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SWITCHGEAR AND PROTECTION (SGP) (403147)iii) Determination of fuse due to oxidation.4] Fusing factor – The ratio of the minimum fusing current & the current rating of fuse is called thefusing factor. As minimum fusing current is more than the current rating. The fusing factor is alwaysgreater than ‘1’.5] Prospective current – The figure shows how the ac current is cut off using a fuse. This iscalled cut-off characteristic of fuse. The rms value of the 1 st loop of the fault current calculated iffuse is assumed to be replaced by a link of a negligible resistance is called the prospective current.6] Cut-off current – The current value at which the fuse melts before fault current achieve its peakvalue is called cut- off current.7] Pre-arcing time – The time between the commencement of the fault current and the instant whenthe fuse melts & the arc starts is called pre-arcing time.8] Arcing time – The time between the end of pre-arcing time and the instant when the arcgets completely extinguished is called arcing time.9] Braking capacity – The braking capacity is the fuse rating corresponding to the rms value of theac component of maximum prospective current at its rated service voltage.10] Pre-arcing (I2 t) – It is the time integral of the square of current passing through the fusingduring pre-arcing time.11] Arcing (I2 t) – It is the time integral of the square of the current through the fuse during the arcingtime.12] Clearing (I2 t) – It is the sum of the pre-arcing I2 t and the arcing I2 t is called clearingI2 t or total I2 t.13] Voltage rating of fuse – The voltage rating of fuse is specified by the manufacture. The ratedvoltage of the fuse must be equal or greater thani) Voltage of a single phase circuitii) Line voltage in case of three phase circuitiii) Voltage between two outer wires in 3 wire dc circuit.CONCLUSION:In this way we have studied the different types of fuses.S.R.C.O.E, PUNEPage 10

SWITCHGEAR AND PROTECTION (SGP) (403147)SHREE RAMCHANDRA COLLEGE OF ENGG. LONIKANDPRACTICAL EXPERIMENT INSTRUCTION SHEETLABORATORY MANUALEXPERIMENT TITLE: STUDY OF FUSE & MCB & TESTING OF MCBEXPERIMENT NO. : SRCOE/ELECT/BE/SGP/02(B)SWITCHGEAR AND PROTECTION (403147)DEPT. : ELECTRICAL ENGINEERINGSEMESTER : II (BE) PAGE:-11AIM :To study Miniature Circuit Breaker (MCB).CONSTRUCTION :As the name indicates, miniature circuit breaker is housed in moldings of insulating materials, whichalso has fire resistance property normally ceramic. The MCB consists of a bimetallic strip forthermal overload protection as well as instantaneous magnetic release type short circuit protection.These releases are fixed on each pole and in case of three phase circuit they are connected to commonconnecting base.FUNCTION :MCB automatically isolates the electrical circuit under sustained overload or short circuit. A bimetallicelement provided on inverse time current characteristic which prevents interruption on normal inrushcurrents or temporary overload. An instantaneous magnetic trips release protects the equipmentsagainst very high currents. MCB‘s are available in 3 tripping circuit via B, C, D.Considering the fusing factor and aging effect of rewirablefuse, the smaller transformer of 25to200 KVA will be better protected with MCB.OBSERVATION TABLE :Sr. No.1.2.3.CurrentTime in Sec.CONCLUSION :Hence we have studied the working of miniature circuit breaker (MCB) and testing of miniaturecircuit breaker MCB is also performed successfully.S.R.C.O.E, PUNEPage 11

SWITCHGEAR AND PROTECTION (SGP) (403147)SHREE RAMCHANDRA COLLEGE OF ENGG. LONIKANDPRACTICAL EXPERIMENT INSTRUCTION SHEETLABORATORY MANUALEXPERIMENT TITLE: STUDY & TESTING OF CONTACTORS.EXPERIMENT NO. : SRCOE/ELECT/BE/SGP/03SWITCHGEAR AND PROTECTION (403147)DEPT. : ELECTRICAL ENGINEERINGSEMESTER : II (BE) PAGE:12-17STUDY & TESTING OF CONTACTORS.AIM:To study & test the contactors.APPARATUS: Switchgear testing kit, contactors & connecting wires etc.THEORY:Contactor is mechanical switching device capable of making, carrying & breakingelectric currentunder normal circuit conditions including operating overload conditions.Acontactor iselectromagnetically operated switches which can be operating remotely.Contactor may be capableof making & braking short circuit currents, if they are designed for short circuit of emerging thecontrol circuit namely.SOME TERMS & DEFINITIONS –Electromagnetic contactor – A contactor in which the opening & closing of mainContacts is achieved by means of an electromagnet.Electropneumatic contactor – A contactor in which the force for closing & openingThe main contacts is provided by an electrically opening pneumatic device.Main Circuit – The conducting parts of the contactor designed to close & open. Thecurrent flows from the supply to load through the main circuit of the contactor.Main Contacts – The contacts in the main circuit to carry the load current when thecontactor is closed position.Control Unit – The circuit which is energized or de-energized electrically for openingS.R.C.O.E, PUNEPage 12

