Basic electrical engineering, Study notes of Electrical Engineering

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Ajay Balar - EE Department Basic Electrical Engineering (3110005) 1

5.1 What is fuse? Define its characteristic.

 Fuse is the simplest device, which break the circuit under abnormal condition.  It is only a current interrupting device under fault condition.  It is not able to make or brake the circuit under normal condition.  A fuse consists of a metal strip of mounted between a pair of electrical terminals, and enclosed by a non-conducting and non-combustible housing.  The fuse is arranged in series to carry all the current passing through the protected circuit.

Figure 5.1 Fuse connected in circuit  The fuse element is made of zinc, copper, silver, aluminum, or alloys.  The fuse element may be surrounded by air or by materials to quench the arc. Silica sand or non-conducting liquids may be used.  It is used for overload short circuit protection in medium voltage (up to 33 kv) and low voltage (up to 400v) installation.  Fuse characteristics are drawn between current and time scale. The curve shows that fault current and operating time is inversely proportional to each other.  The time considered is précising time and current is prospective current.  The fuse characteristics become asymptotic and there is a minimum current below which the fuse does not operate. This is called minimum fusing current. FUSE Characteristics

Current

Time

Figure 5.2 Fuse Characteristics

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They should have a following desirable characteristic:-  Low melting point  High conductivity  Free from deterioration from oxidation  Low cost

5.2 Write short note on HRC fuse.

Fuse Element

Filling Powder

Outer Element Fuse Link Contact

Brass end plate

Figure 5.3 HRC Fuse  When the load capacity is very high the level of fault current will also increase. So faulty clearing device will be under heavy stress.  HRC fuse is preferred for heavy duty and rapid operation.  It consists of heat resisting ceramic cylindrical body having low co-efficient of thermal expansion.  The fuse element is made up of silver or silver alloy to improve fuse life.  It is filled with incombustible powder which absorbs the arc produced at the time of blowing.  The rating is much more accurate. It is widely used because of silent operation and non- deteriorating characteristics.  It is maintenance free and easy to install. Advantages:-  Speed of operation is very high.  Maintenance cost is practically is zero.  They deteriorate with age.  They provide reliable operation.  They cheaper than other protecting devices. Disadvantages:-  Heat produced by arc may affect the associate switches.  They have to be replaced after each operation.

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5.5 Write short notes on MCB.

Figure 5.4 MCB Construction  MCB have the features of good HRC fuse and a good switch.  These are used for switching purpose under normal condition and circuit break under overloading and short circuit condition.  It is normally operated at 1.25 times its rated current.  It is manufactured with quick reset hand operated facility.  It is basically operated on two type of working principles:  Thermal operation:  It is achieved by bimetallic strip which deflects when heated by any over current flowing through it.  Magnetic operation:  When short circuit occurs the rising current energizes the solenoid further operating plunger to strike the trip lever immediately releasing of latch mechanism.

Construction of MCB  The casing of MCB is made of moulded thermoplastic polyester. This material is fire retardant and non-hygroscopic. They are installed directly on Rail in distribution boards, control panels simply by fixing.  The contacts are made of Silver- Copper alloy which ensure longer life of contacts. These have low resistance resulting in low watt loss. The contacts are designed to have zero Bounce during closing operation.  Operating Mechanism of MCB has quick make, quick break, and trip-free mechanism. Application:-  MCB are used extensively in low voltage domestic, commercial and industrial.

Ajay Balar - EE Department Basic Electrical Engineering (3110005) 5

5.6 Write short notes on MCCB.

Definition  MCCB is a switching device which is used in LT electrical system. It provides protection against overload & short circuit. Fault sensing arrangements are installed inside MCCBs & shunt release is provided for remote tripping of MCCB.  It is available between 100A & 630Acurrent. Working Principle  MCCB provides protection against overload through thermal mechanism.  It has bimetallic contacts which expand & contract on temperature changes (same like an automatic iron).  Under normal condition these contacts allow normal current to flow but when current exceeds its trip limit, the bimetallic contacts start heating up & expand until the circuit is isolated/tripped.  When MCCB is tripped, faulty circuit is isolated from circuit & the temperature of bimetallic contacts starts getting normal & MCCB again is ready for next operation.  MCCB provides protection against short circuit – if current is very high, fault current should be interrupted immediately.  This is achieved by electromagnetic induction. Whenever fault occurs, the high current induces a magnetic field in a solenoid coil located inside the breaker, this magnetic induction trips a contact & current is interrupted.  In the tripping process, arc is produced & that is dissipated by taking suitable measures inside the breaker.  These breakers can be manually switched off or on also which is required during maintenance or other purpose.

