475 COMPONENTS OF LT SWITCHGEAR 175.1. Fuses. Fuse is perhaps the s 1 er short circuit, or excessive oven a Sst cheapssn device used for interrupting an electrical circuit seat protection in high voltage (ap to 68 reat magninues. As such its ued for overload andor short- civage circuits their Use is confined to those applications ators tet ped ane aly sabe for current interruption, pplications where their performance characteristics are ¢ action of a fuse is based u i dea het enctane Tewing aoe sinihenaerenlah oan Innormal operating conditions, carrying this current is readily dissipated into the ia ceeds Sana surrounding air, and therefore, fuse element remains ata temperature below its melting point. However, ‘nbn some fault, such as short circuit occurs or when Joad connected in a circuit exceeds its capacity, the current exceeds the limiting value, the heat generated due to this excessive current cannot be dissipated fast. enough and the fusible element gets heated, melts and breaks the circuit. It thus protects a machine or apparatus or an installation from damage due to excessive current. The time for blowing out of fuse depends upon the magnitude of the excessive current. Larger the current, the more rapidly the fuse will blow ie., the fuse has inverse time-current characteristic, as shown in Fig, 17.19. Such a characteristic is desirable for Fig. 1749, TimeCurrem Chavacensis protective gear. ‘200AFUSE Ba § g CURRENT IN AMPERES —> B38 a8 oot 002 38 SSE rime in seconds ——> 0.08 Essentially, a fuse consists of a fusible elemen® 0 the form of a metal conductor of specially selected mall eross-sectional area, a case or cartridge to hold the fusible element, and in some cases, provided with a wreans to aid arc extinction. The part which actually melts and opens the circuit is known as the fuse element. It forms a series part of the circuit to be protected against short circuit or excessive overloads. Fuses have following advantages and disadvantages. Advantages (i It is the cheapest form of protection (ii) It needs no maintenance. ; ot (iii) Its operation is inherently completely automatic unlike a circuit breaker WI pment for automatic action. @) i ae enormous short circuit currents without noise, flame, es ocak sate (0) The minimum time of operation can be made, such smaller than that with Se iret breaker, (vi) The smaller sizes of fuse element impose a current limiting effect under short circuit conditions. (vii) Its inverse time-current characteristic enables its use for overload protection. available. hich requires an elaborate i seine or replacing a fuse after operation. ne is lost in rewiring or repl after ; (i On heavy short circuits, discrimination petween fuses in series cannot be obtained unless there is ° considerable ilfferences in the relative sizes of the fuses concerned. ; (iii) The current-time characteristic of @ fuse cannot always be correlated with that of the protectes i) The current s device. Disadvantages @ Considerable ti Scanned with CamScannerThe function of fuse wire is (i) to carry the normal working current safely without heating and (ity bray the circuit at reine: When the current exceeds the limiting cu ; / il Hcit urrents by fuses or circ Distribution circuits are protected from ground and short-circuit ct y Cult breakers arranged as to disconnect the faulted equipment promptly from its source of sup. 5 ‘USES Are used singy exclusively for the protection of cables in low-voltage light and power circuits oot ved aera Of rating not exceeding 200 kVA, in primary distribution systems. Circuit breakers are employed for larger amoun power and in cases where the operation of the overload device is so frequent as to make the use of fy... impractical. ; . ‘ Necessity of Fuse in an Electric Cireuit, If no fuse or other similar device is provided in the circuit the, a dangerous situation would be created on developing of faults such as overload, short circuit or earth faut, Incase of overload, short circuit and heavy earth faults a heavy current will continue to flow through the consuming apparatus, current carrying cables or wires and other current carrying equipment. Due to continuoys flow of heavy current through the cables or wires, apparatus etc., these wil! get heated up and so get damaged, The fire may also break out. oo. In case of carth leakage fault, (i.e. on the body of the electrical apparatus becoming alive), the body ofthe electrical apparatus will continue to be alive and at much higher potential above that of the earth. In such circumstances any person coming in contact with the metal body of the apparatus is liable to get an electric shock, even if it is earthed. The main function of a fuse is to blow out under a fault and isolate the faulty section from the live side, If the fuse is provided on neutral wire, in place of live wire, then in abnormal conditions though the fuse will blow out but the lamp or other apparatus still remains connected to the live wire and in case of leakage some trouble will arise and cause a considerable damage. In case the earth fault takes on the neutral wire between lamp and fuse provided in it, the fuse will blow out because the neutral wire is slightly at a higher potential with respect to earth and so the fault current flows through the neutral wire and fuse melts itself. The current will flow through the live wire, lamp, neutral wire and earth fault, even after the fuse has blown out and this may cause serious damage to the wiring, the apparatus connected or building itself, If fuses of same capacity are provided on the phase wire and neutral, then in case of short-circuit fault, one of them will blow out first. Ifthe fuse on neutral wire blows out first, the fuse in phase line remains intact and faulty apparatus still remains connected to the live. If some person comes in contact with the faulty apparatus, he is liable to get electric shock. In case the installation is connected to 3-phase 4-wire supply system, and fuses are provided on both live and neutral wire and fuse on neutral wire blows out then voltage of each phase to neutral will become considerably different, which is not desirable. Hence the fuse is provided only in phase or live pole, never on neutral pole, 17.5.2, Fuse Units. A fuse unit essentially consists of the metal fuse element or link, a set of contacts between which it is fixed and a body to support and isolate them. Many types of fuses also have some means for extinguishing the are which appears when the fuse element melts, The various types of fuse units, most