Electrical System Design

Suhail TP MES Collage of Engineering

Design Considerations of Electrical installations

ELECTRIC SUPPLY SYSTEM

250 V or less : LV251 V to 650 V : MV651 V to 33 kV : HVAbove 33 kV : EHV

DISTRIBUTION SYSTEM

Single phase ac supply using a 2 – wire system;Three phase ac supply using 3 – wire system;Supply of Three phase and neutral using a 4 wire system.

DC SUPPLY

A two wire system at 220 VA three wire system with 440 V between the two outer conductors and 220V between the outer conductors and the Centre wire

• SUPPLY VOLTAGE

– Single phase : 240 V, 50 Hz, 2 wire– Three phase : 415V , 50Hz, 4 wire

Consumers with load requirement more than 250 kVA are provided with supply at high voltage with a substation installed in the consumers premises where voltage is stepped down to 415/240 V

– In India and many European countries the frequency of the supply is 50 Hz– In USA 60 Hz

VOLTAGE TOLERANCES

Current carried by the electrical power distribution network varies at different times of the day

It leads to varying voltage drops in the supply cables

Under IE Rules voltage fluctuations may not vary by more than 5 % above or below the declared nominal voltage , frequency 1% from 50 Hz

CONDUCTORS AND CABLES

• All conductor must me surrounded by insulators• In over head tr air act as insulation. Insulation requirement at supports

through which lines are suspended from the ground• The conductor with insulation- A Cable• Mostly used insulation in cables are

– paper (earliest material, works as good insulator as long as it is kept dry) - Rubber ( from 1910 to 1950)- Plastics ( most common PVC,)

For cables for withstanding high temperatures we use silicon- rubbers

STRANDED CONDUCTORS

Stranded conductor consist of no of Cu or Al wires place together like a ropeStranding is employed to make the cable flexibleStranded conductor is expressed as 3/.029, 7/.026 etc first no represent no of strands and second no gives the diameter of each strand in inch or mm

RATING OF CABLES• Cables are always assigned a rating• For rubber and plastic insulated cables rating is based on the • temperature which the insulation can withstand.• When several plastic insulated cables run together in the same duct

or conduit, rating may be reduced since each will tend to heat each other

VOLTAGE DROP• Depends on current , cross sectional area of the cable, and its length• Voltage drop between supply point in any building and any outlet should not be

more than 2.5% of the nominal voltage

CRITERIA FOR SELECTING CABLE• Minimum criteria for selecting a cable

It should be able to carry maximum expected current without the insulation getting damaged• Voltage drop inside the cable should be permissible

PROTECTION OF ELECTRIC INSTALLATIONS AGAINST OVERLOAD, SHORT CIRCUIT AND EARTH FAULT

BASIC CONSIDERATIONS

1. There should be ready means of isolation of a circuit in the event of any accident or for the purpose of servicing. This is provided by means of switches

2. Conductors used should have adequate size. ie. Cross sectional area so that the current they will carry will produce minimum heat and voltage drop.

3. Protection should be provided against excess current and electric shock

Excess Currents

Rated current for a wire is that value of current which does not cause the temperature of the wire to reach a value which would damage the insulation.

Any value of current which causes the insulation to be damaged is called excess current or overload current.

• The wire size selected according to a certain expected load. • If the load exceeds this value, system will draw the excess current.• Value of current depends on the extend of overload

CAUSES OF OVERLOADS

• By switching on heating or light loads of higher power rating.• By switching on a large no of heating or lights than what the wiring designed

for.• Mechanical overloading of motors.• Mechanical faults in the motor.

• Due to large friction at bearings & Jamming of rotor.

SHORT CIRCUITS

• When two phase wires or phase wire to neutral wire make direct contact.• Fuses are used to prevent damage, fire

EARTH FAULT

• When the phase wire makes contact with earth or any conducting material which is earthed, a short circuit exist between the wire and earth. Called earth fault

• Fuses are used to interrupt large current caused by earth fault.

