BEEE 102L LECTURE NOTES Module 2 (1) PDF

Unit II AC Circuits Alternating voltages and currents, – RMS, average, maximum values, Single Phase RL, RC, RLC series circuits, Power in AC circuits, Power Factor, Three phase balanced systems, Star and delta Connections, Electrical Safety, Fuses and Earthing, Residen...

Unit II AC Circuits Alternating voltages and currents, – RMS, average, maximum values, Single Phase RL, RC, RLC series circuits, Power in AC circuits, Power Factor, Three phase balanced systems, Star and delta Connections, Electrical Safety, Fuses and Earthing, Residential wiring. Vellore Institute of Technology, Chennai Campus, Chennai. 100 Alternating voltages and currents The quantities whose magnitude vary with time are called alternating quantities Vellore Institute of Technology, Chennai Campus, Chennai. 101  Contd… Sinusoidal quantities Vellore Institute of Technology, Chennai Campus, Chennai. 102  AC values Peak value (Vp or Vm / Ip or Im )  The maximum value of alternating quantity (i or V) is defined as peak value or amplitude. Root mean square (r.m.s.) value (Vrms / Irms)  r.m.s. value of ac is equal to that value of dc, which when passed through a resistance for a given time will produce the same amount of heat as produced by the alternating current when passed through the same resistance for same time. Vellore Institute of Technology, Chennai Campus, Chennai. 103 Contd… Average value (Iav or Vav)  The average of instantaneous values of current or voltage in one cycle is called it's mean value. The average value of alternating quantity for one complete cycle is zero.  The average value of ac is calculated over half cycle (t = 0 to T/2) Phase angle (φ)  It is an angle of reference of one quantity with that of the other Vellore Institute of Technology, Chennai Campus, Chennai. 104 Contd… Peak factor  Ratio of peak value to rms value Form factor  Ratio of rms value to average value Power factor  Ratio of real power to apparent power Vellore Institute of Technology, Chennai Campus, Chennai. 105 Contd… Vellore Institute of Technology, Chennai Campus, Chennai. 106 Contd… Vellore Institute of Technology, Chennai Campus, Chennai. 107 Classification of ac circuits Single phase circuits  Single Phase RL, RC, RLC series circuits  Single Phase RL, RC, RLC parallel circuits Three phase circuits  Three Phase RL, RC, RLC series circuits  Three Phase RL, RC, RLC parallel circuits Star connected circuits Delta connected circuits Vellore Institute of Technology, Chennai Campus, Chennai. 108 Elemental circuits Pure resistive circuit Vellore Institute of Technology, Chennai Campus, Chennai. 109 Contd… Pure inductive circuit Vellore Institute of Technology, Chennai Campus, Chennai. 110 Contd… Pure capacitive circuit Vellore Institute of Technology, Chennai Campus, Chennai. 111 Single phase circuits Basics Impedance, voltage and power triangles Phase angle and power factor Real, reactive and apparent powers Vellore Institute of Technology, Chennai Campus, Chennai. 112 Single Phase RL series circuit Vellore Institute of Technology, Chennai Campus, Chennai. 113  Exercises 1. A resistor of 25 Ω is connected in series with an inductor of 225.16 mH. Calculate (a) the impedance, and (b) the current taken from a 240 V, 50 Hz supply. Find also the phase angle between the supply voltage and the current. 2. A 10-Ω resistor and 10-mH inductor are connected in series with a 10-kHz voltage source. The rms current through the circuit is 0.20 A. Find the rms voltage drop across each of the elements. Vellore Institute of Technology, Chennai Campus, Chennai. 114 Contd… 3. Obtain the expressions for voltages across each elements and the current in the circuit shown in figure. Assume sinusoidal applied voltage. Vellore Institute of Technology, Chennai Campus, Chennai. 115 Contd… 4. A 200 V, 50 Hz supply is connected to a resistance (R) of 20 Ω in series with an iron cored choke coil (r in series with L). The readings of the voltmeters across the resistance and across the coil are 120 V and 150 V respectively. Find the loss in the coil. Also find the total power factor. Draw the phasor diagram. Vellore Institute of Technology, Chennai Campus, Chennai. 116 Contd… (Assignment 2 Q1) 5. A series circuit consists of a resistor „R‟ and an inductive coil with internal resistance of „r‟. A total current of 5 A flows through the circuit when a supply voltage of 250 V, 50 Hz is applied. The voltage across the resistor is 125 V and across the coil is 200 V. Calculate the reactance and resistance of the coil, power absorbed by the coil and the total power. Vellore Institute of Technology, Chennai Campus, Chennai. 117 Single Phase RC series circuit Vellore Institute of Technology, Chennai Campus, Chennai. 