commonly available are : 1, Round type fuse unit. 2. Kit-kat type fuse unit. 3. Cartridge type fuse unit 4, HRC (High rupturing capacity) fuse units and 5. Semiconductor fuse units Uk Round Type Fuse Unit. This type of fuse unit consists of a porcelain or bakelite box and two separated wire terminals for holding the fuse wire between them. This type of fuse is not in common use on accout its following disadvantages : 1, One of the terminals remain always energised and, therefore, ; ker Ps nak for repl lise either the wor will have to touch the live mains or open the main swit placement of flise Scanned with CamScanner2, Appreciable arcing takes place at-the instant of seer three arcing the five unk basa aera ff fuse and thus damage the terminals. After 2, Rewirable or Kit-Kat Type Fuses, anit . fuse in ‘house wiring’ and small isrent rot sp iriesliietiehos Misrewirable fuse (al80 sometimes known as kitkat one wey consists of a porcelain base carrying the fixed contacts ue ihe ihe incoming and outgoing live or phase wires are conSETES bn a porcelain fuse carrier holding the fuse element, consisting of one or more strands of fuse wire, stretched between its terminals, The fuse carrier is a separate part and can be taken out or inserted in the base without risk, even without opening the main switch, If fuse holder or carrier gets damaged during use, it may be replaced without replacing the complete unit The fuse wire may be of lead, tinned copper, aluminium or an alloy of tin-lead. The actual fusing current will be about twice the rated current, When two or more fuse wires are used, the wires should Fi# 1720 Rewirable or Kit-Kar Type be kept apart and a derating factor of 0.7 to 0.8 should be used to Li arrive at the total fuse rating, The specifications for zewirable fuses are covered by IS : 2086-1963. Standard ratings are 6, 16, 32, 63 and 100 A. A fuse wire of any rating not exceeding the rating of the fuse may be used in iti.e. a 80 A fuse wire can be used in a 100 A fuse, but not in the 63 A fuse. On occurrence of fault, the fuse element blows off and the circuit is interrupted. The fuse carrier is pulled out, the blown out fuse clement is replaced by new one and the supply is restored by re-inserting the fuse cartier in the base. ‘Though such fuses have the advantages of easy removal or replacement without any danger of coming into contact with a live part and negligible replacement cost but they suffer from the following disadvantages: FUSE CASE FUSE CARRIER (@) Unreliable Operation. The operation of the rewirable fuses is unreliable because of the following factors: () There is a possibility of renewal by the fuse wire of wrong size. Gi) ‘The fuse wire deteriorates, over a period, due to oxidation through the continuous heating up of the element. After a relatively short period the current causes the metal to deteriorate and the fuse operates at a lower current than originally rated. (iii) A fuse with normal rated eapacity of say 60 A would fuse at nearly 120 amperes. Should that amount of current damage the apparatus in the circuit, obviously the size of the fuse wire must be smaller but, as its rated capacity will also be lower, the same dependence could not be placed upon it. alas (iv) The protective capacity is uncertain ie, no dependence can be placed upon the wire to interrupt the circuit when a given current flows. For instance although theoretically a No.17 sue ee ircuil is flowing, the circuit may be interrupted when i vritawhen acurrent of 135 Ais flowing, the cireuit may Sen a teva ‘en only a much higher current flows. i ircumstances wh a lower current flows or in other circums! s t () Accurate calibration of the fuse wire is impossible, as a longer fuse operates earlier than one of shorter length. : ; Single phasing of three-phase induction motors is are employed in motor circuits. : at (6) Lack of Discrimination. Due to unreliable operation, discri However, it will be possible to achieve some measure of discri twice the rating of the next fuse ahead of it in series. For example, @ ha fuse of 55 Ai with a fuse of 40 A, but not wil common occurrence where rewirable type fuses mination cannot be always ensured. mination by using a fuse which has fuse of 80 A will discriminate Scanned with CamScanner0 SHUMeBring ©) Smatt Time Lag. Due to the small time lag, such fuses can blow with large transient CUED which = ‘encountered during the starting of motors and swicig Ca neers » Capacitors, worescent lights etc., unless fuses of sufficiently high ratin a it @ Low Rupturing Capacity. Rewirable uses have limited breaking or ripturing capacity, For example, Zccording to IS : 2086-1963 the rewirable fuse of 16 A normal current have a breaking current of 2 KA and those up to 200 A normal current have a breaking current of 4k A. (©) No Current-Limiting Feature. D Slow Speed of Operation, No special means ate employed to extinguish the arc that blows after the fuse melts. Thus arcing time is more in such fuses. . eal 3. Cartridge Type Fuse. This is a totally enclosed type fuse unit, It essentially consists aa insulating container of bulb or tube shape and sealed at its ends with metallic cap known as cartridge enclosing the fuse ¢lement and filled up with powder or geanular material known as filler. There are various types of materials used as filler like sand, calcium carbonate, quartz etc. There is Sometimes a blow out device in the side of the tube to indicate When the fuse is blown. On overloads or short circuits, the fusible clement is heated to a high temperature causing it to vaporize. The powder in the fuse cartridge cools and condenses the vapour . and quenches the arc thereby interrupting the flow of current. Fig, 17.21 Cartridge Fuse Since itis totally enclosed it will not be possible to rewire and, therefore, the whole unit will have to be replaced, once it blows out, It provides complete security against fire risk, as itis a totally enclosed unit. The filling powder provides good insulating path and helps to extinguish the arc at the time of blowing up of fuse This type of fuse is available up to 660 V and the current rating up to 800 A. 4. High Rupturing Capacity (HRC) Fuses. With a very heavy generating capacities of the modern power Stations, extremely heavy currents would flow into the fault and the fuse clearing the fault would be required ‘o withstand extremely high stresses in this process. A rewirable fuse may not be useful in this case and, therefore, high rupturing capacity fases commonly known as HRC fuses, designed and