FUSES AND CIRCUIT BREAKERS

• When the excess current is very high the operating time is very short• When the excess current is small the operating time is longer• When short circuit occurs the resulting current will be extremely high and its

flow should be arrested in shortest possible time• When extra load is switched on which may draw say 25% higher than the rated

current.• This current will not cause any immediate damage, the protective device should

act after a time lag

FUSEIS: 732-1983A device that by fusion of one or more of its specially designed and proportional components , opens the circuit in which it is inserted when the current through it exceeds a given value for a sufficient time. The fuse comprises all the parts that form the complete• A fuse wire must have its rated current I 60 to 90 % of the critical current.• Critical current is the value of current at which the fuse melts.• A fuse wire of 5A rating will have a rating 5.6A to 8A

REWIRABLE FUSE

A re- wirable fuse consists of a length of wire made either of tinned cu or other metal of same size

It will melt and break the circuit if the current rises above the rated value

Cheap and simple in construction

Have the draw back of erratic in operation and of deterioration with time

Rated up to 200A

CATRIDGE FUSE

Fuse wire is enclosed in a sealed cartridge packed with a filler material which will prevent deterioration

a) Diazed fuse (D type cartridge fuse)b) Link type ( HRC fuse link)

D- TYPE CARTRIDGE FUSE

• Rating up to 2 to 63 amperes • Quartz is used as the filler material• Enclosed in a high compression ceramic material designed to withstand the mechanical and thermal stress.• Filler material is used for cooling the arc after the melting of fuse element• A small coloured disc on top of the cartridge fuse is used as the indicator for the fusing of a contact

HRC FUSE

• Available in rating from 2 to 1250 A• High rupturing capacity• Higher speed of operation• Operation is silent and temperature rise is low

PROTECTION AGAINST ELECTRIC SHOCK

EARTHING

Earthing means connecting earth terminals to electrodes installed solidly in the mass of earth.

The objective of earthling is to ensure that a fault to earth produces the same condition as a short circuit between the line and neutral cables.

The path of earth fault current includes the earth wire in the consumers premises and the general mass earth between the earth point and the earth electrode at the substation.

In under ground cables , the armour of the cable can be connected to earth terminal.

EARTHING AND SOIL RESISTIVITY

• Resistance to earth of an electrode of given dimensions is dependent on the electrical resistivity of the soil in which it is installed

• Earth conductivity is electrolytic in nature• It is affected by moisture content, soil chemical composition, and

concentration of salts dissolved in the contained water.• In places with high soil resistivity , the resistivity of soil immediately

surrounding the earth electrode should be reduced.• This is done by using some artificial agents• Sodium chloride, calsium chloride, copper sulphate, Common salt and soft

coke, common salt and charcoal in suitable proportion.

EARTH ELECTRODES

• Under ordinary conditions of soil use of copper, iron or mild steel electrodes is recommended

• When soil conditions are such as are likely to cause excessive corrosion of the electrode it is recommended to use either cu electrode or cu clad electrode, or galvanised iron electrode

• There are two main earth electrodes in use– Rod and pipe electrodes– Plate electrodes

ROD AND PIPE ELECTRODES

• Made of metal rod or pipe having clean surface not covered by paint, enamel, or poor conducting material

• Rod electrodes of steel or GI should be at least 16 mm dia, and that of cu should be at least 12.5 mm in dia

• Pipe electrodes GI or steel should not be smaller than an internal dia of 38 mm, If cu – 10 mm.• Min length of rod or pipe electrodes 2.5m • Pipes and rod should be driven to depth of at least 2.5m. • In rocky area if buried inclined to vertical depth should be 2.5m and inclination

not more than 30 degree from vertical• If it is necessary to reduce the depth, it must be done without reducing the

resistance• This can be achieved by connecting no of pipes or rods in parallel• In such case distance between two electrodes should preferably be not less than

twice the length of the electrodes.

PLATE ELECTRODES

• GI or steel thickness not less than 6.30 mm• If Cu thickness not less than 3.15 mm• The size of plate electrode should be 60 cm X 60 cm• Plate electrode should be burried such that the top edge is at a depth of

not less than 1.5m below the surface of the ground• If plates are used in parallel two plates being separated from each other

by not less than 8.0m

Circuit Breaker

• A circuit breaker is a manually or automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit.

• Its basic function is to detect a fault condition and interrupt current flow.

• Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.

• Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

Operation• All circuit breakers have common features in their

operation.• Once a fault is detected, contacts within the circuit

breaker must open to interrupt the circuit.• Mechanically-stored energy (using something such

as springs or compressed air) or energy from fault current itself are used to separate the contacts.

