Electrical Installations (PDF)

UNIT 9 ELECTRICAL INSTALLATIONS AND CONTROL PANELS Structure 9.1 Introduction Ob~ectives 9.2 Principal Blocks and Components of Electrical Installations 9.2.1 Principal Blocks 9.2.1 Principal Components 9.2...

UNIT 9 ELECTRICAL INSTALLATIONS AND CONTROL PANELS Structure 9.1 Introduction Ob~ectives 9.2 Principal Blocks and Components of Electrical Installations 9.2.1 Principal Blocks 9.2.1 Principal Components 9.2.3 Graphic Symbols used in Electrical Diagrams 9.3 Safety Precautions 9.3.1 Protection of Equipment 9.3.2 Safety of Individual lJsers 9.3.3 Measurement of Earth Resistance 9.4 Residential Wiring 9.5 Starting and Braking of DC Motors 9.5.1 Starters for DC Shunt Motors 9.5.2 Dynamic Braking and Plugging 9.6 Starters for Cage Induction Motors 9.6.1 1:ull- voltage Starring (Direct-on-line Starling) 9.6.2 Auto-transformer Starting 9.6.3 Star-Delta Staning 9.7 Summary 9.8 Answers to SAQs 9.1 INTRODUCTION Electrical installations, irrespective of whether they are big or small, and whetherlhey are meant for residential, commercial or industrial establishments, must meet certain basic requirements. l l e s e concern 1. Safety of equipment and individual users, - 2. Functional efficiency, relating to optimum performance of equipment, operating convenience and ease of maintenance, 3. Life expectancy, ensuring that the installation lasts at least a certain minimum number of years, and 4. Economy, such that the cost of the installation is minimal, while being in conformity with the above requirements and National rules and regulations. In this unit we will begin by considering the principal functional blocks which make up a distribution system meant for a residential unit, a cominercial establishment or an individual plant. This will be followed by a description of the principal components that are used in such installations. Next we consider the protective measures employed to prevent damage to equipment and danger to individuals. Finally we consider, at an elementary level, features of electrical installations for residences and for control of electrical machines. Objectives After studying this unit, you should be able to define functional blocks of electrical installations, describe components used in such installations, explain methods for protecting equipment from damage, describe means of protection from electric shock, describe features of residential wiring, describe starter panels for dc motors, describe star-delta and auto-transformer starters for induction motors, and identity braking and reversal schemes for motors. Electlical Machioes & Measuring Instruments 9.2 PRINCIPAL BLOCKS AND COMPONENTS OF ELECTRICAL INSTALLATIONS 9.2.1 Principal Blocks The block diagrams of Figure 9.1 are greatly simplified functional diagrams of the type of :' distribution systems used for (a) a small unit such as a residence and (b) larger units such as commercial establishments or industries. OUTLETS EQUIPMENT BRANCH CIRCUITS EQUIPMENTI BOARD (a) SMALL UNIT SUCH AS A RESIDENCE BRANCH CIRCUITS EQUIPMENT (b) LARGER UNIT FOR AN INDUSTRY OR COMMERCIAL ESTABLISHMENT Figure 9.1 : Principal blocks of distribution systems Supply Lines are the conductors which extend from the street mains or from transformers to the building containing the electrical installation. Supply Equipment are located at the point of entrance of the supply lines to a building or. other structure, or an area defined in some specific manner to cover the extent of an electrical installation. Supply equipment include a main switch and fuses, or a circuit breaker, together with associated accessories. Metering Equipment include energymeters and recorders for measuring the electrical energy consumed, together with wattmeters and ammeters as required. The Switch Board is a large single panel or assemblage of panels on which are mounted switches, overcurrent and other protective devices, busbars and also instruments as needed. Switchboards are normally accessible from both the front and the rear and are generally not intended to be installed in cabinets. The Panel Board consists of a single panel or a group of panels designed for assembly into a single unit. These include protective equipment such as automatic overcurrent relays together with switches for the control of electrical energy to different branch circuits. The equipment used in the panel is usually designed to be placed in a cabinet against a wall or partition and is normally accessible only from the front. Feeders designate all circuit conductors between different blocks of an electrical installation. Branch Circuits are the circuit conductors between the outlets and the final overcurrent relay protecting the circuit. An outlet is a location on the wiring circuit from which supply is tapped to supply utilisation equipment. Utilisation Equipment (UE) utilise the electrical supply for mechanical, chemical, lighting, heating or other services.  9.2.2 Principal Components Electrical Installations and Control Panels The basic components used in electrical installations are : j Isolating Switches : These are used to isolate or disconnect an item of equipment from the power source. In a three phase system. these consist of three knife-switches and lhrec fuses enclosed in a metallic box (iron clad). The knife-swilchcs open and close simultaneously by means of an external handle. An interlocking nlechanism prevents the cover ftom opening when the switch is in Lhe closed position. Such iron clad switches are designed to carry the rated full-load current indet31lilelyand to withstand a much higher short-circuit current for brief intervals. I Manual Circuit-Breakers : These can be closed and opened manually, but trip (open automatically) when the current exceeds a pre-delermined limit. Aftel tripping, circuit-breakers can be reset inanually. As such circuit breakers require no fuses, they are often used in the place of isolating switches. I i Sequential Switches/Cam Switches : These have a group of fixed contacts and an equal number of moveable contacts. On rotating a handle or knob, the contacts are made to open or close in a predetermined sequence. Push Buttons : These are switches activated by finger pressure. Two or more contacts open or close r when the button is depressed. These are usually spring loaded so as to return to their normal positions (either normally closed or normally open) when the pressure is removed. Control Relays : These are electromagnetic switches which open or close a set of contacts when a relay coil is made to carry current (energised). The coil produces a magnetic field which attracts a moveable part (armature) carrying the contacts. Such relays are generally used in low power circuits. Time delay relays belong to this category, but are designed to operate only after a definite time interval. Magnetic Contactors : These are similar to control relays but are designed to ope11 and close high power circuits of upto several hundred kilowatts. These possess relay coils which on being energised activate sets of contacts designed to handle the requisite high current andlor voltage. r Thermal Overload Relays : These are temperature sensitive devices whose contacts open or close when the current exceeds a preset limit. The current flows through a small heating element which raises the temperature and causes unequal expansion of a bimetallic strip resulting in the opening or closing of conlacls. Thermal relays introduce a time delay as the temperature cannot follow sudden changes in current. Limit SwitchesISpecial Switches : Thcse switches are specifically designed to operate when the pressure, temperature, liquid level, or position of a mechanical part etc. reaches a limiting value. Pilot Lamps : These indicate the ON or O F F state of a component in an electrical installation. Other Components : These include passive devices like resistors, inductors, capacitors and transformers and dynamic devices like various motors and generators. Electrical Machines & 9.2.3 Graphic Symbols used in Electrical Diagrams Memuring Imtruments The symbols used in electrical diagrams to represent various electrical and elecQonic components have been standardised over the years. For a complete listing of such symbols, reference may be made to IS 2032. In Table 9.1 the graphic symbols used to represent some frequently occurring components are givcn. Somc of lhese symbols ;Ire the ones found in lhc IS and some are those used by various authors in booh. Table 9.1 : Some Common Graphic Synibols Explanation : 1. Terminal 2. Crossing conductors, electrically isolated 3. Conductors electrically connected together 4. Group of 3 conductors 5. Earth (ground) connection 6. Lightning arrester 7. Disconnect switch 8. Separable connector 9. Normally open (NO) contact 10. Normally closed (NC) contact 11. Push button, NO 12. push button, NC 13. Circuit breaker 14. Relay coil 15. Fuse 16. Thermal overload element 17. 3-Pole switcl~ with fuses 18.3-Pole circuit breaker with overload relays and separable connectors 19. General symbol for motor 20. General symbd for generator 9.3 SAFETY PRECAUTIONS In order to ensure functional efficiency, satisfactory performance and adequate life expectancy of electrical cquipment, it is necessary to protect equipment froin damage due to overcurrent, overvoltage and environmental hazards. Also, adequate precautions should be taken to ensure that individual users having access to equipment are not subjected to unnecessary risk of injury in general and electric shock in particular. 9.3.1 Protection of Equipment Excluding accidental damage, the life expectancy of electrical apparatus is limited by the temperature of the insulation as high temperatures result in short life spans. This is because higher temperatures lead to more rapid deterioration of the electrical and mechanical  properties of the insulating materials used with consequent reduction in the useful life of the ElectricalIns~lation~ equipment. Tests made on insulating materials have shown that the life span diminishes and Control Panels approximately by half every time the temperature increases by 10°C. This means that a device having a normal life expectancy of ten years at a temperature of 105OC will have a life expectancy of only 5 years at a temperature of 115OC and only 2+ years at a temperature of 125°C. International and National organisations have grouped insulating materials into various classes, with specitied maximum temperature levels, depending on their ability to wiulstand heat. Since ohrnic, magnetic and mechanical power losses in equipment are all sources of heat, the design of the insulating and ventilating systems of the equipment are crucial in deciding the normal operating conditions (full-load Current and power) for a particular operating voltage and speed of the equipment. The main purpose of an insulator is to prevent significantcurrent flow when subjected to a difference of voltage. An ideal insulator would leak zero current through itself whatever the voltage difference. However, no insulator is perfect, and when we apply a moderate voltage LO an insulator a very small current leaks through it. This leakage current is negligible as long as the voltage is not too high. However, if the voltage is increased above a certain critical value, an insulator suddenly loses its insulating properties and breaks down. The breakdown voltage required to produce this catastrophic failure depends upon the material of the insulator and its thickness. The ratio of breakdown voltage to insulator thickness is called dielectric strength. In high voltage equipment it is the dielectric strength that determines the thickness of the insulation used. In low voltage apparatus, however, mechanical and thermal considerations decide the thickness. The safe operating voltages are so selected that the dielectric strength of insulation is much larger than the maximum voltage gradient under rated voltage conditions. Overvoltage Protection F Dangerous overvoltages can occur in electrical installations because of lightning discharges ,and electrical transients (switching surges) caused by faults and switching operations. Lightning currents of several tens of thousands of amperes are quite common, and they last only a few microseconds. However, in flowing from parts of the electric