Figure 5.5 MCCB

Ajay Balar - EE Department Basic Electrical Engineering (3110005) 7

Working of ELCB  It is current operated device designed to operate when a leakage current exceeds the predefined value.  It consists of a operating coil and a trip mechanism which operated the circuit when required.  The coil is supplied through 1-Ф supply so current in phase & neutral wire will be same.  This current will produce flux linkages same in magnitude but of opposite direction. This will result zero net flux in tripping coil of relay.  When fault or leakage current exceeds the limit higher current will flow in phase conductor than neutral current.  Resultant flux now is out of balance in tripping coil of relay. Difference of flux will induce emf in the coil which opens the contact of ELCB and isolate the circuit from the supply.

5.8 Explain types of wire

 There are different types of wire are listed below:  Triplex wire: Triplex wires are usually used in single-phase service drop conductors, between the power pole and weather heads. They are composed of two insulated aluminum wires wrapped with a third bare wire which is used as a common neutral.  Non-metallic sheathed wires: Non-metallic sheath wire is used in most homes and has 2-3 conductors, each with plastic insulation, and a bare ground wire. The individual wires are covered with another layer of non-metallic sheathing. Since it’s relatively cheaper and available in ratings for 15, 20 and 25 amps, this type is preferred for in-house wiring.

Triplex wire (^) Non-metallic sheathed wire

Single strand wire

Figure 5.7 Types of wire

 Single Strand Wire : Single strand wire also uses THHN (Thermoplastic High Heat- resistant Nylon-coated) wire, though there are other variants. Each wire is separate and multiple wires can be drawn together through a pipe easily. Single strand wires are the most popular choice for layouts that use pipes to contain wires.

Ajay Balar - EE Department Basic Electrical Engineering (3110005) 8

5.9 Explain types of Cable

Under ground cable are classified as follow: (1) According to voltage level:  Low voltage (L.T.) cable: It consist of one circular core of tinned stranded copper (or aluminum) insulated by layers of impregnated paper. These cable are used up to 1 kV.  High voltage (H.T.) cable: It consist of either circular shaped or oval or sector shaped 3 core stranded copper or aluminum. These cable are used up to 11 kV.  Super tension (S.T.) cable: The insulation on each core is covered with aluminum foil or own lead sheath or metallized paper. These cable are used up to 33 kV.  Extra high tension (E.H.T.) cable: oil filled cable and gas pressure cable are types of E.H.T. cable. Oil filled cable is consist of oil channel at the center of core by stranding the conductors wire around the hollow cylindrical steel spiral tape.Gas pressure cable is laid in a gas tight steel pipe which is filled with dry nitrogen at 12 to 15 atmosphere pressure produces radial compression and closes the voids. These cable are used up to 66 kV.  Extra super voltage cable: These cable are used up to 132 kV and above. (2) According to insulating material :  Insulation is provided on conducting material to block the path of leakage current from the conductor,thus minimizing the risk shock and fire. Normally cables are classified according to the insulation used over the conductor. The various classification of cable commonaly used for domestic wiring are as follow:  Vulcanized rubber sheathed (V.I.R.) insulated cables: These consists of a copper conductor covered with a insulation layer of Vulcanized Indian Rubber (VIR). A cotton tape covering is provided over this insulation layer to protect the wire from moisture and to provide mechanical strength to the wire. The thickness of the Vulcanized Indian Rubber depends on the voltage.  Cab Type sheathed (T.R.S.) cables: These C.T.S or T.R.S wires consists of vulcanized rubber insulated conductor. This insulation layer is covered by a layer made of tough rubber (or) tough rubber sheathed covering is provided over this insulation layer. This covering will be very hard and protects the wire from moisture and provides mechanical strength to the wire. These wires are available in single core, twin core, triple core etc. As these wires have tough rubber covering no additional protection or strength is required.  Weather – proof cables: These wires consists of conductor provided with an insulation layer made up of hard rubber. Over this cotton sheathed and cotton tape covering is provided especially to protect the wire from moisture. These wires are used where the moisture is present.  Polyvinyl chloride (P.V.C.) insulated cables: These wires consists of a conductor over which an insulation layer made up of Polyvinyl Chloride is provided. These wires cannot resist much heat and they have relatively low melting points, so they aren’t used in hot places and also these wires are not used with heating appliances. PVC wires are available in almost all colors.  Lead sheathed cables: These wires consists of vulcanized Indian rubber insulated conductor over which a Lead sheath is provided which gives mechanical strength to the