SWITCHGEAR AND PROTECTION (SGP) (403147)/closing operation the contactor.Auxillry circuit – The circuit other than main & control circuit is called auxillary circuit.PRINCIPAL:When a definite voltage is applied the coil energizes an electromagnet system comprising of fixed& moving parts, the moving parts get attracted to fixed part. Thus the electrical circuit is completedthrough this contact system. When magnet is de-energized, it gets pushed back by return spring &contacts open & thus electric circuit is broken.CONSTRUCTION OF TYPICAL CONTACTOR:Parts of contactor & their function:Rubber Padding:-It is provided for absorbing shocks during closing and opening the contactor.Magnets :- AC magnets are made up of laminations stacked together to reduce effect of eddy currents.Since in DC frequency is zero, problems of eddy currents losses & hysteresis losses do notexist & hence no laminations are required.There is designed gap between the central limb of fixedmagnet & the moving magnet.When this gap reduces over millions of operations or magnetstarts bulging it indicate thatmechanical life of contactor is over & needs replacement.If duringmaintenance or cleaning, emery or sand paper is used for the magnets the designedgap betweenthe central limb increases as surface material reduces, increasing the air gap.Hence while openingthis remains some residual magnetic flux which does not allowthe moving magnet to separateimmediately, causing severe pitting of contacts. This resultin frequent changing of contacts.Thus wage of emery or sand, paper should be avoided at magnets; it can be cleaned withCRC-2-26 / CTC & rough cloth.Contact System :The contacts are made up of cadmium alloy. These have well anti welding properties. The gapbetween the contacts is less than that between magnets which ensures that contacts areS.R.C.O.E, PUNEPage 13

SWITCHGEAR AND PROTECTION (SGP) (403147)closed before magnet pole faces touch each other.LEAF Spring : The moving contacts have to be held under certain contact pressure which is ensured byproviding leaf springs over the contact carrier bridge. It also helps in bounce free closingReturn Spring :The moving assembly is held against the stored energy of two main springs called as returnsprings which helps the bridge assembly in opening the contact.Shading ring :In A.C. magnets alternating flux (main flux) goes through natural zero point so force ofattraction created becomes zero.This results in repeated de -energization of the magnets & leads tochattering.Some additional force is require to hold the magnet when main flux passes through zero.It does this by generating an auxiliary flux which is out of phase with main flux, theshading ring is embedded in the pole face of the magnet for this.Terminals:- Terminals are made up of silver plated copper and are provided for both powercircuit & control circuit terminators.Auxiliary contacts :Set of normally open (NO) & normally close (NC) contacts are available for indication & alarmpurpose.Arc Chute Set :While breaking high currents between the contacts needs to be effectively quenched.Arc is quenched by following 3 methods:1) Arc is split by providing number of Deion-plates.2) The arc or current when drawn to Deion plates induces a voltage across the plates which3) opposes the cause producing it. (arc) as per Lenz’s Law4) The resistance of the arc specific resistivity x length of arcS.R.C.O.E, PUNEArea of arcPage 14

SWITCHGEAR AND PROTECTION (SGP) (403147)Due to splitting of arc its length increases as resistance of arc is directly proportional tolength. The resistance of arc increases & thus arc is quenched.Housing :Material having good thermal withstand and mechanical strength are used for housing.Mostly ceramic ,SMC ( sheet moulded compound ) & DMC (dough moulded compound )are used for front housing while Die- cast . Aluminum is used for rare housing.SPECIFICATION OF CONTACTOR:1) Rated Voltage – 415 V2) Rated coil voltage – coils are available in various voltage rating eg.24,48,380,240,415 etc(AC/DC)3) Contactor has following main current ratings & is selected as per the type of load ie.(application)Rated current forAC1 – Resistive loadAC2 – Slightly Inductive load ( e.g. slip ring induction motor )AC3 – Highly inductive load (e.g. Squirrel cage I.M. normal operation)AC4 – Inching & highly inductive load like squirrel cage induction motor.Contacts can be fitted with following accessories –1. Auxiliary contact blocks (add on block )2. Mechanical interlock etc.3. Surge suppressor.Advantages of contactors:1. High number of make- break operations2. High making & braking capacity3. Remote ON/OFF4. High mechanical & electrical lifeDisadvantages of contactors:S.R.C.O.E, PUNEPage 15

