EE04 801: ELECTRICAL SYSTEM DESIGN AND ESTIMATION

ELECTRICAL SYSTEM DESIGN AND ESTIMATION

The Electrical Age was born in 1881 when Thomas Edison invented the incandescent lampK.N. Chandra BoseAsst.ProfessorElectrical Engg. Dept.Govt. Engg. College, ThrissurCalculation of length of 25 mm dia. conduit pipe:From MB to HR= 1.5 mFrom SB1 to HR= 1.5 mFrom SB2 to HR= 1.5 mFrom HR to L1, L2 & F1= 0.6 + 2.4 + 2.4 + 1.2 + 2.4 + 1.2 = 10.2 mFrom MB to SB1(HR)= 2 mFrom HR to SB 2= 1.5 + 0.25= 1.75 mFrom HR to L4, L5, F2 & L3 = 0.6+ 0.6 + 1.35 + 1.35 + 0.6 +0.25= 4.75 mFrom HR between Sb2 and L4= 1.8 mTotal= 25 m10% wastage= 2.5 m= 27.5 m say 28 m

78Light/ fan/ 5 A socket points wire (1 Sq. mm)Phase wire:From SB1 to F1, L1 & L2= (1.5 + 0.6 +2.4) x 3 + (2.4 +0.6)x 2 = 19.5 mFrom SB2 to F2, L3, L4 & L5= (1.5 +1.8) x 4 + 0.6 + 0.25 + (1.35 +0.6) x 2 +(1.35 +0.6) = 19.675 mTotal= 39.175 m

78ELECTRICAL SYMBOLS AND DIAGRAMSEach and every part of electrical installation is explained by symbols.Which are very necessary to convey exact meaning to the peoples who read the drawingSymbols are standardized by Bureau of Indian StandardsElectrical Diagrams:The diagrams are classified according to the purpose and the method of representation.Classification of diagrams according to purpose:Wiring diagram: a wiring diagram shows the connections of an installation or part of an installation.It shows how the connections are actually made.

IS 2042 (part-1)-1962 defines a wiring diagramthe wiring diagrams are intended to guide the execution and checking of the internal and external or both connections of an installation or part of the installation.Which also gives the layout of various parts and accessories of wiring.Circuit diagram (schematic): This is an explanatory diagram meant for easy understanding of the operation of an electric circuit.The diagram is arranged in such a way that as far as possible every circuit is drawn in a straight line in order that it may easily be followed.Therefore the diagram should be as simple and clear as possible.IS:2032(part-1)-1962 defines a circuit diagram as an explanatory diagram indented to facilitate the understanding of the details of the operation.

Example:

Important points to be noted while drawing a schematic diagramThe supply line should be drawn horizontal while the different circuit connections should be drawn vertical. Branched circuits of equal importance should be drawn similar to one another.Switches, push buttons and contacts should be drawn in non-actuated position.When there is a sequence of operations the circuit which is actuated first should be drawn more to the left.Crossing of lines should be avoided as far as possible.The phase line must be drawn left pole of the switch.

Design considerations:Electrical installation consists various sorts of controls at various stages.Main switch and distribution board are kept at the place where the KSEB line enters.Other controls, switches and regulators in a particular room for controlling lights, plug points and fans.These must be kept in an appropriate panel boards as per the regulations.The panel board selection is also depend on the type of equipments and nature of the supply given.The panel for the distribution board may be made of well prepared teak wood or iron sheet.The energy meter and other associated main control circuits are mounted on them.The design of a panel board involves, selecting proper material of appropriate dimensions and calculating desired number of switches, regulators and sockets in such a manner as to give good appearance.For large building wirings, number of sub-circuits are incorporated in addition to the main distribution board.The number of circuits and sub-circuits are decide as per number of points to be wired and load to be connected.Fluorescent lamp - 40WBulb, fans, socket outlets 60WPower socket outlets 1000WExhaust fans - as per the capacity

In each sub-circuit, the number of points are limited by the load, max of 750W.Two methods of wiring known as 1. Joint box or Tee system2. Loop systemIn joint box system, the connection to the lamps are made through joints made in joint box, which will be somewhere in horizontal circuit path.This method is not preferred much now a days owing to its draw backs.Extra cost for joint boxReduce flexibility

Loop- in system:Universally employed methodThe switch and light feeds are carried around the circuit in a series of loops from one point to another until the last point on the circuit is reached.The phase or line conductors are looped either in switch board or box and neutrals are looped either in switch board or from the points (light, fan etc)In no case joint is to be made in the run of wire

MeritsJoint box is removedFault identification is easyMore secure than joint box typeNo joints are concealed beneath the floor and in roof spacesDemerits:More wire is required which leads to more dropsLoop in switches and lamp holders is usually difficult.

SYSTEM OF WIRING: Internal wiring in our countryCleat wiringWooden casing and capping wiringCTS or TRS wiringLead sheathed wiringConduit wiringSurface or open typeConcealed type1.Cleat wiring:- Cables used are of VIR or PVC type.Cables are held by porcelain cleats of 6mm.Cleats are made in two halves (base & cap)The whole is then screwed on wooden-plugs previously cemented into the wall.

In this type of wiring, insulated conductors (usually VIR, Vulcanized Indian Rubber) are supported on porcelain or wooden cleats. The cleats have two halves one base and the other cap. The cables are placed in the grooves provided in the base and then the cap is placed. Both are fixed securely on the walls by 40mm long screws. The cleats are easy to erect and are fixed 4.5 15 cms apart. This wiring is suitable for temporary installations where cost is the main criteria but not the appearance13To ensure longer life, cleats must be used at intervals of 30cm and in no case at more than 60cm.Advantages:Cheapest system of internal wiringInstallation and removal is easy and quickMaterial is recoverable after the dismantlementInspection, addition and alterations can be easily madeSkill required is little.Disadvantages:Not good looking Wires are exposed to mechanical injury or damageAny damage to the insulation directly leads to electric shock.

2. Wooden casing and capping wiring:The cables used in this type of wiring is either VIR, PVC or any other approved insulated cables.The cables runs through the wooden casing enclosures.Casing consists of V- shaped grooves.

14Capping is screwed to casing by means of 13mm 4 wooden screws fixed at every 15cm.Two or three cables of same polarity may be run in one groove and in no case the cables of opposite polarity.Suitable for low voltage domestic- installations in dry places and where- there is no risk of fire hazardsAdvantages:Cheap in costEasy to install and rewireProvides good insulation as conductors are good distance apart.Easy to inspect the wiring by opening the capping. Disadvantages:Risk of fireRequires workmanship, so the labour cost is more.

Permanently fixed casing

15Precautions to be taken:The casing-capping used must be of first class seasoned straight, smooth and hard teak wood.Cable of opposite polarity should not run in one grove.Casing must be kept at least 3.2mm apart from the walls or ceilings by means of porcelain pieces of thickness not less than 6.5mm in order to keep the casing dry at the back.Gaps should be avoidedAt bends the grooves must be rounded off in order to save insulation of cable from damage.The casing and capping must be painted with the paint as specified in IS.732 before erection. The wood can of chemically treated to prevent from white ant attack, damage due to aging and other environmental reasons.16CTS or TRS wiringThe cable used must of single core, two core or three core TRS cables with a circular oval shape.TRS cables are sufficiently chemical proof, steam proof but are slightly affected by lubricating oils. TRS cables run on well seasoned, perfectly straight and varnished teak wood batten of thickness 10mm at least.The cables are held on the wood batten by means of tinned brass link clips already fixed on the batten with brass pins.The whole structure is painted as specified in IS 732

CTS ( Cable Tyre Sheathed) / TRS ( Tough Rubber Sheathed ) / Batten wiring: In this wiring system, wires sheathed in tough rubber are used which are quite flexible. They are clipped on wooden battens with brass clips (link or joint) and fixed on to the walls or ceilings by flat head screws. These cables are moisture and chemical proof. They are suitable for damp climate but not suitable for outdoor use in sunlight. TRS wiring is suitable for lighting in low voltage installations

17Merits:Installation is easy and quickLong lifeFire proof up to certain levelAbsolutely free from chemical reactions.Demerits:Good workmanship is requiredCannot be recommended in all palaces, such as open to sun and rain falls.Field of application:- commonly used for low voltage wiring and is extensively used for lighting in domestic, commercial or industrial buildings except workshopsLead sheathed wiring:This type of wiring employs conductors insulated with VIR and is covered with an outer sheath of lead- aluminium alloy containing about 95% lead.Metal sheath give protection to the cable from corrosion, dampness, and mechanical injury.The lead covering is made electrically continuous and is connected to earth at the point of entry.These cables run on wooden batten and fixed by means of linked clips.Merits:Provides better protectionEasy to fixLong lifeFree from electric shock

Metal Sheathed or Lead Sheathed wiring : The wiring is similar to that of CTS but the conductors (two or three) are individually insulated and covered with a common outer lead-aluminum alloy sheath. The sheath protects the cable against dampness, atmospheric extremities and mechanical damages. The sheath is earthed at every junction to provide a path to ground for the leakage current. They are fixed by means of metal clips on wooden battens. The wiring system is very expensive. It is suitable for low voltage installations.

Lead is a main-group element in the carbon group with the symbol Pb (from Latin: plumbum) and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed to air. Lead has a shiny chrome-silver luster when it is melted into a liquid.Lead is used in building construction, lead-acid batteries, bullets and shots, weights, as part of solders, pewters, fusible alloys and as a radiation shield. Lead has the highest atomic number of all of the stable elements.19Demerits:Good workmanship is requiredIt is costlier than TRS wiringIt is not suitable for places where chemical corrosion may occur.Skilled labour and proper supervisions is required.

Precautions to be taken during installationThe clips used to fix the cables on battens should not react with the sheath.Lead sheath should be properly earthed to prevent shocks due to leakage currents.Cables should not be run in damp places and in areas where chemicals (may react with the lead) are used.