Ajay Balar - EE Department Basic Electrical Engineering (3110005) 10

 Lead Sheath: As the cable is placed under ground, soil may present moisture, gases and some other liquids. Therefore, to protect the cable metallic sheath made up of lead or aluminum is provided over the insulation.  Bedding: To protect the metallic sheath from corrosion and some mechanical injury, bedding is provided. It is made up of some fibrous material such as jute.  Armoring: Armoring is used to protect the cable from mechanical injury while handling. It consists of one or two layers of galvanized steel wire or steel tape.  Serving: Serving is provided to protect the armoring from atmospheric conditions. It is made up of some fibrous material like jute.

5.11 Eatrhing System

 What is Eatrhing? Explain the purpose of Earthing. OR Grounding.  “The earthing is the connection of general mass of earth to electrical apparatus in such a manner as to ensure all time an immediate energy discharge without danger”.  The earth is made up of a material that is electrically conductive. A fault current will flow to earth through the live conductor, provided it is earthed.  The Conventional system of Earthing is done by digging of a large pit into which a GI pipe or a Copper plate is positioned with the layers of charcoal and salt.  When system is without earthing and if short circuit occurs and human body touches the metal part, current will get return path through human body.  When system is properly earthed and if short circuit occurs and human body touches the metal part, current will not get path through human body. Because circuit is already provided with low resistance path through earthing.  An effective earthing is made through any wire, pipe, rod or metal plate known as earth electrode.  The connecting wire between electrical apparatus and earth electrode is known as earthing lead or main earthing conductor. Purpose of Earthing  To avoid electric shock to human body.  To avoid risk of fire due to earth leakage current through unwanted path.  Ensure that all exposed conductive parts do not reach a dangerous potential  Maintain the voltage at any part of an electrical system. The qualities of a good Earthing  Must be of low electrical resistance  Must be of good corrosion resistance  Must be able to dissipate high fault current repeatedly

5.12 State the different method of earthing and explain one.

Following are the different methods of earthing.  Pipe earthing  Plate earthing  Coil earthing

Ajay Balar - EE Department Basic Electrical Engineering (3110005) 11

Pipe Earthing:-

Funnel

GI Pipe

Salt Charcoal

Figure 5.9 Pipe Earthing

 Pipe earthing is the most commonly adopted method and is a best system of earthing compared to the others system.  In this method of earthing pipe of sufficient diameter is selected whose size is depend upon (a) Maximum earth current of that installation (b) Type of Soil  As per IS-732-1963, The GI pipe shall not be less than 38mm diameter and 2 meter long for ordinary soil.  If Cast Iron is used then internal diameter should be 10 mm.  The depth at which pipe should be buried depends upon condition of soil and moisture.  For pipe earthing at pit of 40sq.mm is dug in the soil and the pipe having tapered at the bottom is placed vertical in the pit.  The charcoal and salt are filled in that pit alternately in layers about 2 meters from bottom and for a distance of about 15cm around the pipe. This is done to increase the dampness and moisture of soil surrounding pipe.  The pipe placed has 12mm diameter holes drilling in it so that water poured from top is easily spread in the media surrounding the pipe which helps in maintaining the resistance of earth.  At a top a cement concrete work is done to provide protection against mechanical damage.  A water pouring arrangement is provided by a funnel with wire mesh at the top.

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5.13 Write short note on Neutral Earthing.

 Neutral Earthing is the process of connecting a star point of transformer, generator and motor to ground.  Protects system against arcing ground  Keeps balanced voltage with respect to earth  Greater safety to lightning  Maintenance and operating cost is low  Reliable in operation Types of Neutral Earthing  All A.C. power system of today is operated with neutral grounded. It is classified as per voltage level as below.

1. Solid Earthing  Neutral point is connected to earth without intentional resistance or reactance.  It is used below 660 V. 2. Resistance Earthing  Neutral point is connected to earth through resistance.  It is used between 3.3 KV to 11 KV. 3. Reactance Earthing  Neutral point is connected to earth through resistance.  It is good as compared to resistance earthing for same voltage level (i.e. 3.3 KV to 11 KV) 4. Resonant Earthing  The value of reactance is selected so as to neutralise the power frequency capacitive current between line and earth.  It is used for medium voltage transmission line which connected to generator with intervening power transformer.