developed after intensive research for use in medium and high voltage installations, are used for such duties, Their rupturing capacity is as high as 500 MVA up to 66 kV and above. The main advantages of HRC fuses are: © They are cheaper as compared with other types of circuit interrupters of same breaking capacity, (i) No maintenance is required. ii) The operation is quick and sure, (iv) They do not deteriorate with time. () They have inverse time-current characteristic. (vi) They are capable of clearing high as well as low currents, (ii) They are quite reliable and can be selected for proper discrimination, HRC fuses suffer from the following disadvantages : © They are required to be replaced after each operation, Gi) Interlocking is not possible. it) They lack relays in complete discrimination, h the disadvantages to a very large. ; (a) Cartridge Type HRC Fuse. The high rupturing capacity cade fin ‘ie sialac i an Bei form an HRC cartidge fuse consists ofa heat esitng eeranie eae nittalonl-bgs to whicae welded fusible silver (or bimetallic) curentarrying elements "The cthiplese sack ee ee fs the elements is filled with a powder which acts as an are extinguishing agai Shetibs ‘he body suougaicg ‘The process of fusing comprises the following operations ; 1. Pre-arcing operation i.e. melting of silver elements. 2. Arcing operation ie. vaporisation of the elements, Scanned with CamScanner3, Fusion of silver vapours and the filling Brass GIMETAL THERMAL Fume, powder, and END CAP CONTROL ‘contact 4, Extinction of arc under fusion process. — On the occurrence of a fault, short-circuit current flows through the fuse element and element is thus heated up to its melting point When the melting process is completed, an arc js formed. The chemical reaction between silver vapour and filling powder tends to establish FILLING POWDER CARTRIDGE» FUSE OUTER. high resistance. The high resistance acts as an ELEMENT CLEMENT insulator because the fault current decreases Fig. 17.22 HRC Cartridge Fuse along with the high pressure created within the fuse by the fault (excessive) current. Thereafter a transient voltage is created at the instant of fault current interruption on account of sudden release of energy. The physical phenomena associated with the process of fusing include a sudden rise of temperature and the generation of a high internal pressure. (6) Tetra Chloride Type HRC Fuse. Itessentially consists of 2 glass tube filled with carbon tetrachloride solution and sealed at both ends with brass caps. Inside the tube a high resistance fuse wire is sealed at one end of the glass tube and the other end of the fuse wire is held by a strong phosphor bronze spiral spring fixed to the other end of the glass tube. A flexible copper wire is also similarly held between the spring and the other end of cap in parallel with the high resistance wire. On short circuit or overload, the high resistance fuse wire ‘melts and spring pulls back the flexible copper wire. The arc produced is extinguished by carbon tetrachloride vapour. 5, Semiconductor Fuses. These are very fast acting fuses for protection of thyristor and other electronic circuits. 17.5.3. Switch Fuse Units. As per Indian Electricity Rule 50 a suitable linked switch (a switch operating simultaneously on phase or line and neutral wires) is to be provided immediately after the meter board. This rule also stipulates that a suitable cutout must be provided just after the linked switch to protect the circuit against excessive current. The linked main switch and fuse unit may be provided as one unit or as separate units. eo = = FRAME. ye ruses It lh Illi —-pisconecrwounk Ss OPERATING HANDLE cover 8 AUBEER RINGS. Zz (@) DPIC Switch (b) TPIC Switch Scanned with CamScannerAst mou ert a cast xe Cov CONTACTS , Wd WITH AReNaoTS ones ruses -d\leal (la Paysites wow 2 CONTACTS kre ovine 2 ace se corners MOWING cSNaor operas - OpERATIN MECHAM Jf} stron rato coven sumo coven © @ Fig.17.23 Switches and Switch Fuses Switch fuse is a combined unit and is knowa as an iron clad switch, being made of iron. It may be double Pole for controlling single phase two-wire circuits or triple pole for controlling three-phase, 3-wire circuits or triple pole with neutral link for controlling 3-phase, 4-wire circuits, The respective switches are known as double pole iron clad (DPIC); eriple pole iron clad (TPIC) and sriple pole with neutral link iron clad (TPNIC) switches. These switches are shown in Figs. 17.23 and 17.24. Since no fuse is to be provided in neutral (IE Rule 32), in DPIC switch fuses, where provision is made for fuses in both the wires, one fuse carrier is furnished with fuse element and the other with a thick copper wire The specifications of IC switch fuse units are given below as samples : 1. For Two-Wire DC Circuits or Single Phase AC Circuits. 240 V, 16 A, DPIC switch fuse of any make approved by IS. 2. For Three-Wire DC Cireuits, 500 V, 32 A (63/100/150 or higher amperes), IS approved TPIC switch fuse. 3. For Three-Phase Balanced Load Circuits, 415 V, 32 A (63/100/150 or. higher amperes), IS approved TPIC switch fuse. i Nil (6) Triple Pole, Iron Clad Switch (a) Double Pole Iron Clad Switch Scanned with CamScanner(c) 16-Amp. All insulated (A) Al insulated Switch (¢) Three-Phase Iron Clad Main Main Switeh With Moulded Cover Switch For Power Use and Base For Cireuits Up to 16 Amperes Capacity Fig. 17.24 17.54, Miniature Cireuit Breaker (MCB). Isa device that provides definite protection to the wiring instal- lations and sophisticated equipment against overcurrents and short-circuit faults, The outer and interior views of an MCB are shown in Fig. 17.25. Thermal operation (overload protection) is achieved with a bimetallic strip, which deflects when heated by any overcurrents flowing through it. In doing so, releases the latch mechanism and causes the contacts to open. Inverse time-current characteristics result, ie. greater the overload or excessive current, shorter the time required to operate the MCB. On the occurrence of a short Circuit, the rising current energizes the solenoid, operating the plunger to strike the trip lever causing imme- diate release of the latch mechanism. Rapidity of the magnetic solenoid operation causes instantaneous opening of contacts. er fg oven can mouan sane0 vat og AN 5 PLUNGER SaaS 5) er wom | @L roca sos, s INCOMING TERMINAL SILVER GRAPHITE CONTACT TIP FIXED CONTACT Fig. 17.28 Miniature Circuit Breaker (Courtesy Havells) Scanned with CamScannerMiniature circuit breakers are available with different current