20Conduit wiring: Steel tubes are installed on the surface of walls by means of saddles and VIR or PVC cables are drawn into afterwards by means of a GI wire of about 18swg.The conduits must be electrically and mechanically continuous and connected to earth at some suitable point.Conduit size is stated in terms of its outer diameter.The smallest size of conduit is 12mm, and is seldom used in practice except in cheap competitive work.

In general the conduits can be classified as:Light gauge steel plainHeavy gauge steel screwed conduitFlexible conduitPVC conduit

Conduit wiring: In this system PVC (polyvinyl chloride) or VIR cables are run through metallic or PVC pipes providing good protection against mechanical injury and fire due to short circuit. They are either embedded inside the walls or supported over the walls, and are known as concealed wiring or surface conduit wiring (open conduit) respectively. The conduits are buried inside the walls on wooden gutties and the wires are drawn through them with fish (steel) wires. The system is best suited for public buildings, industries and workshops21Advantages:Protection against mechanical damageFire protectionProvides water proofShock proof if proper earthing has been done.

Disadvantages: Very costlyDifficult in erection and requires more timeExperienced and skilled persons are requiredAdvantages:No risk of fire and good protection against mechanical injury.The lead and return wires can be carried in the same tube.Earthing and continuity is assured.Waterproof and trouble shooting is easy.Shock- proof with proper earthing and bondingDurable and maintenance freeAesthetic in appearanceDisadvantages:Very expensive system of wiring.Requires good skilled workmanship.Erection is quiet complicated and is time consuming.Risk of short circuit under wet conditions (due to condensation of water in tubes).

22DESIGN CONSIDERATIONS:

FACTORS AFFECTING THE CHOICE OF WIRING SYSTEM:Depends on technical factors and economic viability1. Durability: Type of wiring selected should conform to standard specifications, so that it is durable i.e. without being affected by the weather conditions, fumes etc.2. Safety: The wiring must provide safety against leakage, shock and fire hazards for the operators. 3. Appearance: Electrical wiring should give an aesthetic appeal to the interiors. contd4. Cost: It should not be prohibitively expensive.5. Accessibility: The switches and plug points provided should be easily accessible. There must be provision for further extension of the wiring system, if necessary.6. Maintenance Cost: The maintenance cost should be a minimum7. Mechanical safety: The wiring must be protected against any mechanical damage.For the sake of completeness, the following points to beConsidered.Specification of Wires:conductor material, insulation, size and the number of coresStandard values of voltagesVoltage limits for AC systemDistance from Electric LinesWiring Installations Standard values of voltages:The standard values of voltages given in IS: 585 1962.For a single phase 2- wire system of AC, the standard voltage shall be 240V.Three phase system it may be:-415 V, 3.3 kV, 6.6 kV, 11 kV, 22 kV, 33 kV, 66 kV, 110 kV, 132 kV, 220 kV and 400 kV The standard dc voltage shall be 220 / 440 V

Voltage limits for AC system:The voltage at any point of the system under normal conditions shall not depart from the declared voltage level by more than the values given;

6% in the case of low voltage (250V or less) and medium voltage (251 to 650 V)6% in the higher side or 9% on the lower side in case of High voltage (651 V to 33 kV)12.5% in case of Extra High voltage ( above 33 kV)

Distance from Electric Lines:

No building shall be allowed to be erected or re- erected, or any additions or alterations made to the existing building unless the following minimum clearances are provided from the over head electric supply lines.

Voltage rangeVertical (m)Horizontal (m)(a) Low & medium Voltage lines2.51.2(b) High voltage linesUp to 11 kV3.71.2Above 11 kV up to33 kV3.72.0(c) Extra high voltage lines3.72.0Note:- For extra high voltage lines apart from the minimum clearance indicated, a vertical and horizontal clearance of 3.0 m from every additional 33 kV or part thereof shall be provided.

Wiring Installations:

A major portion of the fixed installation design in a building relates to wiring installation.

Fittings and Accessories

Reception and Distribution of Main Supply

Conduit wiring1. Fittings and AccessoriesA ceiling rose or any other attachment shall not be used on a circuit.Each socket outlet shall be controlled by a switch which shall preferably be located very near to the socket.Ordinary socket outlet may be fixed at any convenient position at a height above 20 cm (minimum) from the floor level.

In a place where the socket outlet is reachable to children, provide plug socket with shutter facility.

30In an earthed system, the third pin must be connected to earth terminal.All lamps unless otherwise specified shall be hung at a height of not less than 2.5 m above floor level.Unless otherwise specified, the clearance between the bottom most point of the ceiling fan and the floor shall be not less than 2.4 m. The minimum clearance between the ceiling and the plane of the blade shall be not less than 30 cm.

31Reception & Distribution of Main Supply:

A circuit breaker or a linked switch with fuse on each live conductor at the point of entry is mandatory.The main switch is easily accessible and situated near to the termination of service line.Distribution Box (DB) must be provided with fuse unit or MCB (miniature circuit breaker) of adequate rating.

Lights and fans may be wired on a common circuit. Sub-circuit load is limited by max. of 10 points or 800 watts (which ever is lower).Power sub-circuit shall be designed according to the load but in no case shall there be more than two 15 A outlets on each sub-circuit.Load on low voltage sub-circuit should not exceed 3000W and a provision for 20% increase is expected.

Distribution board shall be fixed at the load centre if possible.The DB shall not be above 2 meter from floor level.Wiring conductor can be either copper or aluminium.Minimum cross sectional area for light sub-circuit shall be not less than 1mm2 and 1.5mm2 for copper and aluminium respectively.For power circuit, it shall be not less than 2.5mm2 and 4mm2 for copper and aluminium respectively.For flexible cord, the minimum cross sectional area shall be 0.5 Sq. mm copper.

Conduit wiring:Rigid non-metallic conduits are used for surface, recessed and concealed conduit wiring. Conductors of ac supply and dc supply shall be bunched in separate conduits.Maximum permissible number of 1.1 kV grade single core cables may be drawn through conduits.

Conduit shall be fixed by saddles secured to suitable wood plugs or other plugs with screws at an interval of not more than 60 cm. whenever necessary, bends or diversions may be achieved by bending the conduits or by employing normal bends, inspection bends, inspection boxes, elbows or similar fittings.

36Size of cableSize of conduit (mm)cross sectional area (Sq. mm)Number and diameter (in mm) of wires162025324050Number of cables, maximum11/1.12571320--1.51/1.40461014--2.53/1.06351014--47/0.852361014-67/1.40-25911-107/1.40--479-167/1.70--24512257/2.24---226357/2.50----255019/1.80----23ELECTRIC SUPPLY SYSTEM:Power or electricity is generated at power station, which is located at far away from the consumer.Thus power supply is done in two stages.Transmission Distribution Generation is in three-phase, 50Hz, usually at 11kVStepped up to 132,220 or 400kVAt secondary sub-stations stepped down to 66 or 33kVAgain down to 11, 6.6 or 3.3kV and then to 415V415V to 240V, three phase four wire distribution.The power stations should preferably be situated near the natural sources of energy. eg. Hydro-power stations are installed at places where large quantity of water at high head is available. Coal firing stations near the coal mines and also at places where constant supply of coal can be maintained economically. Another reason to construct stations away from big cities to keep the atmosphere free from pollution.

After stepped upto 132, 220 or 400kV, which is transmitted through 3 wire system and stepped down to 66 or 33kV. Further it is transmitted via 3 wire system and down to 11, 6.6 or 3.3kV and then to 415V.38Small consumers are supplied at 415/240VLarge consumers are supplied at 11, 6.6 or 3.3kV. Such consumers has to provide there own sub-stations.

250V or less:Low voltage251 to 650V:Medium voltage651 to 33kV:High voltageAbove 33kV:Extra high voltage

Small consumers are supplied at 415/240V where they can receive 3 phase and single phase supply. Very small domestic consumers are often provided with single phase supply at 240V. This called secondary transmission or low voltage distribution system. Large consumers are supplied at 11, 6.6 or 3.3kV. Such consumers has to provide there own sub-station to step-down the voltage to 415/240V. This high voltage transmission is called primary transmission.

39

403-, 4 wire distribution system:

Electric power in India is supplied to the consumers by the State Electricity Boards.Standardization of voltage and frequency is extremely important.Supplier and consumer must aware about balanced load on the system. Consumer with load demand more than 250kVA are provided with high voltage supply with substation installed in the consumers premises.For large consumers it is economical.The substation installed in the consumers premises step down the voltage to 415/240V. For large consumers it may be economical to purchase power from the board at high voltage and install his own substation and distribution system. Such a consumer should discuss this first with the board and get details of any special tariffs that may be available before finalizing the project.41Points to be considered:Supply voltageVoltage toleranceConductors and cablesRating of cablesVoltage drop

Under the Indian electricity rule, the voltage fluctuation may not vary by more than 5% above or below the declared voltage and the frequency must be within plus or minus 1% of the declared value.Rubber insulation has been replaced by PVC from 1950.

Under the Indian electricity rule, the voltage fluctuation may not vary by more than 5% above or below the declared voltage and the frequency must be within plus or minus 1% of the declared value.Supply voltage should be as follows:- Single phase: 240V, 50Hz, 2 wireThree phase:- 415V, 50Hz, 4 wire In USA the supply frequency is 60 HzThree phase system consists 4 wires, which are 3 phase line and one neutral wire which is expected to be at zero potential. This wire is normally earthed at substation.Voltage tolerance should be within 5% above or below the declared normal voltageConductors:- All conductors must be surrounded by insulation. Rubber were used as insulator from 1910 to 1950, but recently it was replaced by PVC plastic. Stranded cables are expressed as 3/0.029, 7/0.036 etc. Where numerator represents number of strands in the cable and denominator represents the diameter in inch or mm.Rating of cables:- cables are assigned with a rating, normally it is based on temperature. The cables run through the conduit or ducts must withstand the rise in temperature from all. The rise in temperature reduce its rating as time elapse. The rating of cables are given in IS: 3961.Voltage drop:- The minimum requirement for selecting cable size is that it should be able to carry maximum expected level of current. However this is not only the criteria for cable size selection. The main consideration is on the effective length of wire from the supply side to outlet. Because the voltage drop not only upon the current and cross-sectional area of cable. For short length, the nearest size as per the calculation of current can be selected. But for long length, it is necessary to select 1 or 2 size larger than the minimum rating. Any how the drop of voltage shall not exceed 2.5% of normal voltage.42Cables rating is based on temperature. The rating of cables are given in IS: 3961.

