Introduction
A network of wires connecting various accessories for distribution of electrical energy from the supplier’s meter board to the numerous electrical energy consuming devices such as lamps, fans and other domestic appliances through controlling and safety devices is known as wiring system.
The supplier’s service cable feeding an installation terminates in what is usually called the service fuses. In an ordinary house the service fuse is called as service cutout. Such cutouts including service meters remain the property of the supplier and represent the furthest point of the supplier responsibility. The point at which the consumer's wiring is connected into cutout is known as point of commencement of supply or consumer's terminals. From consumer terminals onwards the supply cables are entirely under the control of consumer's and so laid out as per his selection. A typical house wiring circuit is shown in fig. a
fig (a)
Systems of distribution of electrical energy
Since as per recommendations of ISI the maximum number of points of lights, fans and socket-outlet that can be connected in one circuit is 10 and the maximum load that can be connected in such a circuit is 800 watts, hence in case more load or more points are required to be connected to the supply system, then it is to be done by having more than one circuit.
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Distribution Board System
In distribution board system, which is most commonly adopted for distribution of electrical energy in a building, the fuses of various circuits are grouped together on a distribution board, some times simply known as fuse board.
The two copper strips, known as bus-bars, fixed in a distribution board of hard wood or metal case are connected to the supply main through a linked switch so that the installation can be switched off as a whole from both the poles of supply if required. A fuse is inserted in the + ve or phase pole of each circuit so that each circuit is connected up through its own particular fuse.
In large buildings, however, if only one distribution board were used, some of the points would be at a considerable distance from it and in such cases it is advisable to employ sub-distribution boards either to save cable or to prevent too great voltage drop at the more distant points (lamps or fans or other appliances). In such cases main distribution board controls the circuit to each sub-distribution board from which the sub-circuits are taken, as shown in fig. a
The number of circuits and sub-circuits are decided as per number of points to be wired and load to be connected to the supply system. For determination of load of an installation the following ratings maybe assumed unless the values are known or specified.
a) Fluorescent lamps — 40 watts.
b) Incandescent lamps, fans, and socket outlets — 60 watts.
c) Power socket-outlets — 1,000 watts.
d) Exhaust fans — as per capacity of exhaust fans.
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The Tree System
Another system of distribution of electrical energy in a building is the tree system. In this system smaller branches are taken from the main branch, as shown in fig. b and the wiring system resembles a tree. As each branch is taken off, a fuse is inserted. This system used to be employed in early days. Now-a-days it is no more adopted due to the following draw-backs in this system.
a) The voltage across all the lamps does not remain the same. The lamps in the last branch will have least voltage across them on account of voltage drop in leads,
b) A number of joints are involved in each circuit.
c) Fuses are scattered.
d) In case of occurrence of fault all the joints have to be located and if some of these joints are concealed beneath floors or roof spaces, a lot of
difficulties are to be faced. Sometimes a number of such joints are required to be opened for testing purposes, so damage is caused to installation, conductors and building.
Methods of wiring
There are two methods of wiring known as
a) joint box system (or Tee system) and b) Loop-in system
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1. Joint Box or Tee System:
In joint box system the connections to the lamps are made through joints made in joint boxes by means of suitable connectors or joint cutouts. In this method though there is a saving in the quantity of wire or cable required but the same is offset by the extra cost of joint boxes. The other disadvantage of connections is that the number of T-connections made in a wiring system results in weakness if not properly made. Now-a-days the use of this system is limited to temporary installations only, as its cost is low.
2. Loop- in- system:
This system is universally employed for connections of various lamps or other appliances in parallel. In this system when a connection is required at a light or switch, the feed conductor is looped-in by bringing it direct to the terminal and then carrying it forward again to the next point to be fed, as shown in fig. d. The switch and light feeds are carried round 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 neutral conductors are looped either in switch board or from light or fan. Line or phase should never be looped from light or fan.
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The advantages and disadvantages of loop-in system are as follows;
Advantages
a) Joint boxes are not required.
b) In loop-in system no joint is concealed beneath floor or in roof spaces. As they are made only at outlets so they are accessible for inspection and opening out merely by removing the fitments
concerned. Hence fault location is easy.
Disadvantages:
a) Length of wire or cable required is more and voltage drops and copper losses are, therefore, more.
b) Looping-in switches and lamp holders is usually difficult.