5.14 Safety precautions for electrical appliances

 It is extremely important to take safety precautions when working with electricity. Safety must not be compromised and some ground rules must be followed.  The basic guidelines regarding safe handling of electricity are as given below:  Avoid water at all times when working with electricity. Never touch or try to repair any electrical equipment with wet hands, as it increases the possibility of shock.  Never use any equipment with frayed cords, damaged insulation or broken plugs.  Never work on any receptacle at your home with mains supply ON. Always turn it OFF before working.  Always use insulated tools while working.  Always use standard pins to tap the supply of power from any plug.  Always use the standard lSI marked materials and equipments even though they cost a little more.

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 Avoid electrical hazards by not letting the energized parts and unguarded electric equipment exposed because they are a shock risk especially for children in the house.  Never use an aluminum or steel ladder to work on any appliance at height. Instead use a bamboo, wooden or fiberglass ladder.  Know the wire code of your country.  Always check all your GFCIs (Ground Fault Circuit Interrupters) once a month. These devices are commonly used nowadays to avoid electric shock hazards.  Plug points of high power appliances like refrigerators, wet grinders, washing machine, water heaters etc must have a proper earthing and power should be drawn only through the three pin plugs.  Always use .a circuit. breaker (MCB) with the appropriate current rating, in each electrical circuit.  Take care while removing a capacitor from a circuit. A capacitor stores energy and if not removed properly then it may cause an electric shock.

5.15 Basic introduction of Battery

 It is known that the electrical current can easily flow through the metallic conductors. Current can also flow through same liquids called electrolytes.  When current passes through an electrolytic solution chemical action occur and produced chemical charges.  Thus in this process electrical energy is converted into chemical energy and chemical energy also converted into electrical energy. The device which is converting chemical energy into electrical energy is called electrical cell.

5.16 Explain following terms

(1) Cells

 The smallest element of a battery is a cell. A cell is defined as a source of emf in which chemical energy is converted into electrical energy.  A cell consists of two metal plates of different materials. These plates are immersed in a suitable solution. The value of emf produced by a cell depends on: (1) Material used for the plates or electrodes (2) Types of electrolyte

(2) Battery

 A battery is a group of cells. Depending on the voltage and current requirements, the cells are suitably connected in series parallel configurations. Batteries absorb electrical energy at the time of charging and release it at the time of discharging.  The batteries give out electrical energy due to chemical reaction taking place, while discharging. During the charging process, the batteries chemical changes take place, which absorb the energy.  The entire resistance encountered by a current as if it flows through a battery from the negative terminal to the positive terminal is known as internal resistance of battery.

Ajay Balar - EE Department Basic Electrical Engineering (3110005) 16

5.19 Explain A-h Capacity and W-h Capacity

(a) A-h Capacity

 An ampere hour (abbreviated Ah, or sometimes amp hour) is the amount of energy charge in a battery that will allow one ampere of current to flow for one hour. An ampere is a unit of measure of the rate of electron flow or current in an electrical conductor. 

d d

= Rated current during discharging

I

T = discharging time in hour

A hcapacity I T d d Where

(b) W-h Capacity

 The watt-hour (symbolized Wh) is a unit of energy equivalent to one watt (1 W) of power expended for one hour (1 h) of time. The watt-hour is not a standard unit in any formal system, but it is commonly used in electrical applications. 

d d d

I = Rated current during discharging T = Dischargingtime in hour V= The average voltage during discharging

W hcapacity I T V d d d Where

5.20 Give the classification of Battery (i) Non-rechargeable batteries  Zinc-chloride batteries  Zinc-Carbon  Mercuric Oxide Batteries  Zinc Silver Oxide Batteries (ii) Rechargeable batteries  Lead-acid batteries  Li-ion batteries  Nickel-cadmium batteries  Nickel hybrid batteries

5.21 Explain Lead acid battery with charging & discharging equations

Working of Lead Acid Battery  The storage battery or secondary battery is such battery where electrical energy can be stored as chemical energy and this chemical energy is then converted to electrical energy as when required. The conversion of electrical energy into chemical energy by applying external electrical source is known as charging of battery.  Whereas conversion of chemical energy into electrical energy for supplying the external load is known as discharging of secondary battery. During charging of battery, current is passed through it which causes some chemical changes inside the battery. This chemical changes absorb energy during their formation.