ratings of 0.5, 1,2, 2.5,3, 4, 5,6,7.5, 0,16 20, 25, 32, 35, 40, 63, 100, 125, 160 A and voltage ratings of 240/415 V ac and up (0 220 V de, Operating time is very short (less than 5 ms). So they are very suitable for the protection of important and sophisticatey equipment, ‘such as air-conditioners, refrigerators, computers etc. : ; 175.5. Earth-Leakage Circuit Breaker (ELCB). It is a device that provides protection against earth jegy. age. These are of two types viz, the current operated type and the voltage operated type, Current operated earth-leakage circuit breaker is used when the product of the operating current in amperes and the earth-loop impedance in ohms does not exceed 40. Where such a circuit breaker is used, the consumer's earthing terminal is connected to a suitable earth electrode. A current-operated earth leakage applied to a 3-9, 3-wire circuit is shown in Fig. 17.26. In normal conditions when there is no earth leakage the algebraic sum of the currents in the three coils of the current transformers (CTs) is zero, and no current flows through the trip coil. In case of any earth leakage, the currents are unbalanced and the trip coil is energize and thus the circuit breaker is tripped. Loap LoaD EARTH FAULT METAL SHEATH —+ oR a CURRENT EARTH CONTINUITY TRANSFORMER ‘CONDUCTOR TRIP-COIL = = circu Steel? orcur ya SREAKER BREAKER, hap con. | | | S:WIRE 3-PHASE 2.WIRE SUPPLY AST SUPPLY Fig.17.26 | Cur‘ent-Operated Barth-Leakage Trip Fig. 17.27 Voltage- Operated Earth-Leakage Trip Voltage-operated earth leakage circuit breaker is suitable for use when the earth-loop impedance exceeds the values applicable to fuses or excess-current circuit breaker or to current-operated earth-leakage circuit breaker. Such an earth-leakage trip in a 2-wire circuit is shown in Fig, 17.27. When the voltage between the earth continuity conductor (ECC) and earth electrode rises to a sufficient value, the trip coil will carry the required current to tip the circuit breaker. With such a circuit breaker the earthing lead between the trip-coil and the earth electrode must be insulated; in addition, the earth electrode must be placed outside the resistance area of any other parallel earths which may exist. In both the above types of ELCB the tripping operation may be tested by means of a finger-operated tt button which passes a predetermined current from the line wire through a high resistance to trip the coil and thus to earth. This test operation should be performed regularly. ~ Both types of earth- leakage circuit breakers are arranged to work manually and may take the place ofthe linked switch and fuses, or the excess-current circuit breaker, 36 Molded Case Cireuit real MCC A molded case circuit breaker, abbreviated MCCB, is 11YP® of electrical protection device that can be used for a wide ran; fi i and 60 Ha, The main distinctions between molded-case and mina Ee ae sm egucies oben Or ° ed-cas iature circuit breaker are that the MCCB can hav? gure ratings of up to 2,500 anne and its trip settings are normally adjustable. An additional differen*® is that MCCBs tend to be much larger than MCBs. As with most ircuit brea 5 three ae nadie types of circuit breakers, an MCCB ha * Protection against overload: Currents above the rates al for the application, rated value that last longer than what is norm Scanned with CamScanner© Protection against electrical faults: During a fault such as a short circuit or line fault, there are extremely high currents that must be interrupted immediately. Switching a circuit on and off: This is aless common function of circuit breakers, but they can be used for that purpose if there isn’t an adequate manual switch. ‘The wide range of current ratings available from molded-case circuit breakers allows them to be used in a wide variety of applications. MCCBs are available with current ratings that range from low values such as {15 amperes, to industrial ratings such as 2,500 amperes. This allows them tg be used in both low-power and high-power applications. Operating Mechanism. At its core, the protection mechanism employed by MCCBS is based on the same physical principles used by all types of thermal-magnetic circuit breakers. Overload protection is accomplished by means of a thermal mechanism. MCCBs have a bimetallic contact what expands Fig. 17.28 Molded Case Circuit and contracts in response to changes in temperature. Under Breaker (MCCB) normal operating conditions, the contact allows electric current through the MCCB. However, as soon as the current exceeds the adjusted trip value, the contact will start to heat and expand until the circuit is interrupted. The thermal protection against overload is designed with a time delay to allow short duration overcurrent, which is a normal part of operation for many devices. However, any overcurrent conditions that last more than what is normally expected represent an overload, and the MCCB is tripped to protect the equipment and personnel, On the other hand, fault protection is accomplished with electromagnetic induction, and the response is instant. Fault currents should be interrupted immediately, no matter if their duration is short or Tong. Whenever a fault occurs, the extremely high current induces a magnetic field in a solenoid coil located inside the breaker — this magnetic induction trips a contact and current is interrupted. ‘As a complement to the magnetic protection mechanism, MCCBs have internal arc dissipation measures to facilitate interruption, ‘As with all types of circuit breakers, the MCCB includes a disconnection switch which is used to trip the breaker manually. Itis used whenever the electric supply must be disconnected to carry out field work such as maintenance of equipment upgrades. 