SWITCHGEAR AND PROTECTION (SGP) (403147)1. It cannot sense fault itself.2. It should be installed with relay unit.3. Therefore it becomes costly.Applications of contactors:Remote controlling:Mobile applications, Telecommunication, security, water pumps, green houses,remote power generator, solar systems, traffic control, field control, In agricultural industrieswhere voltage is less & voltage fluctuations are more.Programming controllers.Voltage Monitors - They provide protection against phase loss, phase reversal.Over, under &unbalanced voltage. It will also protect motor against voltagefaults. It will operate when voltage isexceeding the set value.Illumination control.1. Timers2. Starters – Contactor starter from motor1. D.O.L. contactor starter2. Reversing contactor starter3./ contactor starter3. Air break contactors are used in L.V. switchgear. (suitable for large no.of switching operations on load/ over load )4. Key or push button.5. Auxiliary switches.6. Medium voltage vacuum contactors ( 3.6 to 12 KV)(suitable for controllingA.C. loads with high switching rate & unlimited on time)7. Switching off –S.R.C.O.E, PUNE-3 Ø motor in A.C. 3 duty-Transformers-Capacitors-Resistive load (electric furnace )Page 16

SWITCHGEAR AND PROTECTION (SGP) (403147)8. There is also an upcoming ‘Biological contactor technology ‘for Mine efficienttreatment.PROCEDURE :1) Check the rated voltage of contactor coil2) Keep the contactor in vertical position.3) Connect the coil terminals of the contactor to voltage output terminalsof the test panel4) For pick up voltage test, increases the voltage gradually till contactsclose without humming.OBSERVATION TABLE:RatedPick-upDrop-offVoltage (V)Voltage ( V)Voltage (V)VVVCurrent rating APick up voltage V (85 to 110 % of rated coil voltage)Drop off. Voltage V (10 to 70 % of rated coil voltage)CONCLUSION:Thus we have observed the testing of contactor. It is rated for V& it’s pick up & drop offvoltage is V and V respectively.S.R.C.O.E, PUNEPage 17

SWITCHGEAR AND PROTECTION (SGP) (403147)SHREE RAMCHANDRA COLLEGE OF ENGG. LONIKANDPRACTICAL EXPERIMENT INSTRUCTION SHEETLABORATORY MANUALEXPERIMENT TITLE: STUDY & CHARACTISTICS OF THERMAL OVERLOAD RELAYEXPERIMENT NO. : SRCOE/ELECT/BE/SGP/04SWITCHGEAR AND PROTECTION (403147)DEPT. : ELECTRICAL ENGINEERINGSEMESTER : II (BE) PAGE:18-23STUDY & CHARACTISTICS OF THERMAL OVERLOAD RELAYAIM :To study and test thermal overload relay.APPARATUS : Thermal overload relay ( 500 V , 50 Hz , 2 A AC ) Contactors Switch gear testing panelTHEORY :Thermal overload relay prevent an electric motor from drawing excess current and overheating .Thermal overload conditions are the most likely faults to be encountered in industrial motor application, which produces an increase in the motors, Thermal dissipation and temperature . Overload protectionprevents on electric motor from drawing excess current , overheating and finally bumping out.Thermal overload relays can be bimetallic relays , eutectic alloy relays, temperature control or proberelays and solid state relays. A bimetallic device is made up of two strips of different metals . Thedissimilar metals are permanently joined . Heating the bimetallic strips causes it to bend because ,the dissimilar metals expand & contracts at different rates. The bimetallic strips applies tension to aspring on a contact. It heat begins to rise the strip bends and spring pulls the contact apart breaking thecircuit. A melting alloy ( or electric ) overload relay and a mechanical mechanism to activate atripping device when an overload occurs.The relay measures the temperature of the motor by monitoring the amount of current being drawn.S.R.C.O.E, PUNEPage 18