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 When the battery is connected to the external load, the chemical changes take place in reverse direction, during which the absorbed energy is released as electrical energy and supplied to the load.  Now try to understand working principle of lead acid battery and for that we will first discuss about lead acid battery which is very commonly used as storage battery or secondary battery. Materials used for Lead Acid Storage Battery Cells  The main active materials required to construct a lead acid battery are  Lead peroxide (PbO 2 ).  Sponge lead (Pb) and  Dilute sulfuric acid (H 2 SO 4 ). (i) Lead Peroxide (PbO 2 )  The positive plate is made of lead peroxide. This is dark brown, hard and brittle substance. (ii) Sponge Lead (Pb)  The negative plate is made of pure lead in soft sponge condition. (iii) Dilute Sulfuric Acid (H 2 SO 4 )  Dilute sulfuric acid used for lead acid battery has ration of water: acid = 3:1. Discharging process  The lead acid storage battery is formed by dipping lead peroxide plate and sponge lead plate in dilute sulfuric acid. A load is connected externally between these plates. In diluted sulfuric acid the molecules of the acid split into positive hydrogen ions ( H+) and negative sulfate ions (SO 4 −−).  The hydrogen ions when reach at PbO 2 plate, they receive electrons from it and become hydrogen atom which again attack PbO 2 and form PbO and H 2 O (water). This PbO reacts with H 2 SO 4 and forms PbSO 4 and H 2 O (water).       

2 2 2 2 4 4 2

PbO H PbO H O PbO H SO H PbSO H O

Separator

Lead(Pb) anode plate

- +

Lead cathode caoted with Pbo 2

Aqueous sulphuric acid H 2 So 4

Figure 5.12 Lead acid battery

Ajay Balar - EE Department Basic Electrical Engineering (3110005) 19

 A 21% solution of potassium hydroxide (KOH) in distilled water is used as the electrolyte. The specific gravity is approximately 1.2.  The construction of Ni-Cd battery is very similar to that of the nickel-iron battery.  The positive plates are of nickel hydroxide. The negative plates are made of a mixture of cadmium and iron. KOH (potassium hydroxide) is used as electrolyte in this battery.  The internal resistance of Ni-Cd batteries is low due to the use of cadmium. Electrical characteristics are almost same as those of the nickel-iron batteries.

+ -

Negative plate (Cd)

Positive plate Ni(oH) 4

Electrolyte (KOH)

Figure 5.13 Nickel cadmium battery diagram

Action during discharging

  

 

4 2

2

( ) 2 ( ) 2

2 ( )

At anode Ni OH K Ni OH KOH At cathode Cd OH Cd OH

4 ^ ^ ^2 ^  2

arg , 2 ( ) 2 2 ( ) 2 ( ) Positive Negativeplate Positive Negative plate plate plate

Disch ing action can be collectively given as Ni OH KOH Cd Ni OH KOH Cd OH

 The electrolyte i.e. KOH splits into the positive K ions and negative OH ions. During discharging, ions from negative plate force free electrons to positive plates.  When the cell is discharged the positive element becomes nickelous hydroxide while negative element becomes cadmium hydroxide. Action during charging  During charging of the cell, positive element becomes nickelic hydroxide and negative element becomes metallic cadmium.  When cell is fully charged it generates oxygen gas at positive element and hydrogen gas at negative element.

2 ^ ^2 ^4 ^ 

arg , 2 ( ) 2 ( ) 2 ( ) 2 Positive Negative Positive Negativeplate plate plate plate

The ch ing equation is collectively given as Ni OH KOH Cd OH Ni OH KOH Cd

Ajay Balar - EE Department Basic Electrical Engineering (3110005) 20

Applications of Ni-Cd Batteries:  Following are some of the important applications of Ni-Cd batteries:  In helicopters, aeroplanes as auxiliary turbines.  For the traction applications.  For lighting, air conditioning etc. in trains.  In the automobiles.  In the movie cameras, electric shavers, photoflash etc.

5.23 Charging and its methods of Battery

 To charge a lead-acid battery, the following important points should be kept in mind.  A dc source should be used for charging. If ac source is available it should be first rectified.  The dc source should be connected with proper polarities to the battery to be charged.  The dc source voltage should be higher than the maximum terminal voltage of the battery. Typically, the charging voltage should be 2.5 V per cell.  The value of charging current I should be set properly.  The charging current should not be too high in order to avoid 'excessive gassing' and heat during the charging process.  The charging current can be set by taking one of the following two rules:  The charging current should be equal to number of positive plates in a single cell.  The charging current should be such that the full charging be achieved in 8-hours. Charging of battery with AC Supply

To AC line Tranformer^ Rectifier^ Filter^ Regulator^ Battery

Vout

Vout

+

-

Iout

Figure 5.14 Charging of battery from AC supply