47.6 ELECTRIC SHOCK (EFFECTS OF ELECTRIC CURRENTS ON HUMAN BODY) Bruner (1967) states that the threshold of perception of electric shock is about 1 mA. At this level a tingling, sensation is felt by the subject when there is a contact with an electrified object through intact skin. With the increase in magnitude of ac, the sensation of tingling gives way to contraction of muscles. The muscular contractions increase as the current is increased and finally a value of current is reached at which the subject cannot release his grip on the current carrying conductor. The maximum current at which the subject is still capable of releasing a conductor by using muscles directly stimulated by the current is called “ler go current”. The value of this current is significant because an individual can withstand, without serious after effects, repeated exposures to his ‘Jet go current’ for at least the time required for him to release the conductor. Also, currents slightly in excess of ‘let go current? would not permit the individual to release his grip from the conductor supplying current. Based on the experiments conducted on males and females, it is generally accepted that the safe ‘let go Current’ could be taken approximately 9 mA and 6 mA for men and women respectively. At current levels higher than the ‘Iet go current’ the subject loses ability to control his own muscle actions and he is unable to release his grip on the electrical conductor. Such currents are very painful and hard Scanned with CamScannerto bear. This type of accident is called ‘hold-on-type’ accident, and is caused by currents in the range 20-199 mA. These currents may also cause physical injury due to powerful contraction of the skeletal muscles, However, the heart and respiratory function usually continue because of uniform spread of current through, the trunk of the body. If current contacts contact skin and passed through the trunk, at about 100 mA and above, there is q likelihood of pulling the heart into ventricular fibrillation. In this condition, the rhythmic action of the heart cexses, pumping action stops and the pulse disappears. Ventricular fibrillation is a serious cardiac emergency because once it starts, it practically never stops spontaneously. It proves fatal unless corrected within minutes, since the brain begins to die 2 to 4 minutes after it is robbed of its supply of oxygenated blood. At very high currents of the order of 6 A and above, there is a danger of temporary respiratory paralysis and also of serious burns. However, if the shock duration is of only a very few seconds, there is a possibitity of heart reverting to the normal rhythmic action. p The threshold of perception depends largely on the current density in the body tissues. It may vary widely depending upon the size of the current contact. At very small point contact, it is probable that even 0.3 mA current may be felt whereas a current in excess of perhaps 1 mA may not produce sensation if the contacts are somewhat larger. Similarly, depending on the size of contact, the threshold of pain may also be considerably above 1 mA, probably 10 mA if the contacts are large enough. ; Besides the magnitude of current, the current duration and the relationship of current flow resistance are also important. Duration of less than 10 ms typically does not produce fibzillation whereas duration of 0.1 s or longer does. It has been found experimentally that the safe value of current in amperes (rms) which a human body can tolerate is given as 1 = 268 forec3sand1=9mA fors>3s. vi where £ is the time duration in seconds of the flow of current. The tolerable currents mentioned above are for power-frequency currents. It has been found that human body can tolerate about 5 times higher direct current. At high frequencies (3-10 kHz) still higher currents are tolerable. ‘17.7 _EARTHING AND ITS IMPORTANCE Earthing means connections of the neutral point of a supply system or the non-current carrying paris of electrical apparatus, such as metallic framework, metallic covering of cables, earth terminal of socket outlet, stay wires etc., to the general mass of earth in such a manner that at all times an immediate discharge of electrical energy takes place without danger. Earthing is provided 1. to ensure that no current carrying conductor rises to a potential with respect to general mass of earth than its designed insulation, 2. to avoid electric shock to the human beings, and 3. to avoid risk of fire due to earth leakage cuzrent through unwanted path. In an electric installation, if a metallic part of an electric a live wire (that may be due to failure of insulation or otherwise) is charged and static charge on it will accumulate, Now if metallic part, he will get a severe shock, But if the metallic p be transferred to the earth immediately, as the metallic part breakdown occurs. And as the discharge takes place to ippliance comes in direct contact with a bare or the metal being a good conductor of electricity any person comes in contact with this charged arts of the appliances are earthed, the charge Will Comes in direct contact with a bare or live wite of carth, the impedance: of path of the current is low, * Scanned with CamScanner47.8 _METHODS OF EARTHING fizs MER THINGS ‘The various methods of earthing are : 1 Strip or Wire Earthing. In thi ‘ 8. In this system of earthing, strip electrodes of cross section not less than 25 mm x 1.6mm if of copper and 25 mm x 4 mmm if i 7 inimum depth 0.5 metre. If round comtctane svn iron or steel ase buried in horizontal trenches of Tomi? if of copper and 6 mm? if of gal used, their cross-sectional area shall not be smaller than Oo give he required eo eae. H00 oF Sel, The length of buried conductor shall be Sa be as widely distributed as mh resistance, Itshall, however, be not less than 15 metres. The electrodes a Possible, preferably in a single straight or circular trench or in a number of trenches radiating from a point. If conditions requi f parallel trenches of in radial trenches. require use of more than one strip, they shall be laid either in This type of earthing is used at places which ' . ait te cating edb ich have rocky soil earth bed because at such places excavation 2, Rod Earthin; - In this system of earthing, 12.5 mm diameter solid rods of copper or 16 mm diamet solid rods of galvanised iron or steel or hollow section 25 mm GI pipes of length moll less than 2.5 metres a driven vertically into the earth either manually or by pneumatic hammer. In order to increase the embedded Iength of electrodes under the ground, which is sometimes necessary to reduce the earth resistance to desired value, more than one rod sections are hammered one above the other. This system of earthing is suitable for areas which are sandy in character. This system of earthing is very cheap as no excavation work is involved. 