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Electric Supply System (A.C.)

3;7. Electric Supply System (A.C.)

  • Now-a-days power generation is three-phase, 50-Hz, usually at 11 kV. The generated voltage is stepped upto 132, 220 or 400 kV by means of step-up transformers. Then by means of three wire transmission lines electric power is carried to different places where it is received in sub-stations situated near cities. Here the voltage is stepped down to 66 or 33 kV and further carried through three-wire transmission lines to various sub-stations in the cities where the voltage is further stepped down to 11,6.6 or 3.3 kV. These voltages
    are further stepped down to 415 V to make available power to small consumers through 415/240 V, three-phase, four-wire distributors.
  • Small consumers receive power at 415/240 V where they can use both three-phase and single-phase supply. Very small domestic consumers are often provided with single-phase supply at 240 V. Distribution of power at 415/240 V is called secondary or low voltage distribution system. Electric power to large consumers is, however, often supplied direct at 11 kV, 6.6 kV or 3.3 kV. Such consumers install their own sub-station to step-down voltage to 415/240 volts.

3.7.1. Three-phase four wire distribution system

  • ·Electric power in India is supplied to the consumers by the State Electricity Boards. The following are the different systems by which power is distributed to the consumer:

1. Single-phase A.C. supply using a 2-wire system;

2.Three-phase A.C. supply using a 3-wire system;

3.Supply of three-phase and neutral using a 4-wire system.

D.C. supply may also be available in certain areas, on either a two wire system at 220 V, or a three wire system with 440 V between the two outer conductors and 220 V between the outer conductors and the centre wire. However, D.C. supply is no longer commonly used in India and, therefore, we rarely come across D.C. installations.

  • The standard voltages at which the supply authorities deliver power to the consumer are generally as follows:

Single phase: 240 V, 50 Hz, 2 wire

Three phase: 415 V, 50 Hz, 4 wire.

In India and in many European countries the frequency of the supply is 50 Hz. In the USA the power supply frequency is 60 Hz.

  • Standardisation of supply voltage and frequency is extremely important to both electricity supply authorities as well as the manufacturers of electrical equipment. In the 3-phase, 4-wire power distribution system, power is supplied from the sub-station through 4-wires. Three of these wires are called live or phase or line wires. The fourth wire is usually at zero voltage and is called neutral wire. The neutral wire is earthed at the sub-station.
  • The voltage between any two of the line wires in a 3-phase system is..J3 times the voltage between any phase wire and the neutral.
  • Electric loads of the consumers are connected in such a way that all the three phases are equally loaded. If the individual phases of the 3-phase systems are equally loaded the current through the neutral wire will be zero. In case of 3-phase motors, ovens etc. the three phases are equally loaded by their very design. In such cases the neutral wire may be omitted. But in cases where a large number of single phase loads have to be supplied, loads on the three phases are balanced by connecting various single phase consumers or groups of consumers to different phases of the 3- phase supply.
  • Large consumers, particularly those with heavy motor loads, are provided with a 3-phase, 4-wire supply. Motors and heavy heating loads are connected to 415 V, 3-phase supply and are called three phase loads. The load is balanced over the three phases by equally distributing the lighting load and small power loads over the three phases. Consumers with load requirement more than 250 kVA are provided with supply at high voltage with a substation installed in the consumer’s premises where voltage is stepped down to 415/240 V. Fig. 15 shows typical 3-phase 4-wire distribution from a sub-station to the consumer. 7

    Fig. 15. Three phase 4-wire distribution.

    3.7.2. Voltage tolerances

    Current carried by the electrical power distribution network varies at different times of the day. This leads to varying voltage drops in the supply cables. It is thus impracticable to give each consumer exact nominal supply voltage i.e., 415/240 V at his terminals. Supply authorities are, therefore, permitted certain tolerances. Under the Indian Electricity Rules; the voltage fluctuation may not vary by more than 5% above or below the declared nominal voltage and the frequency must be within ± 1% of the declared frequency of 50 Hz. Thus the terminal voltage must be within the range.

    228 V to 252 V for a nominal voltage of 240 V;

    394.25 V to 435.75 V for a nominal voltage of 415 V.

    3.7.3. Service connections

    • The supplier’s distribution system brings power to the consumer through overhead lines or by means of underground cables to a place just outside the consumer’s premises.
    • The line bringing electric power from supplier’s low voltage distributor upto the energy meter installed at the consumer’s premises is called the service connection.
    • Service connection may be by means of underground cables or by means of overhead conductors or cables,

    Fig. 16 shows an underground service connection taken from an overhead distributor. 8

    Fig. 16. An underground service connection taken from an overhead distributor.

