University of Moratuwa, Sri Lanka. EE3231: Design of Electrical Installation

Transcription

1 University of Moratuwa, Sri Lanka B.Sc. Engineering Level 4 Semester 1 Examination EE3231: Design of Electrical Installation Time allowed: 2 Hours January 2010 Additional Material: Not required INSTRUCTIONS TO CANDIDATES This paper contains 4 questions. Answer all the questions. This is a closed book examination. Total marks for the paper is 70 marks. Mobile phones are not permitted. Only equipment allowed is a calculator approved and labeled by the Faculty of Engineering. The maximum marks attainable are indicated in square brackets. Assume reasonable values for any essential data not given in or with the examination paper. Clearly state such assumptions made on the scripts. If you have any doubts to the interpretation of the wording of a question, make your own decision, but clearly state it on the script. Page 1 of 12

2 Question 01 a) Draw labeled schematic diagram for TN-C and TN-S systems. What is the main disadvantage of TN-C system over the TN-S system? [04 Marks] b) Analyze quantitatively the earth loop impedance and the earth fault current of TT and TN- C-S systems [04 Marks] c) An RCD is used as the protective device against electric shock in TT systems. The following figure shows a TT earthing arrangement. The resistance of the earth electrode of the substation neutral is 10 Ω and the resistance of the earth electrode of the installation is 20 Ω. i. What are the two conditions that must be fulfilled by the RCD for correct operation? ii. By considering the worst case, calculate the maximum possible earth fault current within the above installation iii. Calculate the fault voltage U f? Is it dangerous? Why? [12 Marks] N Earth electrode of the substation neutral U f Earth electrode of the installation neutral Question 02 a) A 5 A rewireble fuse and a 10 A mcb are in series in a certain circuit. Which device will operate first in the event of a short circuit fault? Give the reasons for your answer [04 Marks] b) A retail jeweler shop has the following connected load supplied at 230 V/50 Hz: Direct heaters 2 2 kw, kw, kw Cooker 5 kw Water heating (thermostatic) 3 kw (total) Socket outlets 30 A ring circuit Shop and window lighting 2.5 kw (total) Determine the assumed maximum demand of the shop. [08 Marks] c) A single phase 230 V circuit is to be wired in 4 mm 2 phase with 1.5mm 2 circuit protective conductor (c.p.c.) thermoplastic 70 o C copper singles cable. The protective device is a 63 A BS 88 general purpose fuse. Impedance external to the installation (Z e ) for the circuit is 0.23Ω. The circuit is 36m long and has a maximum of 5 s disconnection time. Will this cable comply with the regulation requirements for the disconnection time and thermal constraints? If not find suitable cable size for circuit protective conductor (c.p.c.) [08 Marks] Page 2 of 12

3 Question 03 a) Using the basic equation ( ) derive an equation for resistance of a rod electrode. [03 Marks] b) What would be the expected earth resistance of a 2 m long pipe of diameter 50 mm in a soil of resistivity 100 Ωm. [03 Marks] c) Briefly explain three methods that can be used to improve earth electrode resistance. [03 Marks] d) A domestic premise with a 230 V, 50 Hz supply is protected by a 100 A BS 1361 supply fuse. The total assumed demand of this premise is equal to 140 A. Is this chosen fuse for overload protection correct? Please give the reasons for your answer. [03 Marks] e) Single line diagram of a certain installation is shown in the figure below. A X 50 m/70 mm 2 Cu I B =150 A B X X X 20 m/2.5 mm 2 Cu I B =20 A A 3 phase Copper line (AB) of 70 mm 2 cross sectional area (c.s.a) and a length of 50 m passes a current of 150 A. The line supplies, among the other loads, 3 single phase lighting circuits, each of 2.5 mm 2 c.s.a. Copper 20 m long, and each passing 20 A. What is the percentage voltage drop at the end of the lighting circuits? [08 Marks] Page 3 of 12

4 Question 04 a) Light is defined as visually evaluated radiant energy. Explain what are meant by the terms visually evaluated and radiant energy. b) What do you understand by photometric standard observer (V )? Why do we need such a standard in photometry? c) An isotropic point light source S has a total luminous flux output of 1500 lumen. Determine the illuminance at a point O on a horizontal plane placed 2m directly under this light source. d) Using Lambert s cosine law, find out the illuminance due to the light source S at a point A placed 2m away from the point O on the same horizontal plane in part (c) above. e) What do you understand by correlated colour temperature (CCT) of a light source? The CCT of a fluorescent lamp is specified as 6000K. What colour of light do you expect from this lamp? f) Fluorescent lamps can have different colours while incandescent lamps come with only one colour. Describe the reason for this. g) A room measuring 10m X 6m need to be illuminated for general reading using fluorescent lamps of an efficacy of 80 lm/w. It is known that 35% of the light produced is lost within the fixture and another 20% margin is to be allowed for loss due to aging. i. What is the recommended level of illumination required? ii. iii. If fluorescent lamps of rating 36W are to be used, how many lamps do you need to obtain the recommended illumination? Draw a sketch of the distribution of the lamps on the ceiling. What mounting height do you recommend for the lamps? Page 4 of 12

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