118  Examples 1. A resistor of 25 Ω is connected in series with a capacitor of 45 μF. Calculate (a) the impedance, and (b) the current taken from a 240 V, 50 Hz supply. Find also the phase angle between the supply voltage and the current. [75.03 ohm, 3.20 A, 70.54o leading] 2. An alternating voltage v = 250sin800t volts is applied across a series circuit containing a 30 Ω resistor and 50 μF capacitor. Calculate (a) the circuit impedance, (b) the current flowing, (c) the p.d. across the resistor, (d) the p.d. across the capacitor, and (e) the phase angle between voltage and current. [(a) 39.05 ohm (b) 4.526 A (c) 135.8 V (d) 113.2 V (e) 39.81o leading] Vellore Institute of Technology, Chennai Campus, Chennai. 119  Contd… 3. A 400 Ω resistor is connected in series with a 2358 pF capacitor across a 12 V a.c. supply. Determine the supply frequency if the current flowing in the circuit is 24 mA. [225 kHz] (Assignment 2 Q2) 4. A voltage of 120 V at 50 Hz is applied to a resistance, R in series with a capacitance, C. The current drawn is 2 A, and the power loss in the resistance is 100 W. Calculate the resistance and the capacitance. Vellore Institute of Technology, Chennai Campus, Chennai. 120 Single Phase RLC series circuit Vellore Institute of Technology, Chennai Campus, Chennai. 121  Exercises A coil of resistance 5 and inductance 120 mH in series with a 100 μF capacitor, is connected to a 300 V, 50 Hz supply. Calculate (a) the current flowing, (b) the phase difference between the supply voltage and current, (c) the voltage across the coil and (d) the voltage across the capacitor. Vellore Institute of Technology, Chennai Campus, Chennai. 122  Contd… The following three impedances are connected in series across a 40 V, 20 kHz supply: (i) a resistance of 8, (ii) a coil of inductance 130 μH and 5 resistance, and (iii) a 10 resistor in series with a 0.25 μF capacitor. Calculate (a) the circuit current, (b) the circuit phase angle and (c) the voltage drop across each impedance. Vellore Institute of Technology, Chennai Campus, Chennai. 123 Contd… Find the values of resistance R and inductance L in the circuit of figure Vellore Institute of Technology, Chennai Campus, Chennai. 124  Examples 1. A coil of resistance 5 Ω and inductance 120 mH in series with a 100 μF capacitor, is connected to a 300 V, 50 Hz supply. Calculate (a) the current flowing, (b) the phase difference between the supply voltage and current, (c) the voltage across the coil and (d) the voltage across the capacitor. (Assignment 2 Q3) 2. The following three impedances are connected in series across a 40 V, 20 kHz supply: (i) a resistance of 8 Ω, (ii) a coil of inductance 130 μH and 5 Ω resistance, and (iii) a 10 Ω resistor in series with a 0.25 μF capacitor. Calculate (a) the circuit current, (b) the circuit phase angle and (c) the voltage drop across each impedance. Vellore Institute of Technology, Chennai Campus, Chennai. 125 Three Phase Systems Three phase supply Phase sequence Vellore Institute of Technology, Chennai Campus, Chennai. 126 Advantages of Three-Phase System Transmission lines require much less conductor material. A three-phase machine gives a higher output. A three-phase motor develops a uniform (not a pulsating) torque. The three-phase induction motors are self-starting. Can be used to supply domestic as well as industrial (or commercial) power. The voltage regulation is better. Next Concept of Three-phase Voltages The phase order or phase sequence or phase rotation is abc. Next Next Generation of Three-phase Voltages Next  Whether the coils rotate anti- clockwise or the magnet on the rotor rotates clockwise, the effect is the same. But the latter is safer and easier to make external connections to stationary coils. Next Three windings connected to three loads using six line conductors Next  Thus, the terminals on the periphery appear in the order : R, B’, Y, R’, B, Y’. The three emfs generated eR, eY and eB connected to three respective loads L1, L2 and L3. This necessitates the use of six line conductors. Obviously, it is cumbersome and expensive. Let us now consider how it may be simplified. Next Three-phase Loads There are two kinds of three-phase systems : (i) Star or wye (Y) connection, and (ii)Delta (Δ) or mesh connection If all the three impedances are equal, the load is said to be a balanced load. Next Star (Y) Connected Three-phase System Next Next Unbalanced Three-Phase System The common point M is called local neutral point. Next  Practically, it may not be possible always to make this star-connected domestic load balanced. However, as per KCL, the sum of the three line currents must still be zero. Hence, the voltages across the three loads get adjusted, resulting in neutral shift or floating neutral. This situation is definitely undesirable. Therefore, we use Four-Wire Three-Phase Voltage System. Next Delta (Δ) Connected Three-phase System Note that the ‘finish’ of one phase is connected to the ‘start’ of another phase. Next Voltages And Currents Relations