NATIONAL CERTIFICATE (VOCATIONAL) ELECTRICAL PRINCIPLES AND PRACTICE NQF LEVEL 4 NOVEMBER 2009 (12041004) 23 November (X-Paper) 09:00 12:00 Calculators may be used. This question paper consists of 7 pages.
(12041004) -2- NC700(E)(N23)V TIME: 3 HOURS MARKS: 100 INSTRUCTIONS AND INFORMATION 1. 2. 3. 4. 5. 6. 7. Answer ALL the questions. Read ALL the questions carefully. Number the answers 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. ALL steps and unit of measurements must be indicated in calculations. Write neatly and legibly.
(12041004) -3- NC700(E)(N23)V QUESTION 1: FUNDAMENTALS OF ELECTRICITY 1.1 State TWO main criteria that you would consider when selecting miniature circuit breakers. (2) 1.2 Choose a description from COLUMN B that matches a word in COLUMN A. Write only the letter (A D) next to the question number (1.2.1 1.2.4) in the ANSWER BOOK. COLUMN A COLUMN B 1.2.1 1.2.2 Conduit Trunking A a closed enclosure that comprises a base with a removable cover that is intended to surround insulated conductors and cables 1.2.3 1.2.4 Ducting Flexible conduit B a closed enclosure that allows insulated conductor and cables in electrical installations C tubing that is intended to house and protect electrical wiring D a pipe usually of a diameter not exceeding 50 mm that allows conductors and cables in electrical installations to be drawn in and to be replaced (4 1) (4) 1.3 A 50 Ω resistor, an inductor with an inductive reactance of 60 Ω and a capacitor with a capacitive reactance of 40 Ω are connected in series across a 25V/1 khz power supply. Calculate the following: 1.3.1 The value of the capacitance (3) 1.3.2 The value of the inductance (3) 1.3.3 The total current of the circuit (6) 1.3.4 State whether the circuit is capacitive or inductive and substantiate your answer. (2) [20]
(12041004) -4- NC700(E)(N23)V QUESTION 2: GENERATION AND SUPPLY OF ELECTRICITY 2.1 State where the following type of voltage values are used: 2.1.1 Low voltage less than 1 KV 2.1.2 Medium voltage between 1 KV and 44 KV 2.2 List TWO advantages of a three-phase distribution system. (2) 2.3 Briefly describe the function of the following components with regard to the distribution of electrical power from the generation site to the consumer site: 2.3.1 2.3.2 Lightning arrestor Switchgear (2) (2) 2.4 Indicate whether the following statements are TRUE or FALSE. Choose the answer and write only 'true' or 'false' next to the question number (2.4.1 2.4.2) in the ANSWER BOOK. 2.4.1 2.4.2 Switchgears found in substations are placed only on the high voltage side of the transformer. A strain insulator is used to protect large voltage loads and heavy duty insulation jobs. 2.5 The sketch below (FIGURE 1) refers to the structural layout stages in the transmission and distribution of electrical power. Identify the sections labelled A to G. Do NOT redraw the sketch. Write only A to G with the answer next to the question number in the ANSWER BOOK. C E F A B D G FIGURE 1 (7 1) (7)
(12041004) -5- NC700(E)(N23)V 2.6 Why are overhead high voltage lines preferred to cables buried underground for electricity distribution? (3) [20] QUESTION 3: EARTHING AND LOAD BALANCING 3.1 List ONE function of earthing. 3.2 Define an earth continuity conductor. (3) 3.3 State the SABS 0142 regulation for protecting earthing of metalwork. (2) 3.4 Draw a fully labelled schematic diagram of a core balance earth leakage relay. (7) 3.5 Name TWO methods that are used to protect overhead lines. (2) QUESTION 4: TRANSFORMERS 4.1 Briefly explain how the closed core transformer and the shell transformer are influenced by the electromagnetic interference. (3) 4.2 A 145 KVA transformer has an input voltage of 1 100 V and an output voltage of 320 V, at a frequency of 50 Hz. Calculate the following if the transformer has 80 turns on the secondary winding: 4.2.1 4.2.2 The primary current The secondary current (3) (3) 4.3 State where the following transformers are used: 4.3.1 4.3.2 Star/star connected transformer Delta/star connected transformer 4.4 Draw a fully labelled diagram to show how three single-phase transformers can be connected to form a three-phase delta/star transformer. (4) QUESTION 5: MOTORS, GENERATORS AND ALTERNATOR PRINCIPLES 5.1 Define the following terms: 5.1.1 5.1.2 Armature reaction Commutation (2)
(12041004) -6- NC700(E)(N23)V 5.2 5.3 List ONE factor that can be applied to minimise the effects of the armature reaction. A wound rotor induction motor causes the starter overloads to trip each time the operator attempts to start the machine. List ONE possible cause of electrical defect in the stator section of the starter. 5.4 Name ONE method of starting a three-phase cage type induction motor. 5.5 What is the main purpose of a DC motor starter? 5.6 The sketch below (FIGURE 2) refers to the structure of the wound motor. Identify the sections labelled A to F. Do not redraw the sketch. Write only A to F with the answers next to the question number in the ANSWER BOOK. FIGURE 2 (6 1) (6) 5.7 When a three-phase motor switches from star to delta it is found that the motor hums. Give ONE possible cause for the above problem.
(12041004) -7- NC700(E)(N23)V 5.8 State ONE place where the insulation value of the squirrel cage motor windings can be tested. QUESTION 6: ILLUMINATION 6.1 State ONE application of each of the fluorescent tubes with the following colours: 6.1.1 White and warm white 6.1.2 Natural and daylight 6.1.3 Artificial daylight 6.1.4 De luxe warm white 6.2 Explain what is meant by the term incandescence. (2) 6.3 What is the purpose of the auxiliary electrode in the high pressure mercuryvapour lamp? 6.4 List TWO functions of starters in fluorescent lamps. (2) 6.5 The table below shows the comparison of the efficiency of various lamps. Which lamp has the least efficiency? LAMP TYPE WATTAGE INITIAL EFFICIENCY (LM/W) Incandescent 6 1 500 4-20 Mercury 40 1 000 32-63 Fluorescent 15-215 55-100 High-pressure sodium Low-pressure sodium 35-1000 40-140 18-180 Up to 200 6.6 In what way can stroboscopic rays be dangerous? (2) 6.7 Describe briefly why sodium vapour lights are used only for street lighting. (3) TOTAL: 100