Systems Adopted for House Wiring | Wiring | Electrical Engineering

The following points highlight the nine main systems usually adopted for house wiring. The systems are: 1. Cleat Wiring 2. Casing Wiring 3. Looping-in-System of Wiring 4. Conduit Wiring 5. Concealed Wiring 6. Metal-Sheathed Wiring or Lead-Covered Wiring 7. C.T.S. or T.R.S. Wiring 8. P.V.C. Sheathed Wiring 9. M.I.C System.

1. Cleat Wiring:

Cleat wiring is the cheapest method of wiring. The wires remain exposed to view, and these wires are drawn through cleats made of porcelain or plastic or some other approved material. The wires used are either V.I.R. or single-core P.V.C. wires. The cleats are made in two parts, called base and cap.

The base is grooved to receive the wire and the cap is placed over it. and the whole of it is placed on a wooden plug which is fixed into the wall. The cleats are tightened up on wooden plugs by means of wooden screws which also tighten the grip of the wires between two halves of the cleat. The cleats are usually of two types having two or three grooves so as to receive two or three wires. These cleats are shown in fig. 116.

Cleat wiring is most suitable for temporary job. The wiring can be completed quickly and the wiring materials can be recovered easily while dismantling. Moreover, additions and alterations as well as inspection of wiring system can be easily made.

Cleat wiring is not usually permitted in damp places and also for permanent jobs. After a certain period of installation the wires sag at some places, dust and dirt collect over them and the whole of the wiring system gives a shabby look.

While installing a cleat wiring the following points are to be kept in mind:

(i) Wooden plugs are to be properly cemented in the wall or ceiling, and the distance between two adjacent plugs should be such that the cleats are not more than 60cm. apart horizontally or vertically.

(ii) Cleats shall be of such dimensions that for low voltage installation the distance between two wires shall not be less than 2.5 cm centre to centre for branch lines and 4 cm for sub-main lines.

(iii) In no case two wires shall be placed in the same groove of the cleats. Also the wires shall be laid stretched between the cleats so that they do not touch the wall.

(iv) Joint cut-outs or fuse cut-outs shall not be used in this type of wiring. Where joints become unavoidable, wooden junction boxes with porcelain connectors inside should be used.

(v) Wiring should be enclosed in a conduit when passing through a wall or a floor. The wires should run through a conduit up to a height of 1.5 metres above floor level. In case of a metallic conduit, it should be properly earthed. Wooden bushings are to be provided at both ends of the conduit, otherwise insulation of the wires may be spoiled when drawn through it.

(vi) When two wires cross each other, they should be separated by an insulating bridge piece which should maintain a distance of at least 1.3 cm between the wires.

(vii) The wires should not run near water or gas pipes or structural work.

2. Casing Wiring:

In this system of wiring narrow grooved planks of hard wood are fixed on wooden plugs grouted in the wall instead of cleats and wires drawn along the grooves. These narrow planks are called wood casing. Usually two long grooves are made in each casing, although three- grooved casing is also available.

The top of the casing is covered by a rectangular strip of wood of the same width as that of casing. It is known as capping which remains screwed to the casing. On the surface of the capping a double bed is cut to show the position of wires so that the screws may not be driven through wrong position damaging the insulation of the cables laid under the capping.

Casing wiring is commonly used in office buildings and in residential buildings. This type of wiring is not suitable for places exposed to rain or sun or having dampness. It should not also be used in places where acids and alkalies are likely to be present.

The wood used for casing and capping must be first class seasoned teak wood or any other hard wood free from knots, shakes and shapes. The sides should be well-varnished both inside and outside with pure shellac varnish. The size of casing and capping depends upon the number of wires drawn through the grooves in a particular length of run.

While installing a casing wiring the following points are to be kept in mind:

(i) Any number of wires of the same polarity may be laid in the same groove, but in no case wires of opposite polarities are laid in one groove.

(ii) The casing should be fixed on dry wall and ceiling.

(iii) A clear space of 0.32 cm (1/8″) thick shall be kept between wall or ceiling and the casing by means of porcelain insulators.