• When a current is interrupted, an arc is generated

Arc interruption• Lengthening / deflection of the arc• Intensive cooling (in jet chambers)• Division into partial arcs• Zero point quenching (Contacts open at the zero

current time crossing of the AC waveform, effectively breaking no load current at the time of opening. The zero crossing occurs at twice the line frequency, i.e. 100 times per second for 50 Hz and 120 times per second for 60 Hz AC)

Types of CB

Based on medium in which arc is formed, CB can be classified as • Air circuit breaker• Oil Circuit breaker• Vacuum circuit breaker

Air circuit breaker• Circuit breaker which operates in air at atmospheric pressure.• It is used below 15 kV• Arc voltage is defined as the minimum voltage required in maintaining the

arc• ACB try to increase the arc voltage in three different ways

• By cooling arc plasma• By lengthening the arc• By splitting up the arc into a number of series arcs.

• ACB are two types– Plain air CB– Air blast CB

Operation of ACB• Force arc into contact with as large an area as possible of insulating

material.• air circuit breaker is fitted with a chamber surrounding the contact. This

chamber is called 'arc chute‘.• High temperature plastics reinforced with glass fibre and ceramics are

preferable materials for making arc chute.• lengthening the arc path, is achieved concurrently with fist objective.• Inner walls of the arc chute is shaped in such a way that the arc is not

only forced into close proximity with it but also driven into a serpentine channel projected on the arc chute wall.

• The main arc chute is divided into numbers of small compartments by using metallic separation plates. These metallic separation plates are actually Splitting up the arc into a number of series arcs

Air Blast Circuit Breaker

• These types of air circuit breaker were used for the system voltage of 245KV, 420KV and even more,

• The breaking speed of circuit breaker is much higher during operation of air blast circuit breaker.

• Arc quenching is much faster during operation of air blast circuit breaker.• The duration of arc is same for all values of small as well as high currents

interruptions.• Requires much less maintenance compared to oil circuit breaker.

AXIAL BLAST ACB

What is MCB?• Nowadays we use more commonly Miniature Circuit Breaker or MCB in low

voltage electrical network instead of fuse

• It automatically switches off the electrical circuit during abnormal condition of the network means in over load condition as well as faulty condition. The fuse does not sense but Miniature Circuit Breaker does it in more reliable way. MCB is much more sensitive to over current than fuse.

• The switch operating knob comes at its off position during tripping, the faulty zone of the electrical circuit can easily be identified. But in case of fuse, fuse wire should be checked by opening fuse grip or cut out from fuse base, for confirming the blow of fuse wire.

• Quick restoration of supply can not be possible in case of fuse as because fuses have to be rewirable or replaced for restoring the supply. But in the case of MCB, quick restoration is possible by just switching on operation.

• Handling MCB is more electrically safe than fuse.• Only one disadvantage of MCB over fuse is that this system is more costlier

than fuse unit system.

Working Principle Miniature Circuit Breaker

• There are two arrangement of operation of miniature circuit breaker.• By thermal and electromagnetic effect of over current• The thermal operation of miniature circuit breaker is achieved with a

bimetallic strip whenever continuous over current flows through MCB, the bimetallic strip is heated and deflects by bending. This deflection of bimetallic strip releases mechanical latch. As this mechanical latch is attached with operating mechanism, it causes to open the miniature circuit breaker contacts.

• During short circuit condition, sudden rising of electric current, causes electromechanical displacement of plunger associated with tripping coil or solenoid of MCB. The plunger strikes the trip lever causing immediate release of latch mechanism consequently open the circuit breaker contacts. This was a simple explanation of miniature circuit breaker working principle.

Operation of MCB• There are three mechanisms provided in a single miniature circuit breaker

to make it switched off.

• Main parts are one bi - metallic strip, one trip coil and one hand operated on - off lever.

• Electric current carrying path of a miniature circuit breaker is as follows. First left hand side power terminal - then bimetallic strip - then current coil or trip coil - then moving contact - then fixed contact and - lastly right had side power terminal.

Operation of MCB• If circuit is overloaded for long time, the bi - metallic strip becomes over

heated and deformed.• This deformation of bi metallic strip causes, displacement of latch point.

The moving contact of the MCB is so arranged by means of spring pressure, with this latch point, that a little displacement of latch causes, release of spring and makes the moving contact to move for opening the MCB.

• The current coil or trip coil is placed such a manner, that during short circuit fault the mmf of that coil causes its plunger to hit the same latch point and make the latch to be displaced. Hence the MCB will open in same manner.

• The current coil or trip coil is placed such a manner, that during short circuit fault the mmf of that coil causes its plunger to hit the same latch point and make the latch to be displaced. Hence the MCB will open in same manner.

• In the manual operation, actually the same latch point is displaced and same deformed spring is released