The selected cable size should be able to carry maximum expected level of current. The main consideration is on the effective length of wire. For short length, the nearest size as per the calculation of current can be selected. For long length, it is necessary to select 1 or 2 size larger than the minimum rating. The drop of voltage shall not exceed 2.5% of normal voltage.43 PROTECTION OF ELECTRIC INSTALLATIONPROTECTIVE DEVICESProtection for electrical installation must be provided in the event of faults .The protective circuit or device must be fast acting .FusesRelaysMiniature circuit breakers (MCB)Earth leakage circuit breakers (ELCBBasic consideration:Ready means of isolation of a circuit in the event of accident. Conductors used should have adequate sizeProtection should be provided against excess current and electric shock.

Protection for electrical installation must be provided in the event of faults such as short circuit, overload and earth faults. The protective circuit or device must be fast acting and isolate the faulty part of the circuit immediately. It also helps in isolating only required part of the circuit without affecting the remaining circuit during maintenance. The following devices are usually used to provide the necessary protection:FusesRelaysMiniature circuit breakers (MCB): MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit. MCCB:- molded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Under voltage and under frequency may be inbuilt. Normally it is Used where normal current is more than 100A.Earth leakage circuit breakers (ELCB):- ELCB is a 4-pole one, all phase currents and any current returning through the neutral will go through the summation CT in the ELCB...as long as there is no leakage to earth, it will not trip, since the vector sum of the four currents is always zero.

Every electrical installation and part of an installation should take following conditions: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.Conductors used should have adequate size, so that the current they will produce minimum heat and voltage drop.Protection should be provided against excess current and electric shock.

44Against Excess currentAgainst short circuitAgainst earth faultFuses and circuit breakerElectric shockProtection against over currentAny current beyond the rated value is called excess current.Causes can be overloads, short circuit, earth fault etc.Over currents causes excessive heat.

Protection against over current:- Rated current for a wire is that value of current which does not causes the temperature of the wire to reach a value which would damage the insulation. The wire size of the different parts of an electrical installation are selected according to certain expected loads. Over loads creates excessive current in the system, which will create more I2R heat that damages the insulation of the cable. Which leads to circuit between phases or phase and neutral or phase and ground etc. the purpose of over current protection is to provide a safeguard so that power supply to the circuit will be interrupted before any excess current may damage the system.

45Protection against short circuit: Short circuit may be due to insulation failure or direct contact of phase and neutral line.Fuses or circuit breakers can be used as protector.

To prevent the damage to the wire of the circuit and to prevent out break of fire, fuses or circuit breakers are provided. By blowing the fuse element during over current, the supply can be removed from the service.

ELCB:- An Earth Leakage Circuit Breaker - ELCB for short - was the first name given to what is now called a Ground Fault Circuit Interrupter (GFCI) .

Read more: http://wiki.answers.com/Q/How_does_an_ELCB_work#ixzz1HK36Ws00Why was that important?If there is no fault anywhere in a circuit supplying single-phase alternating power to a unit such as an electrical appliance, machine or other equipment, the current flowing to the unit at any instant in the "hot" or "live" wire should exactly match the current flowing away from the unit in the neutral wire. Similarly, there should be no current flowing in the unit's safety "ground" or "earth" wire.

It is a basic fact of electrical engineering design that all current flowing to an electrical appliance, machine or other equipment from the power generation station via its supply circuit's "hot" or "live" wire should only return to the power station via that same circuit's "neutral" wire. So, as a result of that basic fact, if any current is flowing in the ground wire, it must be caused by a fault condition and the supply of current to the circuit needs to be stopped urgently.An ELCB works by comparing the input current in the "hot" line feeding a load and the return current in the "neutral" line coming back from that loadvery small difference between the input and return currents is what the ELCB is designed to detect and cause it to trip so as to break the flow of current to the load.

46Protection against Earth fault:

Phase wire making contact with earth conductor leads to heavy current flow.For domestic installations fuse can act as protector.High power installation high ratings of fuse or earth fault relay can be usedELCB is the best device for earth fault protection.Very small difference between the input and return currents is what the ELCB is designed to detect.

The importance of earthing

48

Case I When the supply switch S is closed current flows only through the healthy apparatus. No current flows through the person and does not experience a shock.Case II When the supply switch S is closed with defective insulation and apparatus not earthed, a part of the current flows through the body and the personexperiences electrical shock.Case III When the supply switch S is closed with Defective insulation and apparatus being earthed though a small part of the current flows through the body the person will not experience a shock50Electric Shock InjuriesWhat happen when you get the electric shock ? Effects of the shocks:> 1 mA * A normal person will feel a slight tingling sensation.> 5 mA *A mild shock will be felt, but most persons will be able to let go. Not a painful feeling, but definitely disturbing. However, a strong reflexive movement by the victim can cause further accidents and other type of injuries.

>6 to 30 mA*The victim can be paralyzed, or the muscles will freeze (will not be able to release a tool, wire, or other object. Painful, and may not go.At high voltage (above 600 Volt, this current can already cause severe burns).*Women start to suffer the effect at lover current levels (6-26mA), while men can sustain until a bit higher (10 to 30 mA)

>30 mAWill cause respiratory paralysis(The victim stops breathing for a period of time)

Defibrillator in use> 30 mAThis is the most sensitive rating of Earth Leakage Circuit Breakers (ELCB) normally installed in residential home in this country.> 50 to 150 mAThe victim get an extremely painful shock. The breathing stops (respiratory arrest).Severe muscle contraction: flexor muscles may cause holding on, extensor muscles may cause intense pushing away. Death is possible.(At 75 mili-Ampere and above The victim undergo ventricular fibrillation (very rapid, ineffective heartbeat). This condition can cause death within a few minutes. The only way to save the victim is by a special device called defibrillator.)> 1 A and aboveUneven heartbeats occurs (Ventricular fibrillation).The muscles will contract. Damage to the nerves. Death is likely.> 4 AThe victim gets heart paralysis, which means the heart stops pumping.> 5 A and aboveHuman tissues get burned.> 10 A and aboveCardiac arrest and severe burns. Death is probable.> 13 AThe lowest current a typical plug fuse will blow in a socket plug supply connection.> 15 ALowest level of current a normal circuit breaker or fuse will trip at a home distribution board, or a house electrical panel.

Fuses and circuit breakers:

Ideal protective device should be expected to break the circuitAny circuit take initial high current for a fraction of second while switch onSoon it falls to normal value.The severity of high current mealy depends on the time they exists.If high current occurs and persists for a very small interval of time, then it dont affect the system.An important protective device used in electrical installation is the fuse. A fuse wire must have its rated current I 60 to 90% of the critical current I. critical current being the value of current at which the fuse melts. Eg. A fuse wire of 5A rating will have a critical value of 5/0.9 to 5/0.6, ie. 5.6A to 8A.55Residual Current Circuit Breakers (RCCB)In the event of a short to earth the residual current operated system prevents shock-hazard.RCCB monitors the difference between the load currents flowing through it and ensures that it is approximately zero.RCCB operate when the fault current is above residual operating current setting.

In the event of a short to earth the residual current operated system prevents shock-hazard voltages from remaining on the body of Safety Class 1 electrical equipment or machines. For this purpose, the residual current circuit breaker(RCCB) monitors the difference between the loads flowing through it and ensures that it is approximately zero. If, as a result of an earth fault, this difference exceeds the circuit breakers residual operating current rating , then the RCCB will disconnect the faulty equipment. 56Connection Diagram for Single Phase

To use the 4 Pole RCCB in 240V AC single phase network, connect the Neutral at N and Phase at terminal 1 of the RCCB. Confirm the functionality of the RCCB with the help of the Test Push Button. The RCCB should trip when the Test Push Button is pressed.

For this purpose, the residual current circuit breaker(RCCB) monitors the difference between the loads flowing through it and ensures that it is approximately zero. If, as a result of an earth fault, this difference exceeds the circuit breakers residual operating current rating , then the RCCB will disconnect the faulty equipment.

1.2 Additional protection in the event of direct contactIn order to provide additional protection in the event of direct contact with(unearthed) live components, extremely sensitive RCCBs with a rated residual operating current of 30mA or less(< 30mA) are employed in place of more conventional RCCBs with higher residual operating current ratings. In view of this increased range of protection, the VDE standards specify that either an RCCB, as per VDE 0664 Part 1, or an RCCB/MCB, as per VDE 0664 Part 2, with | 30mA must be provided when installing machinery or equipment in areas with particularly high accident risk.This applies for example to the following: socket outlet circuits in rooms with bath or shower (VDE 0100 Part 701)caravans,boats,yachts and their power supply on camping and berthing sites(VDE 0107 Part 721)rooms used for medical purposes(VDE 0107).

