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MINISTRY OF SCIENCE AND TECHNOLOGY Department of Technical and Vocational Education Electrical Power Engineering A.G.T.I Year II (Final Examination) Date:21-10-200 Time: 8:30 am-13:30 am EP 02015 Electrical Wiring And Installation ATTEMPT ANY SIX QUESTIONS 1. (a) Describe the required knowledge on which fields made good electrical installation. (b) State the various range of supply voltage. (c) Define the frequency of supply. 2. (a) What do you understand the single phase installation? (b) What do you understand the 3 phase, 4 wire installation? (c) Draw the circuit diagram of voltage obtains from 3 phase , 4 wires distribution? (d) Draw the wiring diagram of 2-way intermediate switching from four positions? 3. (a) What are the methods of earthing? (b) Explain the method of earthing in electrical installation? 4. (a) What is light? (b) Define regular, scatter and mixed reflection. (c) Which method are base on the modern light of light generation? 5 (a) What is a Parallel Circuit? Explain with its circuit diagram. (b) Explain the termination of PVC Twin and Tree core with earth and properties of the rigid PVC and flexible PVC. 6. (a) A 3-phase motor with a full load current 180A, and a power factor of 0.85 is to be fed by four single-core, pvc-insulated, copper conductor, non-armoured cables, clipped direct on a non-metallic surface at 120-meter run as show in Figures. Determine the size of conductor if the permissible voltage drop from the MCB to the motor terminal is 3%. 50 mm2 cable has tabulated current rating of 134A; TVDr = 0.81 and TVDx = 0.26 70 mm2 cable has tabulated current rating of 171A; TVDr = 0.56 and TVDx = 0.25 95 mm2 cable has tabulated current rating of 207A; TVDr = 0.42 and TVDx = 0.24 MCB IFL = 180A 120 m M 0.85 pf 4 x 1 C/Cu/PVC/NA enclosed in conduit 6. (b) A three-phase circuit consisting of 4 x 10 mm2 Cu/PVC/non-armoured single-core cable is slected to feed an electrical appliance which has a design current of 50 A at 0.8 power factor as shown in Figure. The circuit length is 30 m and the ambient temperature is 25ºC. Determine the voltage drop by taking into consideration a lower conductor temperature. 10 mm2 cable has tabulated current rating of 59 A; TVDr = 3.8 and TVDx = 0 MCB IB = 50A 30 m 25. C Appliance 0.8 pf 4 x 10 C/Cu/PVC/NA clipped direct 7. (a) Explain the Rising Mains Cables systems and Busbar system. (b) Discuss the values of Earth-lppo Impedance. 8. (a) Describe the producer for Load Estimation. (b) Describe the requirements for panelboards. ****************** END * * * * * * * * * * * * * * * * * * 1. (a) Solution To get satisfactory and safety result from installation a person who works in electrical installation field must be known a good knowledge in the following field; 1. Supply system 2. Consumer’s circuit 3. Wiring accessories 4. Conductor and cable 5. Wiring system 6. Testing 7. Instrument and measurement 8. Earthing 9. Illumination 1. (b) Solution The various range of supply voltage are - Low voltage ( not exceeding 250 volt) - Medium voltage ( above the 250 volt but not exceeding 650 volt) - High voltage ( exceeding 650 volt ) 1. (c) Solution The alternating current(A) supply is used as standard. The supply voltage will pass through a complete of positive and negative value cycles, in a given number of every second. This number is known as frequency of supply The SI unit of frequency is Hertz. (1 Hz = 1 C/S) 2. (a) Solution Such simple two wire installation is known as single-phase installation (or) a circuit having one phase (red, yellow, blue) and a neutral. 2. (b) Solution The differential loads such as heating, lighting and power circuit are then distributed over the three-phase, which are identified by colour coading red, yellow and blue, in a given of load balancing are known as 3 phase, 4 wire installation. 2. (c) Solution Figure. Voltage obtains from 3 phase, 4 wire distribution 2. (d) Solution N P A B Figure. Wiringdiagram of 2-way intermediate switching from 4 position 3. (a) Solution The mewthods of earthing are: - direct earthing system - multiple earthed-neutral system - earth leakage circuit breaker system 3. (b) Solution Direct earthing system A system of earthing in which part of the installation are so earthed as specified, but are not connected within installation to earth trip of an earth leakage circuit breaker. Multiple earthed-neutral system A system of earthing in which part of installation, specified so as earthed, are connected to general mass of earth and in addition are connected within installation to the neutral system of supply terminal. Earth leakage circuit breaker system In this system, part of installation are connected to an earth electrode through coil of earth leakage circuit breaker, which controls the supply that of part of installation in addition to equipment earth. 4. (a) Solution Light is defined as visually radiant energy and it has predication. It can travel through space in form of electromagnetic wave at constant speed. 4. (b) Solution Regular reflection The parallel beam of light is reflected as such mirror reflection area called regular reflection. Scatter reflection If, however a beam of light strikes a white wash ceiling, the incident light is scattered in all direction. This is known as scatter or diffuse reflection. Mixed reflection Between the extremes of these two there are numerous transition-mixed or semidiffuse reflection. 