SYSTEMS OF WIRING
The types of internal wiring usually employed in our country are:
1. Cleat wiring:
In this system of internal wiring the cables used are either VIR or PVC type. The cables are held by porcelain cleats about 6 mm above the walls or ceiling. The cleats are made in two halves, one base and the other cap. The base is grooved to accommodate the cables and the cap is put over it and whole of it is then screwed on wooden plugs (gutties) previously cemented into the wall or ceiling. Thus the cables are firmly griped between the two halves of the cleats and secured to the supporting wall or ceiling. The cleats used are of different sizes and different types in order to accommodate cables of various sizes and different numbers of cables respectively. The cleats are of three types—one groove, two grooves and three grooves to accommodate one, two, and three cables respectively.
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Advantages:
a) It is the cheapest system of internal wiring.
b) Its installation and dismantlement is easy and quick.
c) Material is recoverable after dismantlement.
d) Inspection, alterations and additions can be easily made.
e) Skill required is little.
Disadvantages:
a) It is not good looking.
b) It is quite temporary and perishes quickly.
c) The wires are exposed to mechanical injury.
d) The insulation catches dampness from the atmosphere and common salt like substance appears on the insulation which lowers the insulation resistance and Causes leakage. Hence this system of wiring cannot be used in damp places.
e) Oil and smoke are injurious to VIR insulation.
Fields of Application:
The wiring of this type is very suitable for temporary installations in dry places. This is also acceptable where appearance is not so important and cheapness is the main consideration. This system is not suitable for use in domestic premises.
2. Wooden Casing and Capping Wiring:
The cable used in this type of wiring is either VIR or PVC or any other approved insulated cables. The cables are carried through the wooden casing enclosures. The casing consists of V-shaped grooves (usually two to hold the cables of opposite polarity in different groves) and is covered at the top by means of rectangular strip of wood, known as capping, of same width as that of casing. The capping is screwed to the casing by means of wooden screws fixed at every 15 cm on the centre fillet. To protect the casing against white ants first class seasoned teak wood, varnished by shellac varnish is employed. Two or three cables of same polarity (either all phases or all neutrals) may be run in one groove and in no case the cables of opposite polarity should be run in the same groove. The casing ia usually placed 3.2 mm apart from the wall or ceiling by means of porcelain distance pieces of thickness not less than 6.5 mm in order to keep the casing dry at the back.
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3. CTS or TRS Wiring.
In this type of wiring the cables used may be single core, twin core or three core TRS cables with a circular oval shape. Usually single core cables are preferred. TRS cables are sufficiently chemical proof, water proof, steam proof but are slightly affected by lubricating oils. TRS eaoles are run on well seasoned, perfectly straight and well varnished (on all four sides) teak wood batten of thickness 10 mm at least. The width of the batten depends upon the number and size of cables to be carried by it. The battens are available in width of 13,19,25,31,38,44,50,56,63,69 and 75 mm. The wooden battens are secured to the walls or ceiling by flat head wood screws to wood or other approved plugs at an interval not exceeding 75 cm. The cables are held on the wooden batten by means of tinned brass link clips already fixed on the batten with brass pins and spaced at an interval of 10 cm in case of horizontal runs and 15 cm in case of vertical runs. The wiring after erection is neatly painted with two coats of oil-less non-cracking paint as specified in IS 732 and so on.
Advantages
a) Its installation is easy and quick and saving in labor largely compensate for the extra cost of the cable.
b) Its life is long.
c) Within certain limits it is fire proof.
d) It can withstand the action of most chemicals such as acids and alkalies.
e) It is cheaper than other types of wiring except cleat wiring.
f) If the job is carried out with proper attention, it gives a nice appearance.
Disadvantages
a) Good workmanship is required to make a sound job in TRS wiring.
b) This type of wiring cannot be recommended for use in situations open to sun or rain unless preventive steps are taken to preserve the
insulation of cables.
Fields of Application
The TRS wiring is suitable for low voltage installations and is extensively used for lighting purposes everywhere i.e. in domestic, commercial or industrial buildings except workshop where it is liable to mechanical injury.
This type of wiring is suitable in situations where acids and alkalies are likely to be present.
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4. Lead Sheathed Wiring
This type of wiring employs conductors insulated with VIR and is covered with an outer sheath of lead aluminum alloy containing about 95% lead. This metal sheath gives protection to the cable from mechanical injury, dampness and atmospheric corrosion. The whole lead covering is made electrically continuous and is connected to earth at the point of entry to protect against electrolytic action due to leaking current and to provide safety against the sheath becoming a live. The cables are run on wooden batten and fixed by means of link clips as in TRS wiring. The great part of the cable employed is flat twin (the cable having two insulated conductors side by side covered with red and black tape respectively and under one flat covering of lead alloy). Three-core flat type cable is also used in certain cases as well as single core cables under a circular sheath of lead alloy.