57 Earthing:The potential of the earth is considered to be at zero for all practical purposes. The body of any electrical equipment is connected to the earth by means of a wire of negligible resistance.Copper plate is placed at a depth of 2.5 to 3 meters from the ground level

The potential of the earth is considered to be at zero for all practical purposes as the generator (supply) neutral is always earthed. The body of any electrical equipment is connected to the earth by means of a wire of negligible resistance to safely discharge electric energy, which may be due to failure of the insulation, line coming in contact with the casing etc. Earthing brings the potential of the body of the equipment to ZERO. The body of the electrical equipment is not connected to the supply neutral because due to long transmission lines and intermediate substations, the same neutral wire of the generator will not be available at the load end. Even if the same neutral wire is running it will have a self-resistance, which is higher than the human body resistance. Hence, the body of the electrical equipment is connected to earth only. The earth wire is usually connected to a copper plate placed at a depth of 2.5 to 3meters from the ground level.

58The earth resistance is affected by the following factors:Material properties of the earth wire and the electrodeTemperature and moisture content of the soilDepth of the pitQuantity of the charcoal used

Methods of earthing:1. Plate earthingCopper plate of 60cm x 60cm x 3.18cm or a GI plate of the size 60cm x 60cm x 6.35cmThe plate is placed vertically down inside the ground at a depth of 3m .Alternate layer of coal and salt for a thickness of 15cm.

Methods of Earthing:The important methods of earthing are the plate earthing and the pipe earthing. The earth resistance for copper wire is 1 ohm and that of G I wire less than 3 ohms. The earth resistance should be kept as low as possible so that the neutral of any electrical system, which is earthed, is maintained almost at the earth potential. The typical value of the earth resistance at powerhouse is 0. 5 ohm and that at substation is 1 ohm.Plate earthing (plate electrode (IS:3043-1966)In this method a copper plate of 60cm x 60cm x 3.18cm or a GI plate of the size 60cm x 60cm x 6.35cm is used for earthing. The plate is placed vertically down inside the ground at a depth of 3m and is embedded in alternate layers of coal and salt for a thickness of 15 cm. In addition, water is poured for keeping the earth electrode resistance value well below a maximum of 5 ohms. The earth wire is securely bolted to the earth plate. A cement masonry chamber is built with a cast iron cover for easy regular maintenance. When resistance of one plate is higher than the required value, two or more plates of same size should be used in parallel, and is being separated from each other by not less than 8m.

2. Pipe earthing

59

Measurement Type of ElectrodesRodPipeStripRound conductorPlateDiameter(not less than)16 mm (Steel or GI)12.5 mm (copper)38 mm (Steel or GI)100 mm (Cast Iron)Length/ Depth of burial (not less than)2.5 m (ideal 3 to 3.5 m)2.5 m0.5 m1.5 m1.5 mSize--25 x 1.60 mm (copper)25 x 4 (Steel or GI)3.0 Sq. mm (copper)6 Sq. mm (Steel or GI)60 x 60 cmThickness----6.30 mm (copper)3.15 mm (Steel or GI)Standard earth electrodes Design of earth electrode:Earth resistivity, where S = distance between successive electrode in m, R = earth megger reading in

Permissible current density for 3 sec; For Copper = 118 A/mm2Aluminium = 73 A/mm2Steel (GI) = 46 A/mm2Current density permissible at an earth electrode,

where, t = duration of fault current (3 sec)

62For plate electrode, electrode resistance:

where, A= area of both sides of plate in m2

Pipe EarthingGI pipe of 38mm in diameter and length of 2.5m (depending on the current).12mm holes, placed upright at a depth of 3.75m area (15 cms) surrounding the GI pipe is filled with a mixture of salt and coal.GI earth wires of sufficient cross- sectional area are run through a 12.7mm diameter pipe (at 60cms below) from the 19mm diameter pipe.

Pipe Earthing

Earth electrode made of a GI (galvanized) iron pipe of 38mm in diameter and length of 2m (depending on the current) with 12mm holes on the surface is placed upright at a depth of 3.75m in a permanently wet ground. To keep the value of the earth resistance at the desired level, the area (15 cms) surrounding the GI pipe is filled with a mixture of salt and coal.. The efficiency of the earthing system is improved by pouring water through the funnel periodically. The GI earth wires of sufficient cross- sectional area are run through a 12.7mm diameter pipe (at 60cms below) from the 19mm diameter pipe and secured tightly at the top.63

64For pipe or rod electrode, resistance is

where, d= dia of rod and l = length of rod/ pipe in cm For strip or round conductor

where, w= depth of burial of strip electrode in cm and t = width of strip or twice the dia of circular conductor in cm.

When compared to the plate earth system the pipe earth system can carry larger leakage currents as a much larger surface area is in contact with the soil for a given electrode size. The system also enables easy maintenance as the earth wire connection is housed at the ground level.65Specification:The earth rod shall be situated at a distance not less than 1.5 m from the building.The size of the continuity conductor shall be 2.9 mm2 (14 SWG) or half of the installation conductor size.

The permissible value of earth resistance

Large Power Station-0.5 Major Power Station-1 Small Substation-2 In all other cases-5 Earth continuity resistance-1 (between earth plate and any earth conductor)

SpecificationThe earth rod shall be situated at a distance not less than 1.5 m from the building whose installation being earthedThe size of the continuity conductor shall be 2.9 mm2 (14 SWG) or half of the installation conductor size.The permissible value of earth resistance is,Large Power Station-0.5 Major Power Station-1 Small Substation-2 In all other cases-5 Earth continuity resistance-1 (between earth plate and any earth conductor)

66Fault Level Calculations

where d = spacing between conductors and r = radius of conductor

67Earth wire:Minimum permissible size determined by mechanical consideration.All earth wire and earth continuity conductors should be of copper, galvanized iron /steel or aluminium.The neutral conductor should not be used as earth wire, ref. IS: 3043-1966.

Earthing of domestic fitting and appliances have got high importance.

The minimum permissible size of earth wire is determined mainly by mechanical considerations because they are more liable to mechanical injury and should therefore be strong enough to resist any strain put on them.All earth wire and earth continuity conductors should be of copper, galvanized iron or steel or aluminium.Maximum try to avoid interconnections of earth wire/conductors, if it is necessary, then it should be reliable and electrically strong.The neutral conductor should not be used as earth wire, ref. IS: 3043-196668

If we desire to measure only the resistance between the second conductor and the sheath (Rc2-s), then we need to use the megger's "Guard" terminal:

Megger is a portable instrument which is used to measure insulation resistance of the electrical machinery or system. It can be battery operated or mechanically operated (hand crank dc generator) and gives direct reading in ohms. For this reason it is also called as ohm meter. Onboard ship, different systems are present with large voltage ratings and therefore Megger comes in the range of 100V to 5000V.Read more: http://www.marineinsight.com/tech/marine-electrical/construction-and-operation-of-megger-explained/#ixzz1ormARkQk

megger there are 3 ranges 300V, 750V, and 1000 volts. Applying the proper voltage is essential to not damaging the device you are testing. Working voltages up to 240 volts should use the 300 range. Working voltages up to 600 volts use the 750 volt range and working voltages above 600 use the 1000 volt range. As you can see if you used the 1000 volt range on a device that had a working voltage of say 24 volts you could damage the insulation just by testing it.

69Domestic Electric Installations and EstimatesGeneral rules for wiring:Estimator is fully aware about the general rules followed for internal wiring conform with the Indian Electricity Act The worker shall be under the supervision of certificate holder.Every installation must be properly protected near the point of entry of supply cable by a main and fuse.No fuse, switch, or circuit breaker is to be used in earth wire.Selected conductor must carry full load current safely.For the estimation of internal wiring it is necessary that the estimator is fully aware about the general rules followed for internal wiring conform with the Indian Electricity Act and the regulations.IS:732-1983 lays down that all materials , fittings, appliances used in electric installation shall conform to Indian Standard. IS:732-1983 lays down that the work shall be carried out under the direct supervision of a person holding certificate of competency issued by the state government.General rules which are kept in mind in execution of work of an internal wiring are;1. every installation is to be properly protected near the point of entry of supply cables by a two-pole linked main switch and a fuse unit. In a two wire installation if one pole is permanently earthed, no fuse, switch, or circuit breaker is to be inserted in this pole.2. The conductor used are to be of such a size that it may carry the load current safely.70The conductors installed are to be safe in all respects.Every sub-circuit is to be connected to a distribution board. Every line is to be protected by a fuse of suitable rating as per requirements. Switch board must be 1.3 meters above the floor level. All plug and socket outlets are of 3- pin type. Adequate no. of sockets are to be provided in all roomsOnly 3-pin, 5A socket outlets are to be used in all light and fan sub circuits. Only 3-pin, 15A socket outlets are to be used in all power sub circuits.Never install a plug higher than 15A rating.A plug socket of rating higher than 15A , use DP switch.

3. The conductors installed are to be safe in all respects.4. Every sub-circuit is to be connected to a distribution board.5. Every line is to be protected by a fuse of suitable rating as per requirements.6. Switch board must be 1.25 meters above the floor level.7. (a) All plug and socket outlets are of 3- pin type and third pin must be earthed properly. (b) Adequate no. of sockets are to be provided in all rooms so as to avoid the external use of flexible cables. (c) Only 3-pin, 5A socket outlets are to used in all light and fan sub circuits. Only 3-pin, 15A socket outlets are to be used in all power sub circuits. All plugs are to be controlled by individual switches. (d) A plug socket of rating higher than 15A is to used in situations, then it must be connected through double pole switch of appropriate rating. Never install a plug socket higher than 15A.

71Sockets shall above 20cm or 1.3 m above floor level.No socket outlets is to be provided in the bath room at a height less than 1.3m.One or two 3-pin 15A sockets can be in kitchen (if)Lamps are to be hung 2.5m above floor level. IS: 4648-1968Ceiling fans are to be hung 2.75. above floor level.All socket outlets in a room are connected to one phase.At least two lighting sub-circuits in each house.Separate lighting circuits be utilized for all external lightings of steps, walkways, porch, car park, terrace etc. with two way switch control

Balancing of circuit in 3 phase installation shall be planned before hand. It is recommended that all socket outlets in a room are connected to one phase.It is recommended to provide at least two lighting sub-circuits in each house. Separate lighting circuits be utilized for all external lightings of steps, walkways, porch, car park terrace etc. with two way switch control.