(iv) The wooden plugs of approved sizes shall be fixed at a distance of 90 cm apart for casing of sizes up to 63.5 mm. For higher sizes of casing this distance shall not exceed 60 cm.

(v) While passing through floors or walls, heavy gauge conduit of approved sizes shall be used. The conduit should be nicely entered into casing, and it should be extended up to a height of 1.5 m above floor level.

(vi) All joints shall be made with good workmanship as per I.S.I, specification No. 732.

(vii) After the wires are laid in the grooves, the capping is attached to casing by brass screws in a proper way. The screws must not be so fixed as to pierce through the insulation of the wires.

3. Looping-in-System of Wiring:

Junction box or connector box shall not be used for joining two or more wires anywhere in a looping-in-system of wiring.

(a) Loop Wiring with Two-Plate Ceiling Rose:

In this system of wiring earthed neutral wire is drawn from the distribution board, looped, at one terminal of each ceiling rose and ended at the terminal of the farthest ceiling rose. The live wire is also drawn from the distribution board, looped at one terminal of each single-pole tumbler switch and finally ended at the terminal of the switch at the farthest point of the load circuit. (In fact the wiring work begins from the farthest point and ends in the distribution board).

Now, a single-core cable goes from the other terminal of each switch to the other terminal of the corresponding ceiling rose, the switch and the ceiling rose being used for the same load point. The cross-section of this cable is often smaller than that of others, because current of only one point flows through it. Besides, the length of this cable is also very short (see fig. 141).

(b) Loop Wiring with Three-Plate Ceiling Rose:

Use of three-plate ceiling rose in preference to two-plate one is more convenient in loop wiring. Here the live wire is not drawn up to the switch and looped there as per fig. 141. On the other hand both the live and the neutral wires are drawn up to two terminals of the ceiling rose and looped there. This is shown in fig. 142.

A single-core cable is drawn and connected to a switch terminal form the live terminal of the ceiling rose, and another single-core cable is drawn from the other terminal of the switch to a terminal of the third plate of the ceiling rose. Now, a core of the flexible cable connected with the load point is joined with the other terminal of this third plate, while the remaining core of the flexible cable is connected to the free end of the neutral plate of the ceiling rose.

(c) Comparison between Two Systems of Loop Wiring:

It can be seen from fig. 141 and fig. 142 that, while three wires are to be drawn from a switch (other than the farthest one) in case of two-plate ceiling rose, only two wires are to be drawn from a switch in case of three-plate ceiling rose. But at the same time some more length of cables is to be used in order to draw the live main up to the ceiling rose and to loop it there.

However, the distance between a ceiling rose and its switch is considerable, because all switches in a room are usually placed together on a single switchboard. Thus, on the whole, loop wiring by three-plate ceiling rose is a bit more economical than the wiring by two-plate ceiling rose.

4. Conduit Wiring:

Wiring done by insulated wires drawn through iron or steel pipes is known as Conduit Wiring. In our country usually this type of wiring is not adopted for light and fan points because of high expense. This system is mostly used in case of concealed wiring for lamps and fans in the sitting room of well-to-do people. It is widely used for medium pressure installations such as wiring of motors. In many places, in order to reduce expenses, P.V.C. pipes are found to be used now-a-days in place of steel pipes.

5. Concealed Wiring:

In order to install steel conduit pipes or rigid polythene insulated pipes in concealed position, channel of sufficient dimension is at first cut in the wall and conduit pipes are installed there­in. The conduit or P.V.C. pipes should remain well-fixed to the wall by means of stapples or saddles spaced at regular intervals of about 65 cm or 2 ft. The channel is then filled up neatly and properly and the pipes are covered by plastering immediately after installation.

For perfect completion of a concealed wiring, it is necessary to observe the following points:

(i) Conduit pipes or P.V.C. pipes must be thick, i.e. these pipes must be made of heavy gauze.

(ii) The threads of conduit pipes and other accessories shall be painted or shall be preserved by using compound as a precaution against damage or corrosion due to rust.

(iii) The line shall be as straight as possible so that uses of bends and elbows are avoided as far as practicable.