    Fig. 17 shows an overhead service connection-PVC weather proof cable service line.9

    Fig. 17. PVC weather proof cable service-line.

    3.7.4. Service mains

    Having brought the supplier’s service line into the consumer’s premises it is now to be connected with the consumer’s internal wiring. The supply authority have to charge the consumer for the electrical energy consumed. For this purpose the supplier’s service lines will be connected to the input terminal of the energy meter to be provided by the supply authority. 10

    Fig. 18. Block diagram of the meter distribution board.

    Fig. 18 shows the block diagram of the meter distribution board.

    ELECTRIC SUPPLY TO OUR HOUSES AND FACTORIES

    Houses: Fig. 19 shows how supply comes to our houses.

    • ·From a pole of overhead lines through service line or from sub-main in the street (L.T. Pillar) through underground cable to the house, electric supply enters our house.
      • The supply comes direct in energy meter fixed by supply authority.
      • After energy meter the live and neutral wires are controlled in fuse and link supplied by supply authority.
        • Then comes owner’s main switch I.C.D.P. After passing through I.C.D.P. wires are again controlled in distribution box which may be three, four, five or more ways. Separate phase and neutral, from distribution board, goes in different rooms for every eight to ten points. Now-a-days instead of fuse grips, miniature circuit-breakers are used and instead of I.C.D.P. automatic isolators are used. 11

          Fig. 19. Electric supply to houses.

          Factorie: Fig. 20 showing how supply comes to factories. 12

          Fig. 20. Factory wiring.

           

          • In factories or big buildings 3 phase four wire supply comes through overhead lines or cables to the factory in 3-phase meters, supplied by supply company.
            • From there, as shown in Fig. 20 supply comes in ICTP switch from which supply enters in busbar section. From there it is distributed to different sections through ICTP switches to give supply to 3 phase motors etc. and ICDP for single phase to give supply to lighting load.

           

          3.7.5. Electrical wiring systems

           

          There are several wiring systems in use. Each system of wiring has its own particular advantages, no one system is suitable for all installations. Great care, therefore, must be taken in the selection of system which depends upon working voltages, atmosphere and class of building or size of installation.

           

          Systems of wiring may be broadly classified as follows:

           

          1.Tree system

           

          2.Distribution system.

           

          1. Tree system:

           

          • In tree system branch circuits are tapped from the circuits main at convenient places.
          • In this system there are many joints and often fault occurs which is very difficult to locate.
          • This system is not used for installation purpose.

           

          Advantages: Less costly.

           

          Disadvantages:

           

          (i)                           Poor appearance.

           

          (ii)                        The fuses are scattered.

           

          (iii)                      Voltage drop affected.

           

          (iv)                      Fault location is difficult.

           

          2. Distribution system:

           

          • In this system mains are taken to one or more distribution centres and connected to the distribution boards. From those distribution boards connections to the branch circuits are taken.
          • ·In this system no joints are required, each circuit can be readily disconnected from the main without interfering with other circuits.
            • This system is now-a-days widely used for indoor wiring in buildings.

           

          Advantages:

           

          (i)                           Good appearance.

           

          (ii)                        Easy fault location.

           

          (iii)                      Each circuit is protected by a fuse.

           

          (iv)                      Easy fuse replacement.

           

          (v)                         Simple and easy for further extensions.

           

          (vi)                      All the points are maintained almost at the same voltage.

           

          Disadvantage:

           

          It is costlier since more wire will be required to install this system.

           

          I.E. rules recommend such a system.

           

          Systems of Wiring for Domestic Installations

           

          The various systems of wiring for domestic installations are:

           

          1. Cleat wiring
          2. C.T.S. (T.RS.) wiring
          3. Wooden casing and capping wiring
          4. Lead-sheathed wiring
          5. Conduit wiring

           

          -        Surface conduct;

           

          -        Underground or concealed conduct.

           

          1. Cleat wiring:

           

          • This type of wiring is not used practically for permanent wiring and is only suitable for temporary wiring purposes such as for marriages with advantages of saving in labour and overall cost.
          • In this type of wiring, insulated conductors are supported on porcelain or wooden cleats.

          The cleats are very easy to erect and fixed at a distance of 4.5 cm to 15 cm apart. V.I.R (Volcanised India Rubber) or P.V.C. (Polyvinyl chloride) wires are normally used in this system, though T.R.S. (Tough rubber sheathed system) wires can also be installed in special cleats for certain types of installation.13

          Fig. 21. Wires laid in three groove cleats.