72A load more than 1 kW, shall be controlled by an isolator switch or MCBEnergy meters shall be installed at a height where it is convenient to note the meter readingBut not be installed at a height less than 1 m from the groundSteps to be carried out for estimation:Wiring layoutCalculation of total connected loadSelection of Main SwitchDPIC or DPMCBTPIC or TPMCBTPN or TPNMCB or TPNMCCB

Whatever the load to be fed is more than 1 kW, it shall be controlled by an isolator switch or MCBSwitch boards shall not be erected above gas stove or sink or within 2.5 m of any washing unit in the washing room.A switch board shall not be installed at height less than 1.25 m from floor level, unless the front of the switch board is completely enclosed by a door.Energy meters shall be installed at a height where it is convenient to note the meter reading; it should preferably not be installed at a height less than 1 m from the ground.

Wiring layout: Prepare building plan on a suitable scale and mark electrical points, switch boards, main board, meter board, distribution board etc. on the plan using specified symbols. The path of wiring showing connection to each point is marked by a little thick line.Calculation of total connected load: The total connected load and hence the total current is calculated for deciding the cable size, rating of main switch board and distribution board.Selection of Main Switch: Once the connected load is calculated, the main switch can be conveniently selected from the available standard switch list.DPIC (Double Pole Iron Clad) main switch: 5,15 or 30 A, 250V or DPMCB (Double Pole Miniature Circuit Breaker): 5, 10, 16, 32 and 63 A, 250 VTPIC (Triple Pole Iron Clad) main switch: 30, 60, 100, 200 A, 500 V or TPMCB (Triple Pole Miniature Circuit Breaker): 16, 32 and 63 A, 500 V, beyond this TPMCCB (Triple Pole Molded Case Circuit Breaker): 100, 200, 300 and 500 A, 660 VTPN main switch: 30, 60, 100, 200, 300 A, 500 V or TPNMCB: 16, 32, 63A, 500 V, beyond this TPNMCCB: 100, 200, 300, 500 A, 660 V.In TPN, opening & closing of the device will open & close the Neutral. While in SFU, Neutral doesn't automatically opens or closes as it does in case of MCBs or MCCBs. In MCBs & MCCBs, protection is only on Three poles & Neutral is just a isolating pole.

73Main distribution boardAssumptionsCalculation of length of conduitCalculation of length of phase and neutral wireCalculation of length of earth wirePreparation of material table

Selection of Main Distribution Board: The Main Distribution Board is a fuse box or MCB box where different sub-circuits are terminated. Numbers of sub-circuits are decided based on the total connected load or total number of points.

Assumptions: the conditions which are not specified in the question may be assumed conveniently. Eg:- location of main switch board, switch boards, height of building(if not specified)

Calculation of length of conduit: To avoid duplicity in calculating the length of conduit pipe, this may be calculated in three stages. (a) The conduit installed from switch board up to horizontal run (HR) including from main switch or DB to HR. (b) The conduit on walls running parallel to the floor ie, the HR below ceiling. (c) The conduit installed between HR and ceiling, along ceiling and ceiling to last point on HR. The total length of conduit is calculated by adding the length of conduit obtained from the three stages and including 10% wastage.

Calculation of length of phase wire and neutral wire: The phase wire and neutral wire is calculated sub-circuit wise. Once it is calculated, wastage of 15% is included.

Calculation of length of earth wire: The earth wire is run along the conduit. The calculations are carried out in length but it is converted in to weight while preparing material table.

Preparing Material Table: The material table should be prepared with complete specification of each item.

74Number of socket outletsDescriptionNumber of socket outlets5 A15 ABed room2 to 31Living room2 to 32Kitchen12Dining room21Garage11For refrigerator-1For air conditioner-1Verandah1 per 10 m21Bathroom11Recommended levels of illuminationLocationIllumination LevelEntrance Hallways100Living room300Dining room150Bed room:GeneralDress table, bed heads300200Games or recreation room100Table games300Kitchen200Kitchen sink300Laundry200room100room mirror300Sewing700workshop200stairs100Garage70Study30077Selection, rating and installation of equipments on the main switch board Calculate total load and load current in the circuitCalculate load currents in each sub-circuit.Considering future requirements, an iron clad main switch or MCB is suggested.

Example:-A residential wiring has total of 2980 W. one 15A socket, 11 light points, 7 fan points, 9 plug points of 5A. Draw the single line diagram showing cutout, meter, main switch, main distribution board and other equipments. Make your own assumptions for number of electrical points in each sub-circuit and find out the rating of main switch and distribution board.

Example:-In an installation of a house wiring, there is total of 2980 Watts. It consists one 15 A socket. Draw the single line diagram showing cutout, meter, main switch, main distribution board and other equipment. Make your own assumptions for number of electrical points in each sub-circuit and find out the rating of main switch and distribution board.ANS: Load in sub-circuit No. 1Light point= 2 x 60 = 120 WFan point= 2 x 60 = 120 W5 A socket= 4 x 100 = 400 WTotal connected load = 640 W

79Answer:Total connected load: 2860WDecide Number of points in each room and required number of sub-circuits.Assumption if any

sub-circuit No.1Light point= 3 x 60 = 180 WFan point= 2 x 60 = 120 W5 A socket= 3 x 100 = 300 WTotal connected load= 600 W

80Load in sub-circuit No. 2Light point= 5 x 60 = 300 WFan point= 2 x 60 = 120 W5 A plug socket= 3 x 100 = 300 WTotal connected load= 720 WLoad in sub-circuit No. 3Light point= 3 x 60 = 180 WFan point= 3 x 60 = 180 W5 A socket= 3 x 100 = 300 WTotal connected load= 660 W

81Load in sub-circuit No. 4

15 A socket outlet at 1000 W

Total connected load on all sub-circuits = 600 + 720 + 660 + 1000 = 2980 W

Total load in Amp = 2980 / 230 = 12.956 A

Total current coming in DB is 12.956 and considering future expansion select 30A, 250V DPIC or 32A, 250V DPMCB

Q. Draw the wiring layout and estimate the quantity of materials required. Assume the height of ceiling as 3.6 m and one plug point is to be provided in each room.

Soln: Wiring layout

8983Assumption: a) Height of MB from the floor= 1.5 mb) Height of HR from the floor= 3 mc) Height of SB from the floor= 1.5 md) Thickness of wall= 0.25 me) Height of bracket from floor= 2.4 m2. Calculation of total connected loadLight points= 5 x 60=300 WFan points= 2 x 60= 120 W5 A socket= 2 x 100= 200 WTotal connected load= 620 WFull load current= 620 / 230=2.69 Aonly 9 points, hence no distribution board is requireduse DPIC main switch of 15 A, 250 V grade.Selection of wire:use 1.5 sq.mm copper wire for sub-circuit wiringuse 1 sq.mm copper wire for light/fan/5A socket wiring.CALCULATION OF MATERIALSPVC ConduitWiring Cable of desired sizeNeutral wire Earth wireEarth pipes

Selection of wire: PVC insulated copper wire of 1.5 Sq. mm is used for sub-circuit wiring and PVC insulated copper wire of 1 Sq. mm is used for light/ fan/ 5 A socket points. 87Calculation of PVC insulated copper wire:a) Circuit wire (1.5 Sq. mm)From MB to SB1= (1.5 + 2 + 1.5) x 2= 10 mFrom MB to SB2= (1.5 + 2 + 1.5 +0.25) x 2= 10.5 mTotal= 20.5 mWastage 15%= 3.075m= 23.575 mSay 24 m

Neutral wire:From SB1 to F1, L1 & L2= 1.5 + 0.6 +2.4 + (2.4 +0.6)x 2 = 8.1 mFrom SB2 to F2, L3, L4 & L5 = 1.5 +1.8 + 0.6 + 0.25 + 1.2 +1.35+1.35+0.6 = 8.65 m Total= 16.75 m

Total 1 Sq. mm wire= 16.75 + 15 % wastage = 19.2625Say, 20 mEarth wire (14 SWG)

From MB to SB1= (1.5 + 2 + 1.5)= 5 mFrom MB to SB2= (1.5 + 2 + 1.5 +0.25)= 5.25 mTotal= 10.25 m Total 14 SWG bare cu wire + Wastage 15%= 11.78m

say, 12 m

Material Table:Sl No.DescriptionQuantity115 A, 250 V,DPIC switch1 No.225 mm PVC conduit28 m3PVC bend, Tee15 Nos4Saddle clips75 Nos51 Sq. mm ,PVC insulated copper wire20 m61.5 Sq. mm ,PVC insulated copper wire24 m75 A piano switch9 Nos8Ceiling rose2 Nos9Angle bracket5 nos105 A, socket2 Nos11Teak wood box, 25 x 20 cm for SB1 & SB22 Nos12Teak wood box, 25 x 15 cm1 No13Teak wood Batten 7 x 7 cm5 Nos14Wooden screws300 nos1514 SWG bare cu wire12 m16Earthing set(pipe earth)1 Set17Cement, sand etc.93Power distribution in an industryThe power distribution in an industry has different levelsMSB levelSSB levelDB levelLight loads are connected to DBHeavy loads connected to SSB or MSB

DB / DFB (Distribution Fuse Board) / FDB (Fuse Distribution Board) Even number of ways are used in DBsAs per IS, max. number of ways is limited to 12

Power distribution in an industryThe power distribution in an industry has different levelsMain Switch Board (MSB) levelSub Switch Board (SSB) level Distribution Board (DB) levelDB is the last element before the loads. But large loads are directly connected to SSB or MSB

DB / DFB (Distribution Fuse Board) / FDB (Fuse Distribution Board)Usually even numbers of ways are used in DBs (2, 4, 6, 8, 10 and 12). As per IS the maximum number of ways is limited to 12.