3. Pipe Earthing. This is the most common and best system of earthing as compared to other systems suitable for the same earth and moisture conditions. In this method of earthing, a galvanised steel and perforated pipe of approved length and diameter is placed upright in a permanently wet soil, as shown in Fig. 17.29. ‘The size of the pipe depends upon the current to be carried and type of soil. Usually the pipe used for this purpose is of diameter 40 mm and 2.5 metres in length for ordinary soil or of greater length in case of dry and rocky soil, The depth at which the pipe must be buried depends upon the moisture of the ground. The pipe is placed at a depth of 3.75 metres (minimum). The pipe is provided with a tapered casting atthe lower end in brder to facilitate the driving, The pipe at the bottom is surrounded by broken pieces of coke or charcoal for ‘distance of about 15 om around the pipe. Generally alternate layers of coke and salt are used to increase the effective area of the earth and to decrease the earth resistance respectively. Another pipe of 19 min diameter and minimurn length 1.25 metres is connected at the top of Gl pipe through reducing socket In our country in summer season the moisture in the soil decreases which causes increase in earth resistance. So a cement conerete work, 2s shown in Fig. 17.29, is done in order to keep the water arrangement accessible, and in summer to have an effective earth, 3 or 4 buckets of water are put through the funnel connected to 19 mm diameter pipe, which is further connected to GI pipe. . The earth wire (either GI wire or GI strip of sufficient cross section to carry faulty current safely) is carried ina GI pipe of diameter 12 mm at a depth of about 60 cm from the ground. — , Care should be taken that earth wire is well protected from mechanical injury, when itis carried over fron one i a y Plas Bests THis js another common system of earthing, In plate earthing an earthing plate eith of copper of dimensions 60 cm x 60 em x 3 mm or of galvanised iron of dimensions 60 cm x 60 cm x6 mm buried into the ground with its face vertical at a depth of not Jess than 3 metres from re mel ae late is embedded in alternate layers of coke and salt fora minimum, thickness of 15 cm. The a seh | a for GI plate earthing and copper wire for copper plate earthing) is securely aed toan i 2 a mil f help of a bolt, nut and washer made of material of that of earth plate (made of copper in ¢: ‘copper ple earthing and of galvanised iron in case of GI plate earthing). Scanned with CamScanneri ication. As this syst iti ‘5 Fields of Application. ystem of wiring provides protect . . Jampness 30 this is the oly approved sytem of trong a sis fie, mechanealdomage and (@ places where considerable dust or fufTis present such as in textile mills, sawmills, (our Gi) damp situations. s to (ii) in workshops for lighting and motor wirings, (#) places, where there is possibility of fire hazards such as in oil mills, varnish factories ete, () places, where important documents are kept such as a record room, (0) residential and public buildings where appearance isthe prime thing s etc. ‘The recessed type conduit wiring is preferred for residential and public buildings. PVC conduit wiring system (particularly concealed) is cheaper in cost and takes less time but does not vide protection against fire, Insurance requirements stipulate metallic conduit wiring and PVC wiring only for offices. 17.4 TYPES OF WIRES AND CABLES ‘The wires employed for internal witing of buildings may be divided into different groups according to (i) conductor used (if) number of cores used (ff) voltage grading and (iv) type of insulation used. According to the conductor material used in cables, these may be divided into two classes known as copper conductor cables and aluminium conductor cables. According to the number of cores, the cable consists of, the cables may be divided into classes known as single core cables; twin core cables; three core cables; two core with ECC (earth continuity conductor) cables ete. According to voltage grading the cables may be divided into two classes : (i) 250/440 volt cables and Gi) 650/1,100 volt cables. According to type of insulation the cables are of the following types : 1. Vuleanized Indian Rubber (VIR) inselated cables. 2. Tough rubber sheathed (TRS) or cab tyre sheathed (CTS) cables. 3, Lead sheathed cables. 4, Polyvinyl chloride (PVC) cables. 5, Weatherproof cables. 6. Flexible cords and cables. 7.XLPE cables. 8. Multi-strand cables. 1. Vulcanized Indian Rubber (VIR) Cables. VIR, cables are availabie in 240/415 volt as well as in 650/ 1,100 volt grades, VIR cable consists of either tinned copper conductor covered with a layer of vulcanized Indian rubber insulation. Over the rubber insulation cotton tape sheathed covering is provided with moisture resistant Compound bitumen wax or some SINGLE CORE other insulating material for making CONDUCTOR the cables moisture proof. The thickness of rubber insulation ‘depends upon the voltage grade for Which the cable is required. The copper conductor is tinned '© provide protection against corrosion due to presence of traces (8) Seven Strand °F sulphur, zine Oxide and other e it ible ™ineral ingredients in the VIR Fig.17.18 Single Core VIR Cables (a) Single Strand Scanned with CamScannerA single core single strand VIR wire may be employed but larger cables have to be stranded, Single core single strand VIR wire and single core seven strand VIR cables are shown in Figs. 17.154) and 17.15() respectively. 2, Tough Rubber Sheathed (TRS) or Cab Tyre Sheathed (CTS) Cables. These cables are available in 250/440 volt and 650/1,100 volt grades and used in CTS (or TRS) wiring. TRS cable is nothing byt 4 vuleanized rubber insulated conductor with an outer protective covering of tough rubber, which provides additional insulation and protection against wear and tear. These cables are waterproof, hence can be useq in wet conditions, These cables are available as single core, circular twin core, circular three core, flat three core, twin or three core with an earth continuity conductor (ECC). The cores are insulated from each other and covered with a common sheathing. Different types of TRS cables are shown in Fig. 17.16. In wiring of 3 pin plugs separate earth wire may be used as it is cheaper in cost and easier in installation, These cables are cheaper in cost and lighter in weight than lead alloy sheathed cables and have the properties similar to those of lead sheathed cables and thus provide cheaper substitute to lead sheathed cables. OUTER SHEATH OF OUTER SHEATH OF INNER TOUGH RUBBER INNER TOUGH RUBBER Le INSULATION ‘CONDUCTOR INSULATION { ( D i conoucron/” (a) Single Core Single Strand TRS Wire (b) Single Core Seven Strand TRS Cable conpucToR: INNER OUTER SHEATH OF OUTER INSULATION OF INSULATION’ ‘TOUGH RUBBER conoueroR >, ‘TOUGH RUBBER oS c a Liner INSULATION (©) Twin Core Single Strand TRS Cable (d) Three Core Single Strand TRS Cable For Indoor Service For Indoor Service Fig. 17.16 3. Lead Sheathed Cables. These cables are available in 240/415 volt grade. The lead sheathed cable is a vulcanized rubber insulated conductor covered with a continuous sheath of lead. The lead sheath provides very good protection against the absorption of moisture and sufficient protection against mechanical injury and so can be used without casing or conduit system. It is available as a single core, flat twin core, flat three core and flat twin or three core with an earth continuity conductor. Two-core lead sheathed cable is shown in Fig. 17.17. 