(iv) The pipes must be placed in their proper positions before casting the roof of a room.

(v) The pipes, one size higher than that required for fixed number of cables of rated size, shall be used. This will remove any difficulty experienced during drawing some more cables for additional circuits in future.

(vi) Inspection boxes shall be inserted at suitable intervals for drawing and removal of wires.

(vii) In case of steel conduit pipes boxes made of cast iron or sheet metal shall be used in place of ‘tee’.

(viii) In concealed wiring switch boards, switches regulators, wall-sockets etc. are installed inside the walls. Only the knobs of switch and regulator can be operated from outside. It is necessary that the switch boards be made of cast iron or sheet steel and covering lids of Bakelite, plastic or other insulating material. Holes are drilled in the covering lid for exposing wall sockets and knobs of switches and regulators. The front of the switchboard including the lid should be flush with the wall and no part should project further from the wall.

(ix) Switchboard, pipe and other metallic parts shall be properly earthed.

(x) In case of P.V.C. pipes it is better to adopt looping- in-system of wiring as far as possible in place of using joint-boxes.

6. Metal-Sheathed Wiring or Lead-Covered Wiring:

The wiring system completed with wires having metallic (e.g. lead) covering over rubber insulation is known as Metal-sheathed Wiring or Lead-covered Wiring. Here the conductors are rubber insulated and covered with an outer sheath of lead alloy containing about 95% lead which provides protection against mechanical injury. Lead sheath should be properly earthed.

Metal-sheathed cables are costlier than C.T.S. cables. The cables may remain exposed to sun or rain, but it should not be used where acids and alkalies are likely to be present. The cables are laid on wooden battens and remain fixed on it by means of brass or aluminium link clips spaced at intervals not exceeding 10 cm horizontally and 15 cm vertically. The thickness of the batten should not be less than 10 mm (3/8″).

For metal-sheathed wiring it is necessary to observe the following points:

(i) In metal-sheathed wiring sharp bends should be avoided. A round bend of radius not less than 10 cm may be adopted for a change of direction.

(ii) The supporting clips used for the cables must not set up any chemical reaction with the metal sheath.

(iii) The lead sheath must be electrically continuous and properly earthed. For maintaining electrical continuity, bonding of sheaths is necessary at joint-boxes and switch boards.

(iv) When passing through a floor or crossing a wall, the cable must be drawn through conduits. Conduits should go up to a height of 1.5 m above the floor level. Both ends of the conduit should be fitted with ebonite, plastic or hard rubber bushings in order to protect metal sheath and rubber insulation of cables from being damaged.

7. C.T.S. or T.R.S. Wiring:

While dealing with cables, a kind of cable, known as Super-tough Rubber- sheathed cable, has been mentioned. Wiring done with this type of cable is known as C.T.S. Wiring (Cab-Tyre Sheathed Wiring) or T.R.S. Wiring (Tough Rubber-Sheathed Wiring). In practice this type of wiring is adopted only for low voltage circuits.

C.T.S. wiring is used in open space in place of drawing bare conductors. In workshops or in some other places where fumes are generated continuously from acids etc. which may damage the insulation of ordinary cables or wear out conduits etc. or corrode the lead sheath of cables, this system of wiring is very useful. Here no other insulation is applied on the conductor excepting hard rubber sheath, and in the wiring system cable is not drawn through conduit, casing etc.

If this cable is to be drawn through open space, at first two posts are planted at the two ends of the space and a suitable piece of galvanised iron wire is installed between them in a taut condition (if necessary straining screws should also be used to keep the G.I. wire taut). The C.T.S. cable is then bound with the G.I. wire at intervals by means of same link clips as used for lead covered wiring.

If there are walls at the two ends of the open space, two hooks fixed into the walls at the two ends may be used for installing G.I. wire. If, however, a long wall or a parapet along the side of the open space is available, C.T.S. cable may be drawn along this wall or parapet. The scientific system of wiring in a horses’ stable is the C.T.S. system of wiring.

