Installation of Casing Wiring | Systems | Wiring | Electrical Engineering

Casing wiring is generally adopted for low pressure installations. Here V.I.R. wires or P.V.C. wires are laid in grooves.

The two main disadvantages of this system of wiring are:

(i) Very good workmanship is required to make the job perfect, and this results in costlier installation.

(ii) There is every risk of fire from wood.

Casing generally used for installation is about 44 mm wide and 16 mm in thickness (height). However, for cables of higher sizes, 80-100 mm wide and proportionally higher in thickness casings are also in use. Casings may be 5.5 m to 6.0 m long, but smaller lengths are also available. Lengths of about 2.5 m to 3.0 m are convenient for handling.

There are two grooves in each casing. The width of the strip of wood separating the two grooves should be carefully observed so that it is not less than 13 mm, and the portion of wood below each groove shall not be less than 7 mm in thickness. In case the cable has a large cross-section or a number of cables are to be drawn, the size of casing should increase accordingly.

At the time of wiring the cables laid in the grooves are covered by a very thin and long strip of wood which is as wide as the casing. This is known as capping. The thickness of capping should be about 7 mm.

Arrangements for Fixing the Casing:

According to Indian Code of Practice No. 6.2.5. one has to be aware of the following points while installing casing wiring:

(i) Installation of casing wiring is permitted only in dry place. Casing shall not be embedded into cement or plaster. It shall neither be so set as to get contact with a water pipe, nor it shall be laid just below a water pipe. It shall not also be used in a place where moisture accumulates and drips,

(ii) Capping should be fixed on the casing only by screws,

(iii) Provision must be there for easy insertion of cables into the casing.

After the wooden plugs have been grouted in the wall at regular intervals of about 60 cm (for casing wider than 6.35 cm, wooden plugs may be set at intervals of even 90 cm.), a length of casing is placed along the line of wooden plugs and marking is done as to where it should be drilled for fixing on to the wooden plugs. The drilling is done on the strip of wood separating the two grooves. That is why the width of the strip must not be too small.

By drilling the marked points the casing is then fixed on the wooden plugs by means of wood-screws (the screws used by carpenters in woodwork). Casings which are to be vertically fitted should be accurately vertical as checked by the plumb-line, while those which are to be horizontally fitted should be accurately horizontal as checked by the spirit level; otherwise the wiring looks odd (good wiremen are so expert that they can check these by eye estimation).

Before fixing the casing, it is necessary to smear its sides and back properly with two coats of shellac varnish. Otherwise during monsoons or due to damp weather dampness will enter through the wood and damage the insulation of the cables. The varnish used for the casing should be so dense as to be prepared by mixing at least 350 grammes of pure lac with a quart bottle of methylated spirit.

In cheap installations this is practically ignored; but that is not right and proper. That is why after some time the insulation of cables get damaged rather quickly. At the end of the casing line, after fixing the capping on it, either the holes at the end of grooves should be stuffed with suitable pieces of wood, or keeping the casing a bit longer and rubbing the wood on both sides of grooves by a planer, the end grooves are eliminated as well as the capping is made to suit such an end so that no damp from air or moisture from air carrying water particles can reach the cable under the capping. Further protection is provided by painting or varnishing the casing wiring once again on all sides after the wiring is finished. This also prevents entry of external damp.

Spacing insulators are used at a place where the casing is passing below an iron beam. An idea about different sizes of casings available in the market is given in Table No. 16.

Size of casing makes no difference for a.c. or d.c. supply. To keep the casing a little away from the wall, a round piece of porcelain with a central hole is placed between wooden plug and the casing. This is known as spacing insulator. Sometimes, to keep the insulator farther away from the wall, small bobbin insulators are used. It is in damp situations that bobbin insulators are more common.

The screw used for fixing the casing must be long enough to pass through the casing, capping, central hole in the bobbin insulator or spacing insulator and the wooden plug in the wall. The spacing insulators should have a thickness of at least 4 mm. Some people use even cleat (either upper half or lower half) in place of spacing insulator.

With the progress of work when the wires have been laid in the grooves, the capping is fixed on the casing by means of small screws. The screws are round-headed of size 19 mm. If the width of the casing is less than 50 mm, a series of screws are fixed on the central line of the copping, and in case the width is more, two rows (or columns) of screws are fixed on two sides of the capping. For this reason the width of the strips of wood on both sides of the casing shall not be less than 10 mm. In good work screws used for fixing the capping are made of brass.

In casing wiring work starts from the farthest point of the load circuit, gradually proceeds towards the main board and finally ends there. Here the ceiling rose of the lamp bracket is set on a round wooden block. This block should have a thickness (height) of 4 cm (1 1/2″) with two coats of varnish applied on it. It has a saw-cut on one side in such a manner that the tip of the casing closely fits on to it (see fig. 128 and fig. 129). The casing is then cut off to exact measure as to how far it will reach in straight length.

When two pieces of casing or of capping are to be joined together, the joint should be completed as shown in Fig. 130. Such a joint is called Lap Joint. On one side the bottom base of casing is retained and the grooves are eliminated, while on the other side of casing the three strips of wood walling the grooves are retained and the base is removed.