4, Polyvinyl Chloride (PVC) Insulated Cables. These cables are available in 250/440 volt and 650) 1,100 volt grades and are used in casing-capping, batten and conduit wiring system, In this type of cable conductor is insulated with PVC insulation. Since PVC is harder than rubber, PVC cable does aot requit® cotton taping and braiding over it for mechanical and moisture Protection. ; PVC insulation is preferred over VIR insulation because of the following reasons: @ PVC insulation has better insulating qualities. : (i), PVC insulation provides better flexibility. CONDUCTOR LEAD SHEATH ‘VIR INSULATION Fig. 17.17 2-Core Lead Sheathed Cable Scanned with CamScanner(ii) P¥C insulation has no chemical effect on metal of the wire (x) Thin layer of PYG insulation wll provide the desiced insulation level ; @ PVC caste te ees smaller diameter of cable and, therefore, more number‘of wires can be accommodat © conduit of a given size in comparison to VIR or CTS wires. pyc cab a mest vin used for internal wiring these days. ‘Though the insulation resistance of pvCis lower than tof uit its effect is negligible for low and medium voltages, below 600 V. 5. Weather Proo pei ve cables are used for outdoor wiring and for power supply or industrial supply. These © rer PVC insulated or vulcanized rubber insulated conductors being suitably taped (only in case of vulcanized rubber insulated cable) braided and then compounded with weather resting material. These cables are available in 40/415 volt and 650/1,100 volt grades, These L conpuctor WEATHER PROOF INSULATION [INSULATION gables are not affected by heat or sun or rain Seen Weather proof cable is shown in Fig. 17.18, ‘Although TRS cables can be used for outdoor purposes but due to their higher cost, weather proof viples are generally used for outdoor services, Fig. 1718 3-Core Weather Proof Cable 6, Flexible Cords and Cables, The flexible cords consist of wires silk/cotton/plastic covered. Plastic cover is popular as itis available in different pleasing colours, Flexible cords have tinned copper conductors. Flexibility and strength is obtained by using conductors having larger number of strands. These wires or cables are used as connecting wires for such purposes as from ceiling rose to lamp holder, socket outlet to portable apparatus such as radios, fans, lamps, heaters etc. The flexibility of such wires facilitates in handling, the appliances and prevents the wires from breakage. These must not be used in fixed wiring. The flexible cords used for household appliances are available in various sizes and with various thicknesses of coating as very thin/thin/medium/thick/very thick/extra thick etc. 7.XLPE Cables. PVC and XLPE cables are built of insulation made of polymers, Polymers are substances consisting of long macromolecules built up of small molecules or groups of molecules as repeated units. ‘These are divided into homopolymers and copolymers. Homopolymers are built by reactions of identical monomers. Copolymers are built up of at least two different kinds of monomers. The mechanical properties of the polymers e.g. tensile strength, elongation elasticity and resistance ‘against cold depend upon chemical structure. Their resistance against external chemical influences, acids, bases or oils together with their electrical and thermal characteristics are the decisive factors for the usefulness of cables insulated and sheathed with these materials. Advantages of PVC Cables Over Other Types of Cables 1, Non-hygroscopic insulation almost unaffected by moisture. 2. Non-migration of compound allowing vertical installation 3. Complete protection against most forms of electrolytic/chemical corrosion. 4 5. |. Tough/Resilient sheath with excellent fire resisting qualities. . Good ageing characteristics. 6. Not affected by vibrations. Advantages of XLPE Cables Over Both PVC and All Other Types of Cables 1, Higher current rating. Higher short-circuit current rating. Longer service life. ; Can withstand 130°C (maximum) for short time and is favourable to endure short-circuit stresses. Itis less sensitive to the setting of network protection. Because of thermosetting process taking place throuigh cross-linking, crack resistance is increased an ky Scanned with CamScannerssses are reduced, Consequently material is less se 7, Due to chemical cross-linking internal stres cooling gradient. during manufacture, to the setting of the c 8. The.thermal resistivity of cross-linked material is favourably low, compared to thermopia, material. " 9, LF 4 percentage of the ampere-hours available at the 10 90 er hour rate, are given in Fig. 18.5, s LT The capacity of a battery increases with the © ™ increase in temperature (Fig. 18.6) because at high 79 temperature, the chemical reactions taking place Within the cell become more vigorous, the acid © resistance is reduced and diffusion of electrolyte is | 5p improved. However, at high temperature, che paste ‘BB 3 ee a Bets rapidly converted into lead sulphate which is Tig, 18,0 OF DISCHARGE IN HOURS) ——+ always accompanied by expansion of paste Fig. 18.5 Courtesy Chloride india Lid Particularly at positive plates “ausing in buckling and cracking of the grid. At high temperature tke, at sol antimony-lead alloy grid, terminal posts and Wooden SE = Separators are also attacked by the acid. Sot an Bg 00 J | not be advisable to operate lead-acid batteries BS Lt beyond temperature of 40°C. With the fallin) °F Calne (28 chemical rections become alae Se | age | et internal peuslanee increases and diffusion of SEB 60 elite a eS electrolyte becomes poor. Consequently, the Fig. in9 quupeRATURE Inc . ‘ourtesy Chloride India Ltd. Scanned with CamScannercapacity ofthe cell decreases with the fll in temperature till at freezing point (~35°C at specific gravity of 112 of electrolyte) the capacity is reduced to zero even though the battery otherwise be fully charged. (iit) Efficiency. The efficiency of the cell can be given in two ways, enumerated and explained below. 