94Example:-

12 way 3phase DB has 12x3 +1 = 37 cable connectionUsual current rating of DB s are : 16A, 32A and 63A63A, 12 way DB is not common. Here max. input current in 12 way is 756A.Which is not possible to handle by DB.Hence 63A DB is 2 or 4 ways.Motor loads up to 20hp are fed from DB

Eg:- 12 way 3 ph DB = 3 x 12+1 = 37 cable connection including neutral.Usual current rating of DB s are : 16A, 32A and 63A63A, 12 way DB s are not common. Since maximum input current = 63 x 12 = 756A, which is not possible to handle by a DB. Hence 63A DB is 2 ways or 4 ways.Motor loads up to 20 hp are fed from DB s of various rating.

In a designed system 20% spare outlets are kept for future expansion. ie, in each DB, 1 or 2 outlets shall be kept as spares.

95All DBs have isolator or SFU as incomer switch.But in some cases this is avoided if the main switch board supplying the DB is within 3m from the DB20% spare outlets are kept for future expansion.

All DBs have isolator or SFU as incomer switch. But in some case this is avoided if the switch board supplying to the DB is within 3m from the DB.In a designed system 20% spare outlets are kept for future expansion. ie, in each DB, 1 or 2 outlets shall be kept as spares.

96Selection and rating of incomer isolator/SFU and incomer feeder sizeThe connected loads will not put on simultaneously.Hence max. load demand reduces.Then max. demand is expressed through Diversity factor

Average of other outlets are assumed for spare outlets.Commonly accepted and safe value of DF is 1.5.Isolator/SFU is selected based on starting current.

Selection of rating of incomer isolator/SFU and incomer feeder sizeIn any system, all the connected loads will not be put on simultaneously. This reduces the maximum demand from simply computing by adding all connected loads. The maximum demand is expressed through a factor called Diversity Factor,

From the requirement data, the details of connected load on each DB are known to us. For spare outlets, an average of other outlets can be assumed. If the DF is known, we can find the maximum current requirement of the DB to feed all loads including spares. Instead of furnishing the DF, a usual practice is specifying MD. A commonly accepted and safe value of DF is 1.5. this value can be assumed for each DBIf motor loads are connected, for selection of isolator / SFU, the starting current has to be taken in to account rather than continuous current.

97Which takes into account the followings:

Time profiles of electrical loadsCoincidence of electrical loadsMechanical loading of electric motorsSeasonal changes in loading (particularly heating, lighting and cooling)Any special requirements for the particular installation

Example:- 5 hp - 5Nos and 10 hp - 2Nos motors are connected to a DBTotal connected load = 45hp

Hp Calculate corresponding maximum load current.This is the expected continuous maximum current.

Consider Starting inrush current of large motor in the down stream.Consider the method of starting, DOL or Assisted starting.Starting current for DOL is 2.5 timesAssisted starting 1.5 times

Eg:- 5 hp - 5Nos and 10 hp - 2Nos motors are connected to a DBTotal connected load = 45hpMD= 45/1.5 = 30hpCorresponding maximum current is 30 x 1.4 = 42 A. This current is the continuous maximum current

When motors are started we have to account the starting inrush current of large motor in the down stream. Starting current of DOL starting motor is 2.5 times the rated current and for assisted starting (star delta), it is 1.5 times the rated current.So the MD calculation in the above case is as follows:

99MD calculation:One 10 hp (one higher rating) kept aside.Now only MD of 10 hp is existing.Calculate its maximum current, considering method of starting.For one 10 hp alone, maximum current = 2.5 times of its rated current, assuming DOL start.Therefore MD of the DB = sum of the above current.ie, incoming feeder, isolator/SFU of the DB can be rated to above the calculated value.

MD calculation in the above case is as follows:One 10 hp (one higher rating) kept asideNow only MD of 20 hp is existingIts maximum current = 20 x 1.4 = 28 A, here 1.4 (say a constant) is obtained from ratio of power equation p=(hp*735.5)/(sqrt3*v*pf*efficiency)For one 10 hp alone, maximum current = 2.5 x (10 x 1.4) = 35 ATherefore MD of the DB = 28 + 35 = 63 Aie, incoming feeder, isolator/SFU of the DB can be rated to 63 A

100

Grading or Discrimination between Feeder Fuse and DB Fuse:Supply to DB isfrom SSB/MSBThrough feeder.Feeder is protectedby HRC fuseFor better grading of Main fuse and minor Fuse, keep ratio as 2:1

Grading or Discrimination between Feeder Fuse and DB FuseThe feeder to a DB will be fed from an SSB or MSB. This feeder will be protected by the HRC fuse in the SSB or MSB. It is necessary that the feeder protective fuse should not blow off before the motor protective fuse in the DB. This is achieved by proper grading between the fuses. The fuse of SSB/MSB is denoted as major fuse and that of DB is termed as minor fuse. For achieving grading the ratio between major and minor fuses shall be 2:1 or more

102Feeder cable must be 20% excess of MD of DBMajor fuse rating should match with cable selection.Voltage drop should take into account for Cable length above 75 to 100mtrMax drop should be less than 3% in MD condition.

Feeder cable is selected by considering the 20% excess of the MD of DB. Also major fuse rating should match with the cable selection.If the cable length exceeds 75 to 100mtr, the voltage drop condition should be taken in to account. The voltage drop in the feeder should not be more than 3% in the maximum demand condition.

103Example:50 hp, 415 V, 3 ph Induction motor use AYFY 150 m cable.IL = (50 x 735.5)/(3 x 415 x 0.8 x 0.85) = 76 A 50 Sq. mm, 4 core cable is selectedIts voltage drop/Amp/mtr = 1.3 mV% volage drop = (1.3 x 10-3 x 76 x 150 x100)/415 = 3.57 % - exceeds the limitNext higher size cable is 70 Sq. mmIts voltage drop/Amp/mtr = 0.93 mV% volage drop = (0.93 x 10-3 x 76 x 150 x100)/415 = 2.55 % - within limit Therefore 70 Sq. mm AYFY ( PVCAPVC), 4 core cable is selected.

Eg 1:- 50 hp, 415 V, 3 ph Induction motor use PVCAPVC 150 m cable.Ans:IL = (50 x 735.5)/(3 x 415 x 0.8 x 0.85) = 76 A50 Sq. mm, 4 core cable is selectedIts voltage drop/Amp/mtr = 1.3 mV% volage drop = (1.3 x 10-3 x 76 x 150 x100)/415 = 3.57 % - exceeds the limitNext higher size cable is 70 Sq. mmIts voltage drop/Amp/mtr = 0.93 mV% volage drop = (0.93 x 10-3 x 76 x 150 x100)/415 = 2.55 % - within limitTherefore 70 Sq. mm AYFY ( PVCAPVC), 4 core cable is selected. AYFY Means Alluminum PVC Insulated Fltat strip armour and PVC outer sheeth cableA -ALLUMINIUM Y - INNER SHEATH F - FLAT ARMOURED Y - OUTER SHEATH.

104Design of incomer SFU, Cable size and Bus bar rating for SSBs and MSBsSwitch boards in general are power distribution centers with SFUs/MCCBs/ACBs/OCBs for controlling outlets and incomer.switch boards are specified by its total current carrying capacity or incomer current rating.Outlet current rating is the DBs rating.Standard SB rating: 100 A, 200 A, 400 A, 800 A, 1200 A, 1600 A, 2000 A, 2500 A and 3200 A. Incomer controlled with an SFU, called switch fuse controlled board. Other is breaker controlled board.Design of incomer SFU, Cable size and Bus bar rating for SSBs and MSBsSwitch boards in general are power distribution centers with SFUs/MCCBs/ACBs/OCBs for controlling outlets and incomer. Unlike DBs, switch boards are specified by its total current carrying capacity or incomer current rating. Where as in DBs current rating of the outlet is the specified rating. Standard switch board ratings are 100 A, 200 A, 400 A, 800 A, 1200 A, 1600 A, 2000 A, 2500 A and 3200 A. If the incomer supply is controlled with an SFU, the switch board is called switch fuse controlled board and if the incomer is ACB/ OCB controlled, it is called breaker controlled board.

105A switch board having three sectionsOutlet control gearsBus chamberIncomer control gearSSB is used if load is moreFor smaller loads MSB is sufficient.

A switch board having three sectionsOutlet control gearsBus chamberIncomer control gearThe outlet switch, fuse and cable rating are decided by the load that has to be handled through that feeder. If the number of loads is more, SSB is required, which is installed almost at the load centers. In smaller set up SSB may not be necessary and MSB will be the only switch board.106Finding Bus bar size:Bus bar materials: Aluminum or Aluminium alloy working current density, 0.8 A/ Sq.mmCopper working current density, 1.2 A/ Sq.mm250A/0.8 = 312.5 Sq. mmFor neutral bus bar, half the size of phase bus bar size is sufficient.ie, 40 x 8 mm or 50 x 6 mm Al bus bar may be used for phases and 20 x 8 mm or 25 x 6 mm for neutral.Or 31 x 6 mm Cu bus bar may be used for phases and 31 x 3 mm for neutral.small switch boards the distance between the bus supports will be 50 cms.

finding Bus bar size.Bus bar materials are: Aluminum or Aluminium alloy working current density, 0.8 A/ Sq.mmCopper working current density, 1.2 A/ Sq.mmFor the above set up:-250/0.8 = 312.5 Sq. mmFor neutral bus bar, half the size of phase bus bar size is sufficient.ie, 40 x 8 mm or 50 x 6 mm Al bus bar may be used for phases and 20 x 8 mm or 25 x 6 mm for neutral.Or31 x 6 mm Cu bus bar may be used for phases and 31 x 3 mm for neutral.

For small switch boards the distance between the bus supports will be 50 cms.

