with a negative charge. Explain