1. The Quantity or Ampere-Hour (A-H) Efficiency. Since vatiations in terminal potential of the cell uring charge and discharge are not taken into account while working out this efficiency and terminal potential of the cell during charge is inigher than that duving discharge, therefore, quantity efficiency is Elways higher than energy efficiency, in which variations of terminal potential of the celt are taken into account. ’As generally efficiency is defined as the ratio of output to the input, similarly quantity efficiency or ampere-hour efficiency is defined as the ratio of ampere-hours of discharge and ampere-hours of charge. ie Ampere-hour efficiency, ryyy= AMPE-Rous of dscharse ¢ yqq = WT x 100 "Ampere-hours of charge 1.xT, “The quantity efficiency ofthe lead acid cell varies from 90 to 95%. It would be 100 percent if it were not forthe gassing on charge, which represents a non-reversible chemical reaction. Ifthe charging is discontinued each time us soon as the gassing becomes appreciable, the ampere-hour efficiency will. be nearly 100 per tent but ampere-hour capacity will be reduced and itis advisable to give the battery a full charge from time to time in otder to avoid deterioration of the otherwise unused lead sulphate. The quantity efficiency also decreases due to self discharge of the plates caused by local reactions and because of current leakage caused by faulty insulation between the cells and the battery. 2. Energy or Watt-hour Efficiency. Energy efficiency is defined as the ratio of energy delivered in watt- hours by the cell during discharge and the energy drawn in watt-hours during charge. Output in watt ie, Energy or watt-hour efficiency, Nyy i= x 100 Tapot in watt hours Curent, timeof average pd during delivered ™ discharge discharge = “Cunrent ,, time of average pd during * 1° dean ™ charge charge Lyx TyxVg Wt Me We = xTexVa x 199 = 4M x 100 V2 = Me ateve ee tas ayy aod Operation at low rate of charge and discharge and at reduced ampere-hour capacity both tend to raise the wattchour efficiency. Actual wat-hour or energy efficiency obtained in practice ranges from about 75 to 85 per cent, 18.2.7, Battery Ratings. The standards adopted, both by industry and government organisations, are given below: ‘ e 1. Ampere-hour Capacity Rating. ‘The ampere-hour rating of a battery is usually determined from its ability to deliver current continuously for 20 hours at 27°C. A battery that can deliver 5 amperes steadily for 20 hours then its rating will be 100 Ab. > Reserve Capacity. The reserve capacity of a battery is indicated in terms of minutes, a battery is capable of tolerating a drain of 25 A without dropping terminal voltage below 10.5 V (1.75 volts per cell). Higher this rating, better the battery is. + Cole Rating. The cold rating of a baitery indicates the number of minutes a battery can deliver 300 A at =18 °C (O°F). This is applicable in relation to eraft which ply in freezing weather. 4, Cold Cranking Power Rating. This rating is applicable to all 12 V batteries irrespective of their size. “The battery is discharged at ~18°C tll ts terminal voltage falls to 7.2 V. The output current is measured for 30 s. Higher the output, better the battery is. 18.2.8. Uses of Lead-Acid Batteries, The storage batteries are employed for a great variety and range of purposes such as to supply current for electric vehicles and gas exigine ignition, and in telephone exchanges, railway trains, mines, laboratories, hospitals, broadcasting stations, telecommunication systems, emergency Scanned with CamScannerlightings, UPS systems, solar photovoltaic systems, power generating stations and distribution work a5 standby unit. : ‘The storage batteries are employed in central power stations for (i) supplying the whole load during light load periods (ii) supplying peak load during the peak load hours (iif) local lighting for Odd time of breakdown (iv) regulation of load and voltage (v) compensating feeder drop and as a preventive against shy. down, A very important use of storage batteries is in the providing of standby power for various electrical systems. In some elecirical systems, storage batteries are connected in parallel with the generator and load, When the generator is in operation the batteries draw enough current to keep them fully charged. In case of shut down/breakdown of supply from the generator, the batteries supply the load, Railway-car lighting systems are supplied from axle-driven generators when the train is running, with batteries supplying the system when the speed of the train is low or the train is stopped. Automobile electrical systems are similar to the above mentioned system in that the generator, and the load are connected in parallel, the battery supplying power for starting and lighting when the generator is not in operation. Hospitals and other places, where a continuous source of power supply is absolutely essential, often use storage batteries as an emergency (or standby) supply. Applications in which storage batteries supply the normal current are in industrial truck or mine- locomotive propulsion portable lighting equipment, portable radios and other applications in which continuous supply from a generator is impracticable. 18.2.9. Elementary Calculations for Energy Consumption Example 18.1, A battery has taken a charging current of 5.2 A for 24 hours at a voltage of 2.25 V, while discharg- ingit gave a current of 4.5 A for 24 hours at an average voltage of 1.85 V. Calculate the quantity efficiency and the energy efficiency of the battery. Charging current, 1, = 5.2.4 Charging mean voltage, V_ = 2.25 V Charging period, T, = 24 hours Discharging current, |y = 4.5 A Solution: Discharging mean voltage, V, = 1.85 V Discharging period, T, = 24 hours 1, T, (Quantity efficiency, Thay = a a 7g * 100 = 86.54% Ans, ITs. Vg Energy efficiency. Nyy = ogee 3 x 100 = 71.15% Ans. Example 18.2. An alkaline cell is discharged at a steady current of 4 A for 12 hours, the average terminal voltage being 1.2 V. To restore it to original state of voltage, a steady current of 3 A for 20 hours is required, the average terminal voltage being 1.44 V. Calculate the ampere-hour and walt-hour efficiencies in this particular case. Solution: Charging current, I, Charging average voltage, V, Charging period, T. Discharging current, I, Discharging average voltage, Vz Discharging period, T,, . Wy 4x12 Al hour efficiency, Nagy = 4-4 x 100 = = mpere- ncys NaH 1.1 3x20 * 100 = 80% Ans, _ We 12 Watthour efficiency. Myr, = Nyy yt = 80x 7 = 66.7 % Ans. Scanned with CamScanner