During installation of C.T.S. wiring the following points are to be kept in mind:

(i) C.T.S. cables should be laid on well-seasoned, well varnished and perfectly straight hard wood of thickness 10 mm and width sufficient enough to carry the required number of cables.

(ii) The wooden batten should remain fixed to rawl or phil plugs grouted in the wall or ceiling by means of wood screws at an interval not exceeding 75 cm.

(iii) To avoid cracks in the cable bends longer radius should be used.

(iv) While passing through wall or floor, cable must be drawn through conduit pipes. Metal Conduit should be properly earthed.

(v) C.T.S. Cables shall never be buried under plaster. These should be drawn through conduit or wooden channeling.

The Allowable Temperature Rise of Ordinary Insulated Wires and Flexible Cables:

Ordinary insulated cables and flexible cables, which are not specially manufactured for withstanding excessive heat, should not be used in places where the temperature may exceed the limit given in table no. 20.

In cases where the temperature of lamp fittings and other accessories are excessively high, cables and flexible cords which are not specially made to withstand such high temperatures should not be brought near these fittings and accessories. Where there is probability of temperature exceeding 60°C, high temperature resisting cables like flexible cord, specially covered with conditioned asbestos, must be used.

Further, they should be so connected that their temperatures do not exceed 85°C. If, however, the flexible cord is connected with a portable heater with which there is not possibility of excessive rise of temperature, a temperature rise up to 66°C may be allowed, provided that the insulation of wires should remain covered with beads or insulating sleeves suitable for high temperature, and there is no dependence on rubber insulation of cable for the prevention of earth fault of cable conductors or short-circuit among them.

Under such circumstances the upper limit of temperature rise given in table no. 20 may be exceeded. These arrangements are to be specially provided for lamps rated 200 watts or more and for immersion heater.

It should be borne in mind that polythene insulation of cable will be melted if there be excessive rise of temperature even for a very short period.

Where a cable with rubber, P.V.C. or polythene insulation or a flexible cord remains connected with bare conductor or a bus-bar, the insulation of the cable or cord should be peeled off and wires should remain bare for a length of about 15 cm from the point of connection even when the temperature of the bare conductor or the bus-bar is 90°C. But in places where this cannot be done, the current flowing through the bare conductor or the bus-bar should be so reduced as not to allow a rise of temperature above 90°C.

8. P.V.C. Sheathed Wiring:

The cable that is mostly used in these days in place of C.T.S. or V.I.R. wires in named ‘Polyvinyl Chloride Sheathed Cable’ (in brief P.V.C. cable). In fact, nowhere in house wiring the use of C.T.S. cable may be found in these days. P.V.C. cable is in use everywhere. This cable is available in single-core, twin-core or three-core, and its cost is comparatively less than that of other wires.

P.V.C. cable may be used for wiring in open space in place of bare conductor or C.T.S. cable. The rubber sheath of C.T.S. cable deteriorates quickly in places where there is oil, but P.V.C. insulation is not affected by it. For this reason P.V.C. cable is highly suitable in similar places. P.V.C. insulation can withstand acid, alkali, ozone and also direct sun light. Owing to gaps in the sheath it does not dry up, harden and crack like rubber.

But at higher temperatures P.V.C. softens. Hence, it is necessary to see that a P.V.C. cable is never excessively heated. Moreover, P.V.C. insulation becomes brittle in very cold atmosphere. For this reason cables with P.V.C. insulation cannot be used in places where there is ice or snow fall.

There is no difference between wiring systems of P.V.C. wire and C.T.S. wire. However, as the P.V.C. wire is comparatively lighter than C.T.S. wire, link clips are to be fixed on wooden battens at comparatively closer intervals. The, distance between two adjacent link clips should be 6 cm horizontally and 7.5 cm vertically.

As P.V.C. wiring is normally adopted for house wiring and as the process of working is the same, this system of wiring is usually known as C.T.S. wiring. For conduit wiring as well as for concealed wiring, single-core P.V.C. cables are drawn through conduit pipes in place of V.I.R. wires.

