NATIONAL CERTIFICATE (VOCATIONAL) ELECTRICAL SYSTEMS AND CONSTRUCTION NQF LEVEL 2 NOVEMBER 2009

Transcription

1 NATIONAL CERTIFICATE (VOCATIONAL) ELECTRICAL SYSTEMS AND CONSTRUCTION NQF LEVEL 2 NOVEMBER 2009 ( ) 12 November (X-Paper) 09:00 12:00 This question paper consists of 6 pages.

2 ( ) -2- NC710(E)(N12)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)(N12)V QUESTION 1: BASIC ELECTRICAL CIRCUITS AND SYSTEMS 1.1 What is the voltage and frequency values that Eskom supplies to South African households? (2) 1.2 Name TWO ways in which electricity can be distributed to consumers. (2) 1.3 Sketch the IEC symbols for the following components: A lamp A single - pole switch An earth connection A fuse A miniature circuit breaker (5) 1.4 You are required to wire a light circuit in a domestic installation comprising of the following components: x lamps 1 x single-pole switch 1 x distribution board with neutral, live and earth bar 1 x 20A miniature circuit breaker Draw a neat fully labelled sketch of the distribution board and the lighting circuit consisting of TWO lamps controlled from a single - pole switch. Show all necessary components and protective devices. (11) Identify the sub-circuit in FIGURE 1 below. FIGURE Prepare a list to order the components as labelled A G in FIGURE 1. (7) 1.6 Compare an isolator with an earth leakage relay with respect to their use in domestic distribution boards. (2) [30]

4 ( ) -4- NC710(E)(N12)V QUESTION 2: WIRE WAYS AND LOW VOLTAGE CABLES 2.1 What is the purpose of wire ways? (2) 2.2 Choose a description from COLUMN B that matches an item in COLUMN A. Write only the letter (A F) next to the question number ( ) in the ANSWER BOOK. COLUMN A COLUMN B Low voltage cable joint High voltage cable joint A paper insulated lead covered steel/wire armoured cable PILCSWA B flexible conduit Screw connector C soldered joint Wire way D resin pressure joint E used to join a number of conductors together F neutral connector (5 x 1) (5) 2.3 A joint has been made in a cable. You are requested to examine the joint to determine if this joint meets the requirements of a good joint. Which FIVE factors will you take into consideration when doing so? (5) Identify the type of cable in FIGURE 2 below FIGURE 2 With reference to FIGURE 2 above, identify the parts labelled A D. (4) 2.5 You want to make a joint in a cable that is damaged. Which THREE factors will you take into account to ensure the right type of joint to join the cable? (3) [20]

5 ( ) -5- NC710(E)(N12)V QUESTION 3: ELECTRIC MACHINES 3.1 A DC motor consists of various parts. Name FOUR major parts of a DC motor. (4) Identify the type of AC motor in FIGURE 3 below. FIGURE Identify the parts of the motor labelled A F in FIGURE 3 above. Write down the answers next to the letters (A F) in the ANSWER BOOK. (6) 3.3 A faulty motor needs to be tested to determine what the problem is. You have to perform electrical tests to determine the problem. Which THREE important electrical tests would you perform? (3) 3.4 Before you disconnect an electrical machine from the supply, you have to take into account where all the electrical connections are on the machine. You have to remove and connect back all the electrical connections on the machine. State TWO methods that can be used to label the electrical connections of any electrical machine when you disconnect it from the supply. (2) 3.5 You need to perform electrical tests on a three-phase induction motor. The first test that you want to do is the insulation resistance test between windings What tester are you going to use? Which scale will you select on the tester? The reading on the meter is 900Ώ Is this reading acceptable? Motivate your answer. (2) [20]

6 ( ) -6- NC710(E)(N12)V QUESTION 4: FAULT-FINDING AND TESTING 4.1 Name FOUR possible tests that can be carried out on electrical components. (4) 4.2 You are requested to test a circuit breaker that has been removed from a distribution board What tester would you use? How is the circuit breaker tested? (3) 4.3 Compare the operation of an isolator and a circuit breaker. What is the major difference between the operation of an isolator and a circuit breaker? (2) [10] QUESTION 5: PROTECTION AND MEASURING INSTRUMENTS 5.1 What are the symbols/abbreviations of the following: Ammeter Voltmeter Kilowatt-hour meter Frequency meter Power factor meter (5) 5.2 Write down TWO main types of measuring instruments that can be installed or replaced. (2) 5.3 Name FOUR items that need to be communicated to all stakeholders before you can start to complete your task. (4) 5.4 A voltmeter labelled A, and an ammeter labelled B is connected in the circuit below to measure the voltage and current Is the voltmeter correctly connected? If not, how should the meter be connected? Is the ammeter correctly connected? If not, how should the meter be connected? (2) (2) 5.5 After the installation and/or replacement of a metering unit on a panel, certain procedures/steps need to be performed before the job is finalised. Write down these steps. (5) [20] TOTAL: 100