in 3-φ Systems (1) Star-Connected System Next  In a three-phase system, there are two sets of voltages :  the set of phase voltages, and  the other is the set of line voltages. VRN, VYN and VBN denote the set of three phase voltages. The term ‘line voltage’ is used to denote the voltage between two lines. VRY represents line voltage between the lines R and Y. Next VRY  VRNY  VRN  VNY  VRN  VYN  VRN  ( VYN ) VBN -VYN VRY VRY  2(VRN cos 30 ) 30° or VL  2Vph   3 / 2  3Vph VRN VYN VRN cos 30° Click VL  3Vph and I L  I ph Next Star-connected System Next (2) Delta-Connected System I R'R  I Y'Y  I B'B  I ph (say) Next IR  IR'R  I B'B IB’B IR’R cos 30° IR’R 30° IY’Y -IB’B IR I R  2( I ph cos30 )  3I ph Click I L  3I ph and VL  Vph Next Delta-connected System Next Important Points about Three-Phase Systems 1. It is normal practice to specify the values of the line voltages and line currents. 2. The current in any phase can be determined by dividing the phase voltage by its impedance. Next Example 1 A 400-V, 3- supply is connected across a balanced load of three impedances each consisting of a 32-Ω resistance and 24-Ω inductive reactance. Determine the current drawn from the power mains, if the three impedances are (a) Y-connected, and (b) Δ-connected. Next  Click Solution : Z = R + jX = (32 + j24) Ω.  Z  R  X  32  24  40  2 2 2 2 Click (a) Y-connection : VL 400 Vph 400 / 3 10 Vph   V  I ph    A 3 3 Z 40 3 10  I L  I ph   5.78 A 3 Click (b) For Δ-connection : Vph400 Vph  VL  400 V  I ph    10 A Z 40  I L  3I ph  3 10  17.32 A Next  Power In Three-phase System With A Balanced Load Consider one phase only, P1  Vph I ph cos  Click Hence, the total power consumed, P  3P1  3Vph I ph cos  For a star-connected system, V  3 V and I  I Click L ph L ph P  3(VL / 3) I L cos   3 VL I L cos  For a delta-connected system, VL Vph and I L  3 IClick ph P  3VL ( I L / 3) cos   3 VL I L cos  Click Thus, for any balanced load, P  3 VL I L cos  Next Next  In three-phase system with balanced load (such as a three-phase motor), there is no variation of power at all. This is the reason why for driving heavy mechanical loads we prefer a three-phase motor rather than a single-phase motor. Next Example 2 A 400-V, 3- supply is connected to a balanced network of three impedances each consisting of a 20-Ω resistance and a 15-Ω inductive reactance. If the three impedances are (a) star-connected, and (b) delta-connected, in each case determine  (i) the line current,  (ii) the power factor, and  (iii) the total power in kW. Next (a) For star-connected load : VL  3 Vph and I L  I ph VL 400 Click Vph    231 V 3 3 and Z ph  202  152  25  Click Vph 231 (i) I L  I ph    9.24 A Z ph 25 Click Rph 20 (ii) cos     0.8 (lagging) Z ph 25 Click (iii) P  3 V I cos   3  400  9.24  0.8  5.12 kW L L Next (b) For delta-connected load : VL  Vph and I L  3 I ph  VL  Vph  400 V Click Vph 400 (i) I ph    16 A Click Z ph 25  I L  3 I ph  3 16  27.71 A (ii) The power factor is same as above, pf = 0.8 (lagging) Click (iii) P  3 VL I L cos   3  400  27.71 0.8  15.36 Click kW Note that power consumed has become 3 times. Next No. Star-Connected System Delta-Connected System 1. Similar ends are joined together. Dissimilar ends are joined. 2. VL  3 Vph and I L  I ph VL  Vph and I L  3 I ph 3. Neutral wire available. Neutral wire not available. 4. 4-wire, 3- system possible. 4-wire, 3- system not possible. 5. Both domestic and industrial Only industrial loads can be loads can be handled. handled. 6. By earthing the neutral wire, Due to absence of neutral relays and protective devices can wire, it is not possible. be provided in alternators for safety. Next Three Phase connections Star and Delta Connection Vellore Institute of Technology, Chennai Campus, Chennai. 157 Some parameters Line voltages and currents  Delta connection  Star connection Vellore Institute of Technology, Chennai Campus, Chennai. 158 Contd… Vellore Institute of Technology, Chennai Campus, Chennai. 159 Balanced star – star connection Vellore Institute of Technology, Chennai Campus, Chennai. 160  Exercise 1 Obtain the line currents for the circuit shown in figure. Ans: IR1 = 83.33∟0o A, IR2 = 83.33∟120o A, IR3 = 83.33∟240o A Vellore Institute of Technology, Chennai Campus, Chennai. 161  Exercise 2 Find the line currents, total active power, reactive power, apparent power and power factor for the circuit shown in figure. Ans: IA = 6.81∟-21.8o IB = 6.81∟-141.8o IC = 6.81∟-261.8o Vellore Institute of Technology, Chennai Campus, Chennai. 162 Balanced star – delta connection Vellore Institute of Technology, Chennai Campus, Chennai. 163 Exercise 3 (Assignment 2 Q4) Vellore Institute of Technology, Chennai Campus, Chennai. 164 Exercise 4 (Assignment 2 Q5) Vellore Institute of Technology, Chennai Campus, Chennai. 165 Answer Vellore Institute of Technology, Chennai Campus, Chennai. 