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Wiring systems must be selected so that they:< Protect against physical contact with live parts by durable insulationmaterials.< Satisfy the current carrying capacity, voltage drop and other minimum sizerequirements for the conductors.< Protect against mechanical damage, environmental and other externalinfluences.(a) Cables carrying large currents are installed in horizontal cable trays.State why is it necessary to securely fasten the cables in the cable trays?
To prevent movement if a highprospective short-circuit current faultoccurs.
A 400 metre cable supplies a sawmill. Apart from length, state TWO otherfactors that will determine the voltage drop of the cable.
Any TWO of: (2 marks). Load current. Resistivity of the conductors. Cross-sectional area Ambient temperature
TPS flat cable is required to have maximum temperature rating notexceeding 75ºC when operated on full load.Explain why it is dangerous to continuously overload the cable so that itexceeds its maximum temperature rating?
The insulation deteriorates attemperatures over 75ºC.(1 mark) Any ONE of: (1 mark) The cable may become a firehazard. The cable may become ashock hazard.
A 25mm² single-core XLPE insulated copper conductor has a current ratingof 165 amps. A 25mm² single core PVC insulated copper cable has acurrent rating of 143 amps.Explain the reason why the XLPE insulated conductor has a higher currentrating than the PVC insulated conductor.
The XLPE cable insulation is able towithstand a much higher continuoustemperature
A 35 mm² cable is run 600 metres to a pump station. The insulationresistance of the cable is 140M6.Calculate the change in the insulation resistance value if the cable wasshortened to 450m when the pump station was relocated.
R2 = R1 x L1L2= 600 x 140450(½ mark)= 186.67M< (½ mark)= 186.67 – 140 (½ mark)= 46.67 M<
When carrying out testing on a low voltage electrical installation, whichdocument details the tests and checks required to be carried out?
You have isolated a single-phase 230V domestic electrical installation soalterations can be carried out at the switchboard. After completing thealterations you relivened the installation.You are using a voltage tester on the front of a switched socket outlet toensure power is present.Note: The voltage tester is operating correctly.(3 marks)(i) What voltage you would expect to find with the switch OFF:Between Active – Neutral ________________________________Between Active – Earth ________________________________Between Neutral – Earth ________________________________(ii) What voltage you would expect to find with the switch ON:Between Active – Neutral ________________________________Between Active – Earth ________________________________Between Neutral – Earth ________________________________
(i) A-N 0V (1½ marks)To gain marksthe polaritymust becompletelycorrectA-E 0VN-E 0V(ii) A-N 230V (1½ marks)To gain marksthe polaritymust becompletelycorrectA-E 230VN-E 0V
You are using a voltage tester on the front of a 230V switched socket outletto ensure power is present and you get the following results.
Active - Earth 0V 0V
(d) Any TWO of:. The protective earthing conductoris open-circuited.. There is no earth to the socket. The Earth terminal is broken
You are using a voltage tester on the front of a 230V switched socket outletto ensure power is present and you get the following results:
Active - Neutral 0V 230VActive - Earth 0V 0VNeutral- - Earth 230V 230V
Active and neutral are transposed (in thefinal subcircuit or socket outlet).
You have been called to a fixed-wired concrete-making vibrating plant where astaff member received an electric shock off the frame of the plant. The vibratingplant is driven by a 400V, three-phase, 1.5kW induction motor controlled by aDOL (direct-on-line) starter. The motor and starter are controlled by anadjacent isolator.You have isolated the vibrating plant, by turning off the isolator and proving theisolation on the load side of the switch with the prove-test-prove method.The only test instruments you have are a multimeter and clamp type ammeter.(a) (i) What is the first electrical test you would carry out?(1 mark)________________________________________________________(ii) What instrument would you use to carry out the test you have statedin (a)(i).(½ mark)________________________________________________________(iii) Describe how you would carry out the test you have stated in (a)(i)(1 mark)________________________________________________________________________________________________________________(iv) What is the minimum or maximum test result value would youconsider acceptable for the test you have stated in (a)(i)?(1 mark)________________________________________________________(b) (i) What is the second electrical test you would carry out?(1 mark)________________________________________________________(ii) What instrument would you use to carry out the test you have statedin (b)(i).(½mark)________________________________________________________
(iii) Describe how you would carry out the test you have stated in (b)(i)(1 mark)________________________________________________________________________________________________________________(iv) Given that the motor circuit is faulty, what test result value would youexpect for the test you have stated in (b)(i)?(1 mark)________________________________________________________________________________________________________________________________________________________________________(c) Explain why you would carry out the test stated in (a)(i) before the test youhave stated in (b)(i).(1 mark)________________________________________________________(d) In addition to a staff member receiving an electric shock off the frame ofthe vibrating plant, you were also told that in fact the motor had beentroublesome for some time. Sometimes the motor would run and at othertimes it would hum then trip the starter overload unit.State TWO possible reasons why the motor would run and, at other times,it would hum then trip the starter overload unit.(2 marks)(1) ________________________________________________________________________________________________________________(2) ________________________________________________________________________________________________________________
(a) (i) Protective earthing conductor test. (1 mark)(ii) The multimeter set on the lowestohms scale(½mark)To gain thismark the use ofthe multimeterhas to bequalified.(iii) Test between the earth connectionof the isolating switch to thevibrator frame.(1 mark)(iv) 1< or less (1 mark)(b) (i) Earth leakage test (1 mark) With this test,it is presumedthat theisolation isremoved andthe plantrelivened(ii) The clamp-type ammeter set on thelowest scale(½mark)To gain thismark the use ofthe ammeterhas to bequalified.(iii) . Start the motor running and (½mark). Measure the current in theprotective earthing conductor(½mark)(iv) Considerably more than 5 mA (1 mark)Alternative solution(i) Continuity test from each phase toframe of plant.(ii) Multimeter on low ohms.(iii) Test resistance from each phase tothe proven earth conductor (orplant) on both incoming andoutgoing phases of motor starter. (iv) A low enough resistance to indicatea possible connection of a liveconductor to the frame.(c) . To ensure the PEC is continuous (½ mark). And of low resistance (½ mark)(d) Any TWO of: (2 marks). Intermittent open circuit in thewindings on one phase.. Intermittent open circuit in onephase of the starter or isolatingswitch contacts.. Faulty bearings Intermittent mechanical fault indriven load
In a squirrel cage induction motor the rotor bars are skewed at an angleacross the face of the rotor.State TWO reasons why the rotor bars are skewed rather than beingstraight.
Any TWO of:. Smoother running. More even torque. Less noise. To reduce vibration. Increased torque
List THREE types of reduced-voltage starters (other than electronic) thatcan be used with a three-phase induction motor.
Star/delta. Auto transformer. Primary resistance
State the TWO types of fault the MCB provides protection for
What type of fault does the RCD provide protection for?
Any earth fault on the circuit.
A phase to neutral short circuit has occurred in the cable between the lightswitch and the first fluorescent light.
List THREE other checks or tests that should be carried out on the motorbefore reconnecting it to the supply and returning it to service
Any THREE of:/ Protective earthing conductorcontinuity test./ Visual test./ Polarity test./ Check the integrity of the terminations
You have installed a single-phase socket outlet and circuit wiring.List the FOUR tests using test instruments – other than an insulationresistance test or a correct circuit connections test – which should becarried out in the socket outlet and circuit wiring.
Polarity Earth Continuity/ Earth Loop Impedance/ RCD Test
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