4. (c) Solution The molders light are based one of following method of light generation. Temperature radiation For age this had been method avariable, example of temperature radiation are tough, candle, the oil lamp and the gas lamp, etc. The filament lamp too is based. Gas discharge Under certain condition it is possible to pass electrical current through a gas (or) metal vapour. Visible radiation can be generated in this way because free electron knocks the electron bond in the gas atom out of their normal path. This lost is followed back to the original path, give of the energy, in form of radiation. This must more economically than temperature radiation. Fluorescence Certain material can, when exposed an ultra violet or visible radiationb, transform observed energy into radiation with longer wavelength. This is used in fluorescence lamp and in mercury-covered lamp. 5. (a) Solution Circuit can be buit up from simple three phase circuit. For example, if the two light points are controlled from one-way switch. In loop wire is taken from each terminal of first light point and connected to the correspond terminal of the second light point. The result is that when switch is closed the current has two paths. One through each lamp, so that both are energized. This circuit is said to be connected in parallel. When several circuits have to supply from main switch, the switch wire may all be connected to one side of supply and the return wire from apparatus to the other side. They are then in parallel. The full electrical pressure of the supply is applied to each circuit. The amount of power taken by any one circuit is independent of power taken to other side. Fuse Switch Current to flow Phase Light 1 Light 2 Supply terminal Neutral Return wire Figure. Parallel Circuit 5. (b) Solution The Termination of PVC Twin and Tree core with Earth Two methods are used as; (i) Using a sharp knife, slice a cable lengthwise, opened a cable up and cut off the sheathe. (ii) With pair of slide cutter, split end of the supply and hole out the end of the earth conductor. This may then be help with a pair of piler and pulled back along the fuse cable, splitting open the sheathe. Excess sheathe may be cut away. The Rigid PVC The rigid PVC as used in conduit is a thermoplastic polymer and has the following properties. - It has high tensile strength. - It can be bent by hand if warmed up. - It has high electrical resistant. - It has weather resistant. - It does not creale under stress at normal temperature. - It is low flammable. - It is self-extinguished when source of heat is removed. - It is used for special suddle when use of fictuation temperature. The Flexible PVC (used in cable insulation) - It has high electrical resistant. - It has weather resistant. - It should be leept clear of other plastic to avoid of migration of plasticizer. 6. (a) Solution IB = 180 A cos θ = 0.85 L = 120 m 2 70 mm cable has tabulated current rating of 171 A is initially selected. The line-to – line voltage. T V D r cos θ T V D x sinθ x I B xL VdropLL 1000 (0.56 x 0.85 0.25 x 0.53) x 180 x 120 = 1000 13143.6 = 1000 = 13.144 V or 2.6% of 400V So, 95 mm2 cable has tabulated current rating of 207A is selected. 6. (b) Solution IB = 50 A cos θ = 0.8 L = 30 m Ta = 25 C VD =? t 50 t a I ( B )2 tp tr It IB 2 t50 = ta ( ) (t p t r ) It 50 2 = 25 + ( ) (70 30) 59 = 25 + 28.72 = 53.72 C At TVD at t50 = 53.72 C 230 53.72 TVD53.72 = TVD ( ) 230 70 283.72 = 3.8 ( ) 300 = 3.8 x 0.959 = 3.64 T V D r cos θ T V D x sinθ x I B xL VdropLL 1000 (3.64 x 0.8 0) 50 x 30 = 1000 = 4.368 V The voltage drop 4.368 V is actually 95.9% of voltage drop obtain multiplying TVDr value. The conductor temperature at 29.5 A will be IB 2 T29.5 = ta ( ) (t p t r ) It 29.5 2 = 25 + ( ) (70 30) 59 = 25 + 10 = 35 C TVD at 35 C 230 35 TVD35 = TVD ( ) 230 70 = 0.88 TVD The actual voltage is 0.88% of voltage drop obtain multiplying a TVD value. 7. Solution (a) Rising Mains Cables system and Busbar system (10 marks) Please see in page 71-72 of textbook (b) Values of Earth-loop Impedance (6 marks) The go/no-go type of tester simply gives an indication of weather the circuit is suitable for semi-enclosed or cartridge fuse having a fusing factor exceeding 1.5. Since these require the lowest impedance of all types of earth-leakage protection. If values above this are obtained and a check reveats no fault on the circuits then the earth electroder resistance must be tested. The line-earth loop tester and the step-down transformer neutral earth loop tester both relay on a mains supply being available and therefore cannot be used prior to the electric board being requested to provide a supply. The hand generator type of instrument can however, be used to check the impedance of the earth electrode and trace high-impedance section of the installation prior to a supply being connected. It must be remembered however that this type of instrument normally only injects a very a small test current and consequently may not detect earth-continuity paths of low current carrying capacity. 8. Solution (a) Producer for Load Estimation A suggested producer for determining the design currents in various sections of a bulding is given in the following steps. • Determine the quantity of each type of load and the power requirements. Estimate the design current in each final circuit based on the rating of the connected load. Apply an appropriate demand factor to obtain the maximum demand of earth final DB. • Determine the type of connection from the final DB to the main DB and continue up to the incoming switchboard. Estimate the design current in each distribution circuit using an appropriate coincidence factor. • Determine spare capacity to be provided for load growth. (b) The Requirements for panelboards Please see in page 113 of textbook azlazlazlazlazlazlazalazlazlazlazlazlazl END azlazlazlazlazlazlazlazlazlazlazlazlazlaz
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