166 Balanced delta – star connection Vellore Institute of Technology, Chennai Campus, Chennai. 167 Exercise 5 (Assignment 2 Q6) Vellore Institute of Technology, Chennai Campus, Chennai. 168 Balanced delta – delta connection Vellore Institute of Technology, Chennai Campus, Chennai. 169 Contd… Vellore Institute of Technology, Chennai Campus, Chennai. 170 Exercise 6 (Assignment 2 Q7) Vellore Institute of Technology, Chennai Campus, Chennai. 171  Exercise 7 (Assignment 2 Q8) The circuit shows a balanced Δ–Δ connected system. Estimate the phase impedance and all line currents if the load power factor is 0.8 assuming inductive load. Vellore Institute of Technology, Chennai Campus, Chennai. 172  Three Phase Power Measurement One wattmeter method Vellore Institute of Technology, Chennai Campus, Chennai. 173  Contd… Three wattmeter method Vellore Institute of Technology, Chennai Campus, Chennai. 174  Contd… Two wattmeter method Vellore Institute of Technology, Chennai Campus, Chennai. 175  Electrical safety – insulation and fuses (Seminar 1 (2)) Insulation is used to prevent „leakage‟. Maximum voltage (peak values ) present must be taken into account for determining the type of insulation Fuses are the weak link in a circuit Used to break the circuit if excessive current (lead to fire) is drawn. Fuses rely on heating effect of current. Hence r.m.s values of current must always be used for calculating the appropriate fuse size. Vellore Institute of Technology, Chennai Campus, Chennai. 176  Notes – Personal and equipment safety in home 1. Avoid water at all times when working with electricity. Never touch or try repairing any electrical equipment or circuits with wet hands. It increases the conductivity of electric current. 2. Never use equipment with frayed cords, damaged insulation or broken plugs. 3. If you are working on any receptacle at your home then always turn off the mains. It is also a good idea to put up a sign on the service panel so that nobody turns the main switch ON by accident. 4. Always use insulated tools while working. 5. Electrical hazards include exposed energized parts and unguarded electrical equipment which may become energized unexpectedly. Such equipment always carries warning signs like “Shock Risk”. Always be observant of such signs and follow the safety rules established by the electrical code followed by the country you‟re in. Vellore Institute of Technology, Chennai Campus, Chennai. 177  Contd… 6. Always use appropriate insulated rubber gloves and goggles while working on any branch circuit or any other electrical circuit. 7. Never try repairing energized equipment. Always check that it is de- energized first by using a tester. When an electric tester touches a live wire, the bulb inside the tester lights up showing that current flows through that wire. Check all the wires, the outer metallic covering of the service panel and any other hanging wires with an electrical tester before proceeding with your work. 8. Never use an aluminium or steel ladder if you are working on any receptacle at height in your home. An electrical surge will ground you and the whole electric current will pass through your body. Use a bamboo, wooden or a fibreglass ladder instead. 9. Know the wire colour code of your country. 10. Always check all your GFCI‟s once a month. A GFCI (Ground Fault Circuit Interrupter) is a RCD (Residual Current Device). They have become very common in modern homes, especially damp areas like the bathroom and kitchen, as they help avoid electrical shock hazards. It is designed to disconnect quickly enough to avoid any injury caused by over current or short circuit faults. Vellore Institute of Technology, Chennai Campus, Chennai. 178  Contd… 11. Always use a circuit breaker or fuse with the appropriate current rating. Circuit breakers and fuses are protection devices that automatically disconnect the live wire when a condition of short circuit or over current occurs. The selection of the appropriate fuse or circuit breaker is essential. Normally for protection against short circuits a fuse rated of 150% of the normal circuit current is selected. In the case of a circuit with 10 amperes of current, a 15 ampere fuse will protect against direct short circuits whereas a 9.5 amperes fuse will blow out. 12. Working outside with underground cabling can be dangerous. The damp soil around the cable is a good conductor of electricity and ground faults are quite common in the case of underground cabling. Using a spade to dig at the cable can damage the wiring easily so it is better to dig at the cable by hand while wearing insulated gloves. Vellore Institute of Technology, Chennai Campus, Chennai. 179  Contd… 13. Always put a cap on the hot/live wire while working on an electric board or service panel as you could end up short circuiting the bare ends of the live wire with the neutral. The cap insulates the copper ends of the cable thus preventing any kind of shock even if touched mistakenly. 