installation to earth, because of the impedance of the path, dangerously high voltages can be generated. "Lightriing arresters" or "Surge diverters" are used to protect equipment from such transient overvoltages. A lightning arresler is connected in parallel across the equipment it is to protect, and is designed to breakdown and provide an alternative path lo earth for surge currents at voltages higher than the normal working voltage, thereby protecting lhe equipment. Earthing or grounding of electrical installations is an important means of providing protection. The term earth or ground in an electrical installation loosely refers to the mass of the earth beginning with the moist regions below its surface. For all practical purposes t h ~ general s mass of the earth may be regarded as being at a uniform potential - the earth or ground potential - which is often taken to be the zero reference potential. For safety reasons, r electrical installations are connected to earth at specific points such as the neutral of a star connected 3-phase system and the frames of electrical equipment. Special care is taken to ensure that the impedance to earth offered by such connections is as small as possible. In the case of lightning strokes, low resistance will ensure that the voltage of the equipment with respect to earth does not become too high. Also, in the event of a fault (i.e. failure of insulation) to the ground, low earth resistance ensures a high fault current and thus protective devices which respond to such fault currents will quickly isolate the equipment from the supply reducing damage to the equipment. The resistivity of the earth is quite high and ranges between 5 to 5000 ohm-meters depending on the composition (clay, sand etc) and the moisture content. (By comparison, at 20°C copper has a resistivity of only 17.2 X ohm-meters and iron about ohm-meters). In spite of its high resistivity, the earth is an excellent conductor. This is due to the enormous cross-section it offers to current flow and is illustrated in Figure 9.2. The figure depicts two iron rods driven into the earth some distance apart with a voltage V applied across them. The resulting current I which enters rod A, spreads out and flows through the volume of the earth before converginx on to rod B and returning to the voltage source. The resistance offered by the earth, even if the rods are spaced apart by several metres, is small. Most of the resistance restricting the current I is concentrated in the soil in a small region around each rod and therefore a change in the distance between the rods does not change the resistance between them unless the rods are brought very close together. The resistance offered by the small region at each rod constitutes the earthing resistance of the meetlical Machines & rod. This earthing resistance of the rods can be reduced by driving the rods deeper into the Measuring Instrmnenq ground and by soaking the soil near the electrodes with chemicals such as copper sulphate. -.. EARTH CURRENTS Figure 9.2 : Illustrating earth resistance between two rods driven into the earth When a point in an installation is to be earthed, the point in question may be connected to an earth electrode. Such an electrode is often produced by driving a galvanized iron pipe into a pit dug into the earth containing alternate layers of charcoal and salthand which is kept moist. Water mains, coilsisting as they do of long lengths of metallic piping buried underground are essentially at earth potential. The same is true of the massive steel reinforcements used in many buildings. Earthing can be achieved by connection to earth electrodes fixed to such structures. However, since earth leakage currents can lead to electrochemical action and corrosion, such practices are not encouraged. SAQ 1 A steel structure in an electrical installation is earthed and the resistance to earth offered by Lhe earthing scheme is estimated to be In. (a) Calculate the rise in potential of the steel structure if it is hit by a 50 kA lightning stroke. (b) If the potential rise is to be restricted to 20 kV for lightning strokes of less than 50 kA, what is the maximum permissible value of h e earth resistance ? Over-Current Protection When items of equipment such as transformers, motors and generators are overloaded, these draw currents which exceed the rated (i.e. designed full-load) values of current. Since ohmic losses ( z ~ Rincrease ) as the square of the current the increased heat produced because of increased current, causes overheating of the electrical windings leading to reduced life expectancy. All equipment, therefore, requires to be protected against overcurrent. Fuses and overload trips in starters and circuit breakers are the means used to ensure such protection. Overload relays and fuses disconnect electrical circuits and equipment from the main supply whenever the current exceeds a preset safe value. Maintenance procedures can then come into operation to rectify the situation and ensure that such overload conditions do not persist when the equipment is next energised. Fault conditions can lead to severe overcurrents. A fault condition exists whenever there is insulation failure and the normally high resistance between supply lines, between lines and ground and between conductors and the frame of electrical equipment offered by insulation falls to a small value. Such fault currents generally add on to the load carrent, and if preset Electrical Installatiom values of tripping mechanisms or fuses are exceeded, overcurrent protection will isolate the and Control Panels equipment from the supply so that Ule equipment can be replaced or rectified. As already mentioned, low earthing resistance, by ensuring higher values of fault current, helps in the rapid isolation of faulty equipment. 9.3.2 Safety of Individual Users From an electrical point of view, protection of an individual user is equivalent to protecting him from electric shock. Electric shock is actually caused by the current that flows through the human body. The current depends on the resistance offered by the skin contact resistance, thc resistance of the bulk of the body