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, ThrissurWhy is electrical system design and estimation?

It is the language of engineers which providing proper communication to the peoples around the world, so that a particular word means the same thing to every one and every where.This is possible through graphical symbols.In India symbols are standardized by Bureau of Indian Standards.IS:2032 gives a list of standards.

2Role of National Electric codeWhat is National electric code or NEC?

The National Electrical Code (NEC), or NFPA 70, is a United States standard for the safe installation of electrical wiring and equipment. It is part of the National Fire Codes series published by the National Fire Protection Association (NFPA). "National Electrical Code

http://en.wikipedia.org/wiki/National_Electrical_Code#Structure_of_the_NEC

Why it is necessary?In single word, it is for security measures, in all respects3ELECTRICAL 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.

20