14. Take care while removing a capacitor from a circuit. A capacitor stores energy and if it’s not properly discharged when removed it can easily cause an electric shock. An easy way to discharge low voltage capacitor is that after removal from the circuit is to put the tip of two insulated screw drivers on the capacitor terminals. This will discharge it. For high voltage ones a 12 Volts light bulb can be used. Connecting the bulb with the capacitor will light up the bulb using up the last of the stored energy. 15. Always take care while soldering your circuit boards. Wear goggles and keep yourself away from the fumes. Keep the solder iron in its stand when not in use; it can get extremely hot and can easily cause burns. Vellore Institute of Technology, Chennai Campus, Chennai. 180  Wire colour code of India (Seminar 1 (1)) Electrical wires follow standard colour coding that helps classify each wire function in the circuit. In India wires are RGB mode i.e. Red- Green- Black. Each of these RGB wire have different functions. Red – Red wire signifies the phase in electric circuit. It is he live wire which cannot be connected to another red wire or black wire. Red is used in some types of switch leg. Switch leg is the wire that comes off from the bottom terminal of a switch and when the switch is turned on becomes hot. This is the leg that turns the load off and on. Black – Black wires signifies neutral wire in electric circuit. The neutral wires is connected to neutral bus bar inside an electric panel. A bus bar is and conductive metal bar that attracts the electric current for distribution purpose.) Black wire can be connected to black wire only and no other colour wire. Black wire being neural, it does carry charge/current. It mainly carries the unbalanced load i.e. the return current that we call. Return current is the electricity/current not being used and the return current to the electrical board/panel. Green – Green wire stands for grounding/ earthing in electric circuit. A green wire should be on can be connected to green wire only (no other wire). Grounding wires are usually not meant for lights and fan purposes. Green wires are chiefly used for socket purpose. Socket could be for AC, geyser, TV, microwave, etc. Normally, switches have only 2 wires i.e. neutral and phase. Vellore Institute of Technology, Chennai Campus, Chennai. 181 Earthing (Seminar 1 (4)) One of the safety methods. Earthing or grounding is a circuit which connects the metallic body of the electrical apparatus with the Earth's conductive surface. Purpose of Earthing is to avoid or minimize the danger of electrocution, fire due to earth leakage of current through undesired path Vellore Institute of Technology, Chennai Campus, Chennai. 182  Need of Earthing To protect human lives as well as provide safety to electrical devices and appliances from leakage current. To keep voltage as constant in the healthy phase (If fault occurs on any one phase). To Protect Electric system and buildings form lighting. To serve as a return conductor in electric traction system and communication. To avoid the risk of fire in electrical installation systems. Vellore Institute of Technology, Chennai Campus, Chennai. 183 Types of Earthing Strip or wire earthing Rod earthing Pipe earthing Plate earthing Vellore Institute of Technology, Chennai Campus, Chennai. 184  Strip or Wire Earthing A copper strip of minimum cross-section 25 mm × 1.6 mm is buried horizontally inside the ground at minimum depth 0.5 m and alternatively a galvanised iron strip of minimum of cross-section 25 mm × 4 mm can be buried horizontally at a same depth inside the ground. For this purpose around conductor can also be used and at that case the minimum cross-sectional area for copper conductor would be 3 mm² and for galvanised iron conductor it would be 6 mm². The buried portion of the electrode that is either script or round conductor should be long enough to provide required minimum resistance to the earth path. Generally the length of the conductor inside the ground is maintained more than 15 m. The buried conductor should be widely distributed as possible preferably in a single straight trench or in a circular trench or in a number of trenches radiating from a point. This type of earthing is mainly used in rocky area where excavation work is quite difficult. Vellore Institute of Technology, Chennai Campus, Chennai. 185 Contd… Vellore Institute of Technology, Chennai Campus, Chennai. 