          Fig. 21 shows wires laid in three groove cleats.

          Advantages:

          (i)                           It is easy to retrieve the materials used.

          (ii)                        Rare fire accidents.

          (iii)                      Cheaper in cost comparatively.

          (iv)                      Reduced risk of short-circuit (since the conductors are insulated from each other).

          (v)                         Inspections are easy.

          (vi)                      Alterations can be made easily.

          Disadvantages:

          (i)                           Appearance not attractive.

          (ii)                        Regular cleaning is needed.

          (iii)                      Due to the exposure of the conductors, there is a risk of mechanical injury.

          1. 1.   C.T.S. T.R.S. (Batten Wiring) :
          • In such a wiring, the wires used are sheathed in tough rubber of P.V.C. wires and they are clipped on wooden button with clips. The button is fixed on the wall or ceiling.
          • This wiring is suitable for damp climate, but cannot withstand much heat and so is not suitable for places of very hot weather and there is also danger of mechanical damage and fire hazard. C.T.S. wires are not suitable for outdoor use. Therefore, they should not be exposed to direct sunlight and where there are corrosive acid fumes. At such places now P.V.C. wire can be easily used.

          Clips used are of the following two types:

          1. Link clips

          2. Joint link clips.

          Advantages:

          (i)                           Less risk of short circuit.

          (ii)                        Least risk of mechanical injury.

          (iii)                      Cheaper than wooden casing and capping wiring.

          Disadvantages:

          (i)                           Skilled workmen are required to do the wiring.

          (ii)                        Need to be protected from sunlight, rain etc.

          (iii)                      Sharp bends should be avoided.

          3. Wooden casing and capping wiring:

          • This type of wiring is most commonly used for indoor and domestic installations.
          • Here, insulated conductors are laid inside rectangular teak wood boxes having grooves inside it. A rectangular strip of wood called capping, having same width as that of casing; is fixed over it.

          The casing is attached to the wall or ceding (and they can be painted matching with walls and ceiling). In place of wood, these days PVC is used.

          • The obvious objections to the wooden casing and capping system are that wood is very inflammable. This type of wiring should not be used where there is a danger of mechanical damage or fire hazard according to I.E. Rules. Positive and negative wires should be taken in different channels and there should be no crossing of + ve and – ve wires in the channel,

          Advantages:

          (i)                           Less costly.

          (ii)                        Wires are separated in respective grooves of the casing.

          (iii)                      Easily accessible for inspection, repairs and alterations.

          (iv)                      Suitable for low voltage installations.

          Disadvantages:

          (i)                           Wood may get damaged by white ants etc. (if wood is not properly seasoned)

          (ii)                        There is a risk of damage due to fire.

          (iii)                      For making proper casings and cappings, skilled carpenters are required.

          4. Lead-sheathed wiring:

          • In lead-sheathed system the conductor, either twin core or three core, each being separately insulated and covered with the common lead sheath, is used. The lead-sheathed wires arc easily fixed by means of metal clips on wooden batten and form a good surface system.
          • Whenever the wires are broken for connections to switch or light point, a junction box must be used. The junction boxes are usually of metal and provided with some means of ear thing so that continuity of the sheath is obtained.
          • Being very costly, these wires are not used now-a-days.

          5. Conduit wiring. Refer Fig. 22. 14

          Fig. 22. Rigid conduit coupling (Screwed).

          • In conduit wiring V.1.R. wires, or P.V.C. wires are carried through steel or iron pipes going good protection from mechanical injury or fire risks.
          • This system is the best and most desirable system of wiring for workshops and public buildings. It gives very good appearance when concealed.
            • In this wiring the pipes are cut with hacksaw and are threaded with die set for bend, tee junction box etc. and are then fixed on the walls on wooden gutties or plugs with saddles. Then wires are drawn with the help of fish wire (steel wire).
            • Now-a-days P.V.C. conduit pipes are also available which do not require threading.

          Jointing is done with a special made solution. These are flexible and can be bent easily. For concealed wiring, the pipes are directly buried in the wall and roofs and then wires are drawn through them.

          Advantages:

          (i)               No risk of fire.

          (ii)            Protection against mechanical injury.

          (iii)          The lead and return wires are carried in the same tube/pipe.

          (iv)          Earthing and electrical continuity is easily assured.

          (v)             It is shock proof if earthing and bonding is properly done.