SWITCHGEAR AND PROTECTION (SGP) (403147)This is done indirectly through a heater coil. Temperature control relays are used to protect the motorby directly sensing the temperature of the windings using thermister of RTD probes . The motor musthave on or more positive temperature co-efficient thermister probes embedded in its windings.When nominal operating temperature of the probe is reached, its resistance increases rapidly .This increase is detected by a threshold circuit which controls a set of relay contacts solid staterelays have no moving or mechanical parts. The relay calculates the average temperatures within themotor by monitoring its starting and running current. A solid state relay is also another type of overcurrent relay.PRINCIPLE OF OPERATION :A bimetal is formed by two metals with unequal coefficient of linear expansion brazed together in theform of strip. When heated strip bends one end is fixed and other bends when heated under overload/single phasing.CONSTRUCTIONS :The over current relay operates on thermal effect of electric current. Basically , it consist of a bimetallicstrips . The bimetallic strip is nothing but two metal strips having different co-efficient of thermalexpansion joined together . Here one end is supported and fixed and uneven expansion caused bendingof strips. This operates the relay contacts . The fig. shows the construction of overload thermal relay.It consists of following partsUpper Slider bar :One end of slider bar is connected to spring for pull action and other end is connected to spring for pullaction and other end is connected to contact tripping level and top pivot points when bimetallic stripbends due to overload / unbalance current, this upper slider bar moves and pushes the contact trippinglevel , i.e. The upper slider bar is movable.Lower Slide bar :-S.R.C.O.E, PUNEPage 19

SWITCHGEAR AND PROTECTION (SGP) (403147)One end is connected to spring which has push action and other end is connected to bottom pivot pointand contact tripping level. This bar has slots for carrying bimetallic strip. The spring apposes movementof contact tripping lever towards contact carrier i.e. lever slide bar is fixed one.Bimetallic strips :It is made up of metal strips having different co-efficient of thermal expansion. They are three innumber and housed in slots of lower slider. It gets supply from CT’s for high currents.WORKING :Under normal conditions :Strips are strained; contacts tripping lever is in vertical position and relay contacts are in normalposition.Under overload condition :Currents are greats than normal but balanced and hence all three bimetal and bend equally. Thelower slider moves towards contact right and pushes control carrier. Hence NC contacts open whichgivessignal to the contactor and supply is cut off.Suppose the R phase opens therefore any bimetalsof Y and B phases operates due to bending of other two bimetals. Thus for the same power, the othertwo phase bimetals operates and trip the circuitproviding the single phasingproduction.Effect of over loading / over heating :The effect of overloading is an incipient fault which generally cause the damage of the insulation issupported to withstand certain temperature without damage. If the temperature increases beyondthis for long periods , then the insulation deteriorates . The final stage is breakdown of insulation, thusthis reduces the life of motor and so it becomes necessary to prevent this overheating due tooverloading.Causes of overheating : Overload high currentS.R.C.O.E, PUNEPage 20

SWITCHGEAR AND PROTECTION (SGP) (403147) Supply variation – low voltage result in high current Heavy starting – high starting current. High starting frequency – motor is required to start often. High duty factor – motor remains on for long periods and off for short periods. Single phasing complete load comes on two windings hence high current. Faulty bearings and other mechanical reasons. Defect in cooling systems for very large machines.Advantages of Thermal overload Relays :As the motor gets damaged due to higher temperature and causes of higher temperature arebasically higher currents, the most commonly used protection is of current sensing devices ,bimetallic thermal relay is used extensively.The main advantages are :Thermal overload relay gives protection for both balance and unbalance faults.They give protection against overloading & single phasing .The relay is compensated per ambient temp. variation from 5 c to 55 c so that it can protect themotor for both cold start & hot start conditionThese relays come as attachment with contactorsThey are quite economical.Application of Thermal Overload Relay :Overload Protection :The relay protects motor against damage resulting from overloading as per international standardestablished for the protection of motor . Adjustment of relay to match the full load current of motor is alever which moves against a calibrated scale marked in full load current. The range of current settingon any relay is between 100 % & 160 % of its minim

SWITCHGEAR AND PROTECTION (SGP) (403147) S.R.C.O.E, PUNE Page 9 In such fuse, the fuse carrier drops out once, the fuse operates, the dropping out of fuse carrier provides the necessary isolation between the terminals. 5. Liquid Fuse – When fuse operates, in case of high current th