186  Rod Earthing A metallic rod of sufficient length is driven vertically into the ground normally by hammering on the top. Normally galvanised iron rod of 16mm diameter of minimum length 2.5 m are used for this purpose. The electrical installation which to be earthed, is connected to the top of the earth rod or pipe by means of copper or aluminium earth continuity conductor of sufficient cross-section. The rod earthing system is mainly used where soil has sandy characters and also it is often used for temporary earthing purpose. This is cheapest and easiest method of earthing as this method does not require any excavation work. Vellore Institute of Technology, Chennai Campus, Chennai. 187 Contd… Vellore Institute of Technology, Chennai Campus, Chennai. 188  Pipe Earthing Pipe earthing system is most commonly used and reliable system. In this method of earthing, a galvanised steel pipe of suitable length and diameter is buried vertically in the permanent wet soil under the ground. The length and diameter of the pipe are determined by the conditions of soil and the current to be carried. Normally minimum diameter and length of the pipe is maintained 40 mm and 2.5 m respectively for ordinary condition of soil and greater length is used for rocky and dry soil conditions. The depth under ground level at which the pipe is buried, depends upon the moisture condition of soil but it should not be less than 3.75 m under the ground. The earthing pipe is surrounded by alternative layers of charcoal and salt to keep moisture and thereby reduces the earth resistance. Another galvanised iron pipe of lesser diameter (19 mm) is fitted vertically on the top of the earthing pipe by means of reducing socket. The top of this pipe is projected in a cement concrete work on the ground. One or more GI plates are welded on this pipe by keeping the pipe openings clear to facilitate the connections of earth continuity conductors from different electrical installations. The cement concrete work is done to keep the water arrangement accessible and in dry season to keep the earth resistance minimum, 3 to 4 buckets of water are put in the concrete work or through the funnel if it is fitted to the top of the 19 mm diameter pipe. Vellore Institute of Technology, Chennai Campus, Chennai. 189 Contd… Vellore Institute of Technology, Chennai Campus, Chennai. 190  Plate Earthing A metallic plate of sufficient size is buried in wet soil vertically under the ground. If copper plate is used for this purpose the minimum dimensional of the plates should be 60 cm × 60 cm × 3 mm and if it is GI plate, then minimum dimensional should be 60 cm × 60 cm × 6 mm. In case of copper plate, a copper earth continuity conductor is connected to the plate with the help of copper nuts bolts and washers whereas in the case of GI plate, GI earth continuity conductor is connected to the plate with help of GI nut bolts and washers. This earthing plate along with connected earth continuity conductor, is buried vertically at minimum 3 m depth under the ground. The surroundings of the plate are filled with alternative layers of charcoal and salt of minimum 15 cm thickness of each layer. From the buried plate, the earth continuity conductor is passed through a GI pipe of 12 mm diameter. These GI pipe is used to protect the earth continuity conductor from direct contact of soil. Now another GI pipe of 19 mm diameter is driven vertically to the GI plate. Top of this 19 mm diameter pipe should be projected vertically on the ground level. A concrete chamber is made around the projected 19 mm diameter pipe and this chamber is covered by cast iron shutter. The 19 mm diameter pipe is used to keep the water arrangement accessible to the earthing plate. In this type of earthing, 1 to 2 buckets of water is poured on every 3 to 4 days through a funnel at the top of the 19 mm diameter pipe to facilitate the moisture content of the surroundings of the earthing plate. Vellore Institute of Technology, Chennai Campus, Chennai. 191 Contd… Vellore Institute of Technology, Chennai Campus, Chennai. 192  Notes What is earthing? Earthing means connecting any non-current carrying conductor part of an electrical system with general mass of earth in such a manner that there is an immediate discharge of electrical energy to the earth in the event of electrical potential developed at that part of the system. For example, metallic frame work of electrical appliances, metallic covering of electrical cables, the earth terminal of three pin socket outlets, stay wires and also neutral point of single phase and three phase supply systems must be properly earthed. Earthing is done to ensure that no current carrying part of the system rises to be potential beyond its normal value, no non-current carrying conducting part of a system rises to a potential beyond earth potential that is zero. Proper earthing also helps to avoid electrical shock to the human beings also to avoid the chance of fire hazard due to leakage current through unwanted path. Why earthing is required in an electrical installation? Properly designed, constructed and installed electrical equipments and appliances should not have any of the non- current carrying conducting parts