107If DF is not given, assume, DF as 2 for all switch boards.Standard switch ratings are: 32A, 63A, 100A, 200A, 250A, 400A, 630A AND 800A.

If DF is not given, we can assume, DF as 2 for all switch boards.The term ampacity is some times used to denote the maximum current rating of the feeders. If DF is not clearly known, the total ampacity of outlet feeders shall not be more than two times the ampacity of the incomer feeder.The feeder cables need to be selected for the fuse used in the SFU.Eg:- when we want 125A feeder, the fuse and cable corresponding to 125A. But the switch may be 200A, since above 100A, only 200A switch is available.Standard switch ratings are: 32A, 63A, 100A, 200A, 250A, 400A, 630A AND 800A. Some manufactures makes 125A and 320A also.

109Electrical installation for small industries:Every motor/large load must be controlled by a main switch and starter.Size of SFU and starter will depend upon the rating of the motor.large capacity motors, three phase underground cables are used.The current rating of cables for motor supply may be based upon the normal full load current, but the rating of the fuse should be based upon the starting current.The sequence to be followed:Motor currentDecide cable sizeDecide size of conduitDecide fuse ratingDecide starter, DB and Main switch

Deciding the cable size:Current carrying capacity not less than 150% of the motor full load current rating.

\dc motors:

Current required by a 400v 15hp dc motor at 80% efficiency is= Rated hp x 735.5/(V* efficiency)= 15 x 735.5/(400 x 0.8) =33.2A

Cable should be 1.5 times the full load current = 33.2 x 1.5 = 49.8ASelect a cable of 7/1.63 mm copper conductor capacity 53A

112\single ac motor:5hp motor working at 240V at 75% efficiency and 0.8 power factorRated hp x 735.5/ (efficiency x V * pf)5 x 735.5/(0.75x 240x 0.8) = 25.5A.Select a cable with 1.5x25.5 = 38.23A, ie 7/1.4mm (10mm2) copper conductor with 43A capacity.\three phase motor:Motor of 3 phase 415v 10 hp at 85% efficiency and 0.8 pf10x 735.5/(3xVLxpfx efficiency) = 15ACapacity of cable = 15x1.5 = 22.5ASelected cable is 7/1.06 mm copper cable of 28A

Table\single ac motor:5hp motor working at 240V at 75% efficiency and 0.8 power factorRated hp x 735.5/ (efficiency x V xpf)5 x 735.5/(0.75x 240x 0.8) = 25.5ACurrent carrying capacity of cable should be 25.5 x 1.5 = 38.23ASize of cable selected is of current carrying capacity of 43A 7/1.7 aluminium or 7/1.4mm copper.of cable selected is of current carrying capacity of 43A 7/1.7 aluminium or 7/1.4mm copper.

\three phase motor:Motor of 3 phase 415v 10 hp at 85% efficiency and 0.8 pf10x 735.5/(3xVLxpfx efficiency) = 15ACapacity of cable = 15x1.5 = 22.5ASelected cable is 7/1.06 mm copper cable of 28A

113Deciding the starter, DB and main switchSerial NoTypes motorMethod of starting1IM of very small ratingDOL2IM of medium rating upto 15hpStar-delta3IM of high ratingAuto transformer4Slip ring IMRotor resistance5DC series2 point 3 point6DC shunt/ compound motor3 pointDifferent types of motors have different types of startingDeciding the starter, DB and main switch

Example:A small work shop has following motors. Prepare wiring scheme forOne 5hp, 400v 3 phaseOne 3hp, 400v 3 phaseOne 1/2hp, 230v 1 phaseOne 1hp, 400v 3 phaseFix the positions of the motor on the floor, or make floor plan firstDraw the wiring diagram for electrical connections starting from MS. Determine the quantity of material required and cost of the same for the power distribution arrangement. For illuminating the workshop area 32 bulbs are proposed to be used. Sketch the proposed arrangement of lamps. Draw the wiring diagram. Assume 85% efficiency and 0.8 pf

115Example:

A small work shop has following motors. Prepare wiring scheme for,One 5hp, 400v 3 phase, One 3hp, 400v 3 phase,One 1/2hp, 230v 1 phase, One 1hp, 400v 3 phase.

Fix the positions of the motor on the floor, or make floor plan, and Draw the wiring diagram for electrical connections starting from MS. Determine the quantity of material required and cost of the same for the power distribution arrangement. For illuminating the workshop area 32 bulbs are proposed to be used. Sketch the proposed arrangement of lamps. Draw the wiring diagram. Assume 85% efficiency and 0.8 pf

Steps:Motor circuit:Floor planMotor currentCable size for each motor and main supplySize of conduit for each motor and length of conduitRating of fuse unitLength of cableLength of earth wire requiredDecide starter, DB and Main switchLabour costFor lighting circuit:Decide number of sub circuitsSize of cable and lengthConduit size and lengthRating of fusesRating DB and Main switchlabour cost

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Motor installation: Main supply: Fix the cable rating required as follows for all machinesLine current of each 3 phase motor =Line current of each single phase motor= Total full load current of all motors = 15.75A (say)Cable rating 1.5 x 15.75 = 23.6 ASelect 7/1.06mm copper cable of rating 28A.The size of conduit, from meter board to MB shall be 25.4 mm having capacity to accommodate 3 runs of 7/1.06mm.Current rating of main switch is the starting current of one motor of highest rating plus full load current of remaining motors, ie = (7.5x1.5+4.5+2.25+1.5) = 19.5,So 30A,500V grade (Al) TPIC switch shall be used.

Motor installation: Main supply: fix the cable rating required as follows for all machinesLine current of each 3 phase motor =Line current of each single phase motor= Total full load current of all motors = 15.75ACable rating 1.5 x 15.75 = 23.6 ASelect single core 7/1.06 mm copper cable having current rating 28A may be used from meter board to main board.The size of conduit used from meter board to MB shall be 25.4 mm having capacity to accommodate 3 runs of 7/1.06mm Current rating of main switch is the starting current of one motor of highest rating plus full load current of remaining motor= 7.5x1.5+4.5+2.25+1.5) = 19.5, here 150% of highest rating has takenSo 30A,500V grade TPIC switch shall be used

119Starting current of highest rated motor is 11.25A.Number of circuits = 4So 4 way 500v 15 A per way, IC DB with neutral linkThe MS and MDB are assumed to be mounted at a height f 1.5m above the ground level.Find size of cable for each motorFind size of conduit for each motorRating of switch fuseLength of conduit requiredLength of wire requiredLength of earth wire requiredLabour cost

Starting current of circuit of highest rating is 11.25 ie.1.5x7.5Number of circuits = 4So 4 way 500v 15 A per way, IC DB with neutral linkThe MS and MD are assumed to be mounted at a height f 1.5m above the ground level.Find size of cable for each motorFind size of conduit for each motorRating switch fuseLength of conduit requiredLength of wire requiredLength of earth wire requiredLabour cost120Lighting distribution:No. of sub circuitsSize of cableSize of conduitRating of SFU and DBLength of wireLength of conduitLabour cost

Lighting distributionNo. of sub circuitsSize of cableSize of conduitRating of SFU and DBLength of wireLength of conduitLabour cost

121Number of lamps proposed = 32Assume lamps be 100watts mounted at equal intervals.Number of lamps along length= 8Number of lamps along width= 4Number of sub-circuits= 4 with 8 points.

122

EARTH BUS, BUSBAR AND CHAMBERSEARTH BUS:All earthing conductors shall be of high conductivity G.I./ Aluminum/copper and shall be protected against mechanical injury or corrosion.Main earthing conductors shall be taken from the earth connections at the main switchboards to an earth electrode. Sub earthing conductors runs from MSB to Sub DB.

All earthing conductors shall be of high conductivity G.I./ Aluminum/ copper and shall be protected against mechanical injury or corrosion.CONNECTION OF EARTHING CONDUCTORS:(i) Main earthing conductors shall be taken from the earth connections at the main switchboards to an earth electrode with which the connection is to be made. Sub-main earthing conductors shall run from the main switchboard to the subdistribution boards. Final distribution boards earthing conductors shall run from sub-distribution boards.(ii) Circuit earthing conductor shall run from the exposed metal of equipment and shall be connected to any point on the main earthing conductor or its distribution boards or to an earth leakage circuit breaker. Metal conduits, cable sheathing and armouring shall be earthed at the ends adjacent to switchboards at which they originate or otherwise at the commencement of the run by an earthing conductor ineffective electrical contact with cable sheathing. Where equipment is connected by flexible cord, all exposed metal parts of the equipment shall be earthed by means of an earthing UGVCL-Specification EarthingSeal & Signature of Bidder Page 3 conductor enclosed with the current carrying conductors within the flexible cord. Switches, accessories, lighting fitting etc. which are rigidly secured in effective electrical contact with a run of metallic conduit shall not be considered as a part of earthing conductor for earthing purposes, even though the run of metallic conduit is earthed.(iii) All metal clad switches and other equipment carrying single phase current shall be connected to earth by a single connection. All metal clad switches, carrying medium voltagesand high voltage shall be connected with earth by two separate and distinct connections. The earthing conductors inside the building, wherever exposed, shall be properly protected frommechanical injury by running the same in GI pipe of adequate size.(iv) Earthing conductors, outside the building, shall be laid 600 mm below the finished ground level. (v) In case of copper earthing strips, the cover lapping at joints (wherever required), shall be of minimum 75 mm. Sweated lugs of adequate capacity and size shall be used for ail terminationof wires above 6 sq. mm size and bare copper wire above 2.5 mm dia. Lugs shall be bolted to the equipment body to be earthed after the metal body is cleaned of paint and other oily substance and properly tinned.(vi) Neutral conductor, sprinkler pipes or pipes conveying gas, water or flammable liquid, structural steel work, metallic enclosures for cables and conductors, metallic conduits and lightningprotection stem conductors shall not be used as a means of earthing an installation or even as a link in an earthing system. The electrical resistance of metallic enclosures for cables andconductors measured between earth connections at the main switchboard shall be low enough to permit the passage of UGVCL-Specification Earthing Seal & Signature of Bidder Page 4current necessary to operate fuse or circuit breakers and shall not exceed one ohm. 124A good earthing system is required for:Protection of buildings and installations against lightning Safety of human and animal life by limiting touch and step voltages to safe valuesElectromagnetic compatibility (EMC) i.e. limitation of electromagnetic disturbancesCorrect operation of the electricity supply network and to ensure good power quality.A good earthing system is required for: protection of buildings and installations against lightning safety of human and animal life by limiting touch and step voltages to safe values electromagnetic compatibility (EMC) i.e. limitation of electromagnetic disturbances correct operation of the electricity supply network and to ensure good power quality.