9. M.I.C. System:

M.I.C. system is the abbreviation of Mineral Insulated Copper-Covered wiring system. The cable used in this system has magnesium oxide insulation over its current carrying copper conductor and this insulation is tightly packed into its copper sheath. The cable can be drawn along and across the wall. Wiring is done by copper clips, saddles etc. suitable for the cable.

The cable can be turned or bent at the time of wiring, but the radius of curvature shall not be less than six times its diameter. It is strong enough to bear the weights when wiring is done vertically or when pendant lamps etc. are suspended from it. It can also be drawn through steel conduits. As, however, moisture gets easy ingress into magnesium oxide, the open and the jointed ends of cables are sealed. Special glands etc. are available for these types of sealing.

Equipment for Sealing Cable Ends:

In order to seal, a cable end it is necessary to remove the sheath and to take out the internal magnesium oxide first upto certain length of the cable from the end. For this purpose a special kind of instrument is to be used. Next, the threaded side of the sealing pot (fig. 166) is slung over the conductor and mounted on the outer copper sheath. The pot is first turned by hand and then by means of pliers so that its thread cut the outer copper sheath and the pot sets on it (fig. 167).

A kind of plastic compound is available in the market. This compound is then thrust and pressed into the sealing pot and the sealing cap (fig. 168) is put on the outer end before finally tightening on the thread by means of a kind of instrument so that the compound is well-pressed. At the same time the pot-mouth also comes folded on the outer surface of the cap and grips it tightly. The sealed end then looks as shown in fig. 169.

For drawing this type of cable through a conduit, arrangements for setting a gland and the manner of fixing it with the conduit are shown in fig. 170. The gland is shown separately in fig. 171. This gland is slung over the cable before sealing the cable-end and is fixed with the conduit-box etc. by means of lock-nuts (fig. 172). Arrangements for earthing this system of wiring is the same as that of earthing in a lead-sheathed wiring.

Special Rules for using Flexible Cable or Flexible Cord:

Flexible cable or flexible cord may be used in too low voltage and high voltage circuits.

Flexible cable or cord can be used only with portable appliances, apparatus or lamp fittings, but these should be of circular type with three cores.

Only in cases of fixed lamp fittings or where flexible cord is not out of sight or where the cord is not rubbed against rough surfaces, twisted flexible cord or parallel-twin flexible cord can be used. But for pendant lamps, only twisted cord shall be used.

Where there is possibility for the cables of being rubbed against rough surfaces or having mechanical injury or coming into contact with water, tough rubber sheathed or P.V.C. sheathed flexible cord has to be used. These types of cable are commonly known as Workshop Flexible.

It may, however, be noted that the use of unlinkable flexible cord is acceptable in all cases where the cord is not exposed to too many twists or mechanical stress and strain e.g. electric iron, hair drier and similar other domestic appliances.

In workshops or factories where rubbing against rough surfaces and mechanical stress and strain are less severe and not too frequent, braided and compounded circular flexible cords may be used in place of T.R.S. or P.V.C. sheathed cords.

N.B. Where there is probability of oil or petrol coming into contact with a flexible cord or where ‘flame-proof’ is a necessary specification, flexible cords with a sheath of polychloroprene compound may be suitable for use.

Thinly insulated or thinly sheathed flexibles may be used only in cases of house wiring carrying low current or for such light work in office where the cord is not exposed to rubbing against rough surfaces or other forms of mechanical stress and strain.

Joints in Cables:

Where two or more cables are to be joined together, their joint should be a lasting one and should not have any fault from electrical point of view. Moreover, all joints must remain in view excepting those laid underground. The joint must either be soldered or so tied (crimped) by means of clamps etc. that all the conductors are tightly fixed together.

Care should be taken to see that the resistance of the jointed conductor does not exceed that of the unjointed conductor.

Every joint of cables with soldered joint should be so insulated that it is no less efficient than the Insulation of the core at other parts of the cable. Moreover, the joint should be protected from moisture and mechanical injury. No acid or other corrosive material should be used in the process of soldering a joint.

While using metal-sheathed paper insulated cables or metal-sheathed varnished cambric cables, joint box filled with some type of compound or similar other metal sleeve may be necessary.