which is in contact with any current carrying part. But accidentally may be due to failure of insulation between current carrying and non-current carrying conducting parts of the equipment/appliance, if any of the non-current carrying conducting parts comes in contact with any of the current carrying parts of the equipment/appliance, there will be a static electrical charge developed in the non-current carrying conducting part. Now if any human being touches that non-current carrying conducting part of the equipment or appliance, the accumulated static charge will get a path to the earth through his body and hence it is discharged immediately, as a result he gets an electrical shock. But if the non-current carrying conducting parts that are metallic frameworks parts of the equipment or appliance properly earthed, then at the occurrence of touching between any current carrying part or live part to the non-current carrying part of the equipment/appliance, the live part of the equipment/appliance gets low impedance path to the earth through the properly earthed metallic frameworks and hence there will be a huge current drawn from source, passing to the earth through this path. As a result the circuit breaker or MCB, or fuses associated with this equipment/appliance will immediately break to discontinue the supply to the equipment/appliance. Thus proper earthing of non-current carrying metallic parts of electrical equipments and appliances provides safety of operation. Vellore Institute of Technology, Chennai Campus, Chennai. 193  Contd… What should be the distance of earth from a building? An article electrode should not be situated within a distance of 1.5 m from the building whose installation system is being earthed. What should be the size of earth continuity conductor? The conductor by which a metallic framework of an electrical equipment/appliance is connected to the earth is referred as earth continuity conductor. The cross-section of earth continuity conductor should not be either less than 2.9 mm2 or half of the installation conductor size. What should be the earth resistance of an electrical installation? Earth resistance is defined as the resistance between actual earth and the earthed body of the installation. This is nothing but the resistance of the path connecting the body of the installation to the actual earth. This resistance should be low enough to carry sufficient current to the earth to ensure proper operation of protective relays or blowing of fuses associated to the installation. The earth path consists not only the earth continuity conductor but also it includes soil in between the end of the earth continuity conductor inside the ground and actual earth. As the resistivity of soil depends upon its moisture content, it varies time to time throughout the year. As a result earth resistance of an installation is not constant throughout the year, it varies with weather conditions. Earth resistance of an installation is minimum in rainy season whereas it is maximum in dry season. Although this resistance varies time to time, but there are some standards of maximum allowable earth resistance. 1. Maximum allowable earth resistance of a large power station is 0.5 ohm 2. Maximum allowable earth resistance of major power station is 1.0 ohm 3. Maximum allowable earth resistance of small substations is 2.0 ohm 4. Maximum allowable earth resistance for all other cases is 5 ohm. This should be noted that, the resistance between any point on the earthed body and the earth pit should be less than 1 ohm. Vellore Institute of Technology, Chennai Campus, Chennai. 194  Residential Wiring Circuits (Seminar 1 (3)) One lamp controlled by one switch Vellore Institute of Technology, Chennai Campus, Chennai. 195 Contd… Two lamps controlled by two switches Vellore Institute of Technology, Chennai Campus, Chennai. 196 Contd… Staircase wiring Vellore Institute of Technology, Chennai Campus, Chennai. 197 Contd… Godown wiring Vellore Institute of Technology, Chennai Campus, Chennai. 198 Contd… Tunnel wiring Vellore Institute of Technology, Chennai Campus, Chennai. 199 Contd… Hospital wiring Vellore Institute of Technology, Chennai Campus, Chennai. 200 Types of wiring (Seminar 1 (3)) Cleat wiring CTS wiring or TRS wiring or batten wiring Metal sheathed wiring or lead sheathed wiring Casing and capping wiring Conduit wiring Vellore Institute of Technology, Chennai Campus, Chennai. 201  Cleat wiring In this type of wiring, insulated conductors (usually VIR, Vulcanized Indian Rubber) are supported on porcelain or wooden cleats. The cleats have two halves one base and the other cap. The cables are placed in the grooves provided in the base and then the cap is placed. Both are fixed securely on the walls by 40mm long screws. The cleats are easy to erect and are fixed 4.5 – 15 cms apart. This wiring is suitable for temporary installations where cost is the main criteria but not the appearance. Advantages: Easy installation Materials can be retrieved for reuse Flexibility provided for inspection, modifications and expansion. Relatively economical Skilled manpower not required. Disadvantages: Appearance is not good Open system of wiring requiring regular cleaning Higher risk of mechanical injury. Vellore Institute of Technology, Chennai Campus, Chennai. 