"Step potential" is the voltage between the feet of a person standing near an energized grounded object. It is equal to the difference in voltage, given by the voltage distribution curve, between two points at different distances from the "electrode". A person could be at risk of injury during a fault simply by standing near the grounding point.

"Touch potential" is the voltage between the energized object and the feet of a person in contact with the object. It is equal to the difference in voltage between the object and a point some distance away. The touch potential could be nearly the full voltage across the grounded object if that object is grounded at a point remote from the place where the person is in contact with it. For example, a crane that was grounded to the system neutral and that contacted an energized line would expose any person in contact with the crane or its uninsulated load line to a touch potential nearly equal to the full fault voltage.IEEE Std. 80-2000 is a standard that addresses the calculation and mitigation of Step & Touch Potentials to acceptable levels.

125The electrical properties of earthing depend essentially on two parameters:earthing resistanceconfiguration of the earth electrode.The potential of the earth grid Vgrid can be calculated using Ohm's Law if the fault current (If) and resistance of the grid (Zgrid) are known.Vgrid = If * ZgridThe potential at a distance is the analysis of a driven rod electrode in homogeneousEarth. Then Voltage Profile is

Differences between steady-state and pulsed properties of an earthing system are caused mainly by: flow of currents with very high values, up to a few hundreds of Ka. Very fast current rise times - typical lightning strikes reach a few hundred kA/s.126Measurement MethodAverage earth resistivity at the site:The resistivity of soil varies from site to site and with depth of the soil and horizontal distance.For correct earth resistivity, it is desirable to get information about horizontal and vertical variation of resistivity at the site under consideration.The vertical variations may be detected by repeating the measurements at a given location in a chosen direction with different electrode spacing. The spacing's may be increased in steps of 2, 5, 10, 15, 25 and 50 metres or more.

Average earth resistivity at the siteThe resistivity of the soil at many sites have been found varying with the depth of the soil and also with horizontal distances. Variation of the resistivity with depth is mainly due to stratification of earth layers and is found predominant when compared to the variation with horizontal distances. For the correct computation of earth resistivity, it is desirable to get information about the horizontal and vertical variations of earth resistivity at the site under consideration. The vertical variations may be detected by repeating the measurements at a given location in a chosen direction with different electrode spacing. The spacings may be increased in steps of 2, 5, 10, 15, 25 and 50 metres or more.

The horizontal variations are studied by taking measurements in various directions from the centre of the station. If the variation in the earth resistivity readings for different electrode spacings in a direction is within 20 to 30 percent, the soil is considered to be uniform. When the spacing is increased gradually from low values, a stage will be reached at which the resistivity readings become more or less constant irrespective of the increase in the electrode spacing. This value of the resistivity is noted as the resistivity in that direction. Similarly, the resistivity for at least eight equally spaced directions from the centre of the site are measured. These resistivities are plotted on a graph sheet. A closed curve is plotted on the graph sheet joining the resistivity points to get a polar resistivity curve. The radius of the equivalent circle is the average earth resistivity of the site under consideration. The value will be reasonably accurate when the soil is homogeneous.

127The horizontal variations in measurements in various directions from the centre of the station is noted. If the variation in the earth resistivity readings for different electrode spacings in a direction is within 20 to 30 percent, the soil is considered to be uniform. When the spacing is increased gradually from low values, a stage will be reached at which the resistivity readings become more or less constant irrespective of the increase in the electrode spacing.

The resistivity for at least eight equally spaced directions from the centre of the site are measured.These resistivity's are plotted on a graph sheet and a closed curve is plotted on the graph sheet joining the resistivity points to get a polar resistivity curve. The radius of the equivalent circle is the average earth resistivity of the site under consideration. The value will be reasonably accurate when the soil is homogeneous.

The fault current from a distribution or transmission system can usually be estimated with precision,But the calculation of the earth grid resistance is more complicated. Difficulties arise from the extended and irregular shape of practical ground grids, and the varying resistivity of soil at different depths.At points outside the earth grid, the potential rise decreases. The simplest case of the potential at a distance is the analysis of a driven rod electrode in homogeneous earth.While the fault current from a distribution or transmission system can usually be calculated or estimated with precision, calculation of the earth grid resistance is more complicated. Difficulties in calculation arise from the extended and irregular shape of practical ground grids, and the varying resistivity of soil at different depths.At points outside the earth grid, the potential rise decreases. The simplest case of the potential at a distance is the analysis of a driven rod electrode in homogeneous earth. The voltage profile is given by the following equation.

130

FORMULAE FOR CALCULATION OF EARTH RESISTANCES131Properties of earthing at high impulse currents

Differences between steady-state and pulsed properties of an earthing system are caused mainly by: flow of currents with very high values, up to a few hundreds of kA. Very fast current rise times - typical lightning strikes reach a few hundred kA/s

Maximal length lmax of lightning earth electrode as a function of resistivity

BUSBAR and BUSBAR CHAMBER:An electrical conductor, maintained at a specific voltage and capable of carrying a high current, usually used to make a common connection between several circuits in a system.

an electrical conductor, maintained at a specific voltage and capable of carrying a high current, usually used to make a common connection between several circuits in a system134

Bus bar chamber consists of bus bars of copper or aluminium strips.Incoming lines are connected here through SFU.Chamber size depends on 1) size and number of strips used 2) number and rating of switches to be mounted on it.

Switch Fuse Unit comprises of vitreous steatite porcelainrewireable fuses or HBC fuse fittings complete with theirconducting parts. The switch is fitted with sturdy side operating&Front operating handle with quick make-break type mechanismContacts are made of electrolytic copper,Brass, silver-plated. Thefixed contacts are provided with removable shield.Switch fuse units are providedwith rewireable fuse or HBC fuseSwitch Fuse units are pvovided with rewirable fuse or HBC FuseLinks.EnclosuresThe Enclosure is made of sheet steel duly phosphatised andpower-coated. They are pvovided with conduit knock-outs andhave detachable gland plates. Door interlock is provided toprevent opening when the switch is in ON condition.135

SFU Switch Fuse Unit (H.R.C.TYPE)

Switch Fuse Unit S.P.N. & D.P. Rewwirable / HRC TYPE As Pers IS : 13947

Switch Fuse Unit (Star Series-K-Type)

137SubstationsClassification based on design:Outdoor typePole mounted (single stout pole/ H-type/ 4-pole structure employed for transformers of 25 kVA, 100 kVA and above 100 kVA)Foundation mounted (For transformers above 250 kVA and voltage of 33 kV and above)Indoor type (In this the substation apparatus are installed within the building)

On the basis of design substations may be classified in to Outdoor typePole mounted (single stout pole/ H-type/ 4-pole structure employed for transformers of 25 kVA, 100 kVA and above 100 kVA)Foundation mounted (For transformers above 250 kVA and voltage of 33 kV and above)Indoor type (In this the substation apparatus are installed within the building)

138Outdoor substation: When transformers are installed out door, certain clearances must be maintained.Clearance between suppliers and consumers structure should not be less than 3 meters. This is for maintaining the minimum sectional clearance of 206 m at 11 kV. Suppliers and consumers structure shall be braced together when the clearance between them is 5 m or less.The ground clearance of the live parts of CTPT unit shall not be less than 3.7 m.Phase to phase clearance at the AB switch shall be 915 mm

Outdoor substationWhen transformers are installed out door, certain clearances must be maintained.Clearance between suppliers and consumers structure should not be less than 3 meters. This is for maintaining the minimum sectional clearance of 206 m at 11 kV. Suppliers and consumers structure shall be braced together when the clearance between them is 5 m or less.The ground clearance of the live parts of CTPT unit shall not be less than 3.7 m.Phase to phase clearance at the AB switch shall be 915 mm

139Phase to earth clearance at the AB switch shall be 610 mm. It is the clearance between the operating rode of the AB switch and the jumpers of 11 kV down conductorsThe supported length of 11 kV jumpers shall be limited to 1.5 m for standard conductors and 2.44 m for solid conductors (No. 2 or No. 0 SWG copper). Where there is a cable end box with open terminations, the clearance of the live pars to ground shall not be less than 305 mThe ground clearance of ht parts, usually 11 kV at the transformer bushings shall not be less than 2.75 m.The ground clearance of AB switch handle shall be between 1 and 1.2 m

Phase to earth clearance at the AB switch shall be 610 mm. It is the clearance between the operating rode of the AB switch and the jumpers of 11 kV down conductorsThe supported length of 11 kV jumpers shall be limited to 1.5 m for standard conductors and 2.44 m for solid conductors (No. 2 or No. 0 SWG copper). Where there is a cable end box with open terminations, the clearance of the live pars to ground shall not be less than 305 mThe ground clearance of ht parts, usually 11 kV at the transformer bushings shall not be less than 2.75 m.The ground clearance of AB switch handle shall be between 1 and 1.2 m

140

142Indoor Substation:11 kV/415 V are quite frequently installed at industrial areas and other load areas like multistoried buildings, telephone exchange, Hospitals etc.Indoor station