          (vi)          Replacement/alteration of defective wiring is easy .

          (vii)        The whole system is water proof.

          Disadvantages:

          (i)                           As compared to other systems, it is costlier.

          (ii)                        In order to avoid drop in voltage due to induction, bunching of wires needs to be done.

          (iii)                      This type of wiring requires skilled workers.

          (iv)                      There is risk of short circuit under wet conditions (due to condensation of water in the tubes/pipes).

          (v)                         Its erection is not so easy and requires time.

          Comparison of wiring in domestic use:

          S. No. Aspect Cleat wiring Wooden casing capping wiring C.T.S. Wiring Metal sheathed wiring Conduct wiring
          l. Cost Low Medium Medium Medium High
          2. Life Short Fair Long Long Very long
          3. Possibility of lire Nil Good Poor Poor Nil
          4. Mechanical protection Nil Fair Fair Good Very good
          5. Protection from dampness Nil Slight Good Good Poor
          6. Type of personnal required Semi-skilled Highly skilled Skilled Skilled Highly skilled

           

          Wiring Accessories:

          The wiring accessory is any device associated with the wiring and electrical appliance of an installation, such as a fuse, plug, switch etc.

          In a complete electrical wiring system the following wiring accessories are used:

          1. Cut-outs
          2. Switches

          (i)                           One-way switch

          (ii)                        Two-way switch

          (iii)                      Double pole switch

          (iv)                      Table lamp switch

          (v)                         Two-way centre off switch

          (vi)                      Push button switch

          (vii)                    Bed switch.

          1. Meter
          2. Distribution board
          3. Socket 5A
          4. Socket 15A
          5. Plug top
          6. Holder

          (i)                           Bracket holder

          (ii)                        Batten holder

          (iii)                      Pendant holder

          1. Ceiling rose

          10. Junction boxes

          11. Fuses

          (i)                           Round type fuse unit

          (ii)                        Rewirable or kit-kat type-fuses

          (iii)                      Carbridge type fuses

          (iv)                      High rupturing capacity (HRC).

          Conventional symbols:

          WIRING

          General Wiring

          Wiring on Wall surface

          Wiring below wall surface (concealed

          Surface Conduit wiring

          Concealed conduit wiring

          Wiring going upward

          Wiring going downward

          Wiring passing vertically through a room.

          Two way switch

          Intermediate switch

          Period limiting switch

          Time switch

          Pendent switch

          Push button or bell push

          Luminous push button

          Restricted Access push button

          SWITCHES:

          Single pole switch

          Two pole switch

          Three pole switch

          Single pole pull switch

          Multiposition switch

          Interlocking switch and socket outlet

          SOCKET OUTLETS:

           

          Socket outlet 5 amperes

          Socket outlet 15 amperes

          Socket outlet 5 amperes with switch

          Socket outlet 15 amperes with switch

          Interlocking switch and socket outlet 5 ampere

          Water tight light fitting

          Specifications of wires:

          • For ordinary wiring the wires are of single solid conductors or multistrands.
            • Where flexibility is required, flexible cords are used.
            • Following types of wires are commonly used:

          1.Volcanised India Rubber (V.LR.) and tough rubber sheathed (T.R.S.)

          2.Polyvinyl chloride (P.V.C.) insulated

          - Polyvinyl chloride (P.V.C.) insulated polyvinyl chloride (P.V.C.) sheathed

          3.Weather proof wires

          4.Flexible wires.

          3.7.6. D.C. distribution

          In a D.C. system the electrical energy may be fed and distributed either by two wire or by three wire system.

          Two-wire D.C. distribution systems:

          Fig. 23 shows a two-wire D.C. distribution system from generating station or substation to the consumer’s terminals. 15

          Fig. 23. Two-wire D.C. system.

          • This system consists of t\VO wires, one outgoing (known as positive wire) and another returning one (negative wire).
          • Untapped feeders run into bus bars in suitable feeding points in the distribution area.
          • Distribution cables are connected to the bus bars through fuses or links.
          • Each separate consumer is fed from the distributor by a service cable tapped on to the distributor at the nearest convenient point.
            • The electrical appliances and motors etc. are connected in parallel between the two wires.

          Three-wire D.C. distribution systems:

          • This system consists of two outers and an earthed middle wire known as neutral wire.
          • The lamps or consumer apparatus are connected between the neutral and one of the outer.
            • The voltage between the outers being twice that of consumer’s terminals increases the transmission efficiency and reduces the copper cost.