202 Contd… Vellore Institute of Technology, Chennai Campus, Chennai. 203  Cable Tyre Sheathed (CTS) wiring or Tough Rubber Sheathed (TRS) wiring or batten wiring In this wiring system, wires sheathed in tough rubber are used which are quite flexible. They are clipped on wooden battens with brass clips (link or joint) and fixed on to the walls or ceilings by flat head screws. These cables are moisture and chemical proof. They are suitable for damp climate but not suitable for outdoor use in sunlight. TRS wiring is suitable for lighting in low voltage installations Advantages: Easy installation and is durable Lower risk of short circuit. Cheaper than casing and capping system of wiring Gives a good appearance if properly erected. Disadvantages: Danger of mechanical injury. Danger of fire hazard. Should not be exposed to direct sunlight. Skilled workmen are required. Vellore Institute of Technology, Chennai Campus, Chennai. 204 Contd… Vellore Institute of Technology, Chennai Campus, Chennai. 205  Metal sheathed wiring or lead sheathed wiring The wiring is similar to that of CTS but the conductors (two or three) are individually insulated and covered with a common outer lead-aluminum alloy sheath. The sheath protects the cable against dampness, atmospheric extremities and mechanical damages. The sheath is earthed at every junction to provide a path to ground for the leakage current. They are fixed by means of metal clips on wooden battens. The wiring system is very expensive. It is suitable for low voltage installations. Precautions to be taken during installation The clips used to fix the cables on battens should not react with the sheath. Lead sheath should be properly earthed to prevent shocks due to leakage currents. Cables should not be run in damp places and in areas where chemicals (may react with the lead) are used. Advantages: Easy installation and is aesthetic in appearance Highly durable Suitable in adverse climatic conditions provided the joints are not exposed Disadvantages: Requires skilled labor Very expensive Unsuitable for chemical industries Vellore Institute of Technology, Chennai Campus, Chennai. 206 Contd… Vellore Institute of Technology, Chennai Campus, Chennai. 207  Casing and capping wiring It consists of insulated conductors laid inside rectangular, teakwood or PVC boxes having grooves inside it. A rectangular strip of wood called capping having same width as that of casing is fixed over it. Both the casing and the capping are screwed together at every 15 cms. Casing is attached to the wall. Two or more wires of same polarity are drawn through different grooves. The system is suitable for indoor and domestic installations. Advantages: Cheaper than lead sheathed and conduit wiring. Provides good isolation as the conductors are placed apart reducing the risk of short circuit. Easily accessible for inspection and repairs. Since the wires are not exposed to atmosphere, insulation is less affected by dust, dirt and climatic variations. Disadvantages: Highly inflammable. Usage of unseasoned wood gets damaged by termites. Skilled workmanship required. Vellore Institute of Technology, Chennai Campus, Chennai. 208 Contd… Vellore Institute of Technology, Chennai Campus, Chennai. 209  Conduit wiring In this system PVC (polyvinyl chloride) or VIR cables are run through metallic or PVC pipes providing good protection against mechanical injury and fire due to short circuit. They are either embedded inside the walls or supported over the walls, and are known as concealed wiring or surface conduit wiring (open conduit) respectively. The conduits are buried inside the walls on wooden gutties and the wires are drawn through them with fish (steel) wires. The system is best suited for public buildings, industries and workshops. Advantages: No risk of fire and good protection against mechanical injury. The lead and return wires can be carried in the same tube. Earthing and continuity is assured. Water proof and trouble shooting is easy. Shock- proof with proper earthing and bonding Durable and maintenance free Aesthetic in appearance Disadvantages: Very expensive system of wiring. Requires good skilled workmanship. Erection is quiet complicated and is time consuming. Risk of short circuit under wet conditions (due to condensation of water in tubes). Vellore Institute of Technology, Chennai Campus, Chennai. 210 Contd… Vellore Institute of Technology, Chennai Campus, Chennai. 211