Advantages
a) It provides protection against mechanical injury better than provided by TRS wiring.
b) It is easy to fix and looks nice as it can be run in building without damaging decoration and can be painted to suit colour scheme of the surroundings.
c) Its life is long if proper earth continuity is maintained throughout.
d) It can be use din damp situations provided protection against moisture effect on the ends of the cable is given.
e) It can be used in situations exposed to rain and sun provided no joint is exposed.
Disadvantages
a) It is costlier than TRS wiring.
b) It is not suitable for places where chemical corrosion may occur.
c) In case of damage to insulation the metal sheath becomes alive and
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Fields of Application
This wiring system is suitable for low voltage (up to 250 volts) installations. It may be used in places exposed to sun and rain provided no joint is exposed. It may also be used in damp places with a suitable protective covering. It should not be used in places where chemical corrosion may occur.. This type of wiring is not very common in use these days except for some small installations and distribution boards etc.
5. Conduit Wiring
In this system of wiring steel tubes, known as conduits, are installed on the surface of walls by means of saddles or pipe hooks or buried under plaster and VIR or PVC cables are drawn into afterwards by means of a GI wire of size of about 18 SWG. In damp situations the conduits can be spaced from the walls by means of small wooden blocks fixed below the pipes at regular intervals. In order to facilitate drawing of wires numbers of inspection fittings are provided along its length. The conduits should be electrically and mechanically continuous and connected to earth at some suitable point.
The conduits used for this purpose are of two types namely (i) light gauge (or split type) conduit and heavy gauge (or screwed type) conduit. Light gauge or split conduit with a seam along its length is used for cheap work. It is not water tight or even damp proof and is not permitted on medium voltage (i.e. on voltages higher than 250V). Screwed conduit (solid drawn or with welded seam) is used for all medium voltage (250 V or 600 V) circuits and in places where good mechanical protection and absolute protection from moisture is desired. In general the finish of the conduit is black stove-enamelled, there being a smooth coating of enamel both on the inside and outside surface of the tube.
Galvanized conduit is also employed, especially in damp situation when the conduit is on the surface but under ordinary conditions buried in walls it offers little, if any, advantage over good enamelled conduits.
Advantages
a) It provides protection against mechanical damage.
b) It provides complete protection against fire due to short-circuits etc.
c) The whole system is water proof.
d) Replacement and alteration of defective wiring is easy.
e) Its life is long if the work is properly executed.
f) It is shock proof also if earthing and bonding is properly done.
Disadvantages
a) It is very costly system of wiring.
b) Its erection is not so easy and requires time.
c) Experienced and highly skilled labour is required for carrying out the job.
d) Internal condensation of moisture may cause damage to the insulation unless the system outlets are properly drained and ventilated.
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Fields of Application
As this system of wiring provides protection against fire, mechanical damage and dampness so this is the only approved system of wiring for:
a) Places where considerable dust or puff is present such as in textile mills, saw mills, flour mills etc.
b) Damp situations.
c) In workshops for lighting and motor wiring.
d) Places, where there is a possibility of fire hazards such as in oil mills, varnish factories etc.
e) Places, where important documents are kept such as a record room.
f) Residential and public buildings, where the appearance is the prime thing. The recessed type conduit wiring is preferred for residential and public buildings.
CHOICE OF WIRING
The following factors should be considered before selecting a particular type of wiring.
a. Safety: The first and foremost consideration is safety to a person using electricity against leakage or shock. Where there is a possibility of fire hazard, conduit wiring is used.
b. Mechanical Protection: The wiring must be protected from mechanical damage during use.
c. Permanency: The wiring must not deteriorate unduly by action of weather, fumes, dampness etc.
d. Appearance: The wiring should he good looking.
e. Durability: The wiring must be durable.
f. Accessibility: In wiring system there should be facilities for extension, renewal or alterations.
g. Initial Cost: The wiring selected should suit the pocket of the owner of the building.
h. Maintenance Cost: The wiring should have, as far as possible, the lowest maintenance cost.
The other factors, in addition to above, to be kept in view while making the choice of wiring is load voltage to be employed, type of building etc.
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