Now, on screwing these two sides together, the whole thing will be as strong as a continuous one. The joint of capping shall be an oblique one [Fig. 130(b)]. Care must be taken to see that capping’s are not joined together at a point where there is already a joint of casing, and also no screw for fixing the capping pierces any side wall of the casing.

Joint at the Corner:

The kind of joint necessary at the corner round which the casing wiring must proceed is shown in Fig. 131. The two tips of casings that are to be joined together are placed on the floor, cut at an angle of 45° with a saw and finally screwed to each other. In order that the corners of the grooves do not injure the insulation of the cables, these should be chiselled to flush with each other.

The appearance of the joint where the casing is taken from one wall to another is either as shown in Fig. 132 or somewhat similar. Here each casing is placed on the floor resting on one side, then cut at an angle of 45° with a saw, and finally the joint is made flush with each other.

Later, just for the corner point, a small piece of casing is taken and from it a piece as shown in Fig. 133 is cut off by a saw and this smaller piece is fixed to the casings on both sides by means of screws. The shape of such a joint should be such that the radius of curvature of the joint should not be less than 75 mm. The purpose of so doing is that, just at the corner the insulation of cables is not damaged due to twist etc. and the sharp bend (about 90°) of cables is also avoided.

Bending on Cables:

To bend a V.l.R. or P.V.C. cables the internal radius of bend shall be at least four times the diameter of the cable. Where the casing will go from one wall to another on the external side, the joint should be as per fig. 134.

T-Joint:

From a point in the continuous run of the casing, sometimes connection is to be taken out for lamp point, fan point etc. through a joint of the casing, known as T-Joint. Where such a joint is to be adopted, a V-shaped piece of casing is to be cut off upto the middle of the casing used in the continuous run.

Later, the tip of another casing to be joined to it is cut off in the shape of V and is made flush with the V-groove of the former casing. This is shown in Fig. 135.

Bridge:

When it is required to draw one circuit over another, a small piece of casing, named Bridge, is used so that the cable of one circuit does not come in touch with that of another. At first the bridge is fixed on the casing and then the second cable is drawn over it.

Where a T-joint is necessary, an one-half bridge is fixed there along with a full bridge. This additional one-half portion is known as Half-Bridge. A bridge is also used where cable of one circuit crosses that of another circuit. The joint of casings at this point is called Cross Joint. T-joint and cross-joint of casings are shown in Fig. 137 and Fig. 138 respectively.

Leading a Cable from One Room to Another:

When leading from one room to another, a cable may be drawn either through a casing or through a wall-tube. If casing is used, the hole in the wall must be large enough to leave a clearance of at least 25 mm (1″) all around the casing. The purpose of this clearance is to keep the casing dry through ventilation of air. If a wall-tube is to be used, the two ends of the tube project a little from the wall.

The partition wall between the grooves at the end of casing remaining in contact with the wall-tube must be cut off to the same extent as the amount of projection of the tube form the wall (Fig. 139). This will keep the wall-tube properly fitted with the casing. But in case the diameter of the tube is larger than the height of the casing or where more than one wall-tubes are used, it will not be possible to fix the capping over the casing.

In such cases, therefore, the height of the casing is increased with the help of a half bridge. Now-a-days (specially in a.c. circuits) continuous earthing system is in practice. In this system a single No. 6 or No. 8 S.W.G. galvanized iron wire is drawn continuously outside the casing along with the cables and finally earthed. This is called ‘earth continuity conductor’.

The outer metallic covers of fan regulator iron-clad distribution box. Earth terminal of the wall socket etc. remain connected with this wire. Usually a separate wall-tube is used for leading earth continuity conductor through the wall. For this work a half bridge on the casing near the wall is indispensable.

It may be mentioned here that, just as the metallic covers of all equipment’s which are liable to come in contact with human body are to be connected with earth continuity conductor, so also there should be three sockets instead of two in every wall socket and three pins instead of two with every wall plug. Also the flexible cables used in this system must have three lengths of insulated wires instead of two.

Leading a Cable through the Floor:

In a casing wiring if cables are to be drawn from a lower floor to an upper floor, a piece of conduit is pushed through a hole made in the floor. The sizes of wires of all the circuits to be drawn from lower to upper floor are calculated at first, and then the size and number of conduits are determined accordingly.

If continuous earthing system is adopted, another extra conduit is to be provided for drawing earth continuity conductor. At the ceiling of the lower floor all conduits must project at least 25 mm. At both ends of a conduit insulating bushes are to be fitted. In the upper floor conduit will rise up to a height of 1.5 metre (5 ft.) above the floor level.

At this end of the conduit one end of a casing should remain properly fitted. For proper fitting the lower end of the casing is cut to size as shown in fig. 140. If necessary, the spacing of the casing from the wall may be increased by using a half bridge. Besides, every piece of conduit should remain well-earthed.

Modes of Casing Wiring; Utility of Looping-in-System:

Like cleat wiring, casing wiring can also be done by means of connectors inside junction boxes (formerly this was the practice; even now it is found in many places) as well as by looping-in- system. Loop wiring has many advantages. No joint is necessary and the insulation resistance is better retained by this system than any other system of wiring.

The reason for no joint is that, one piece of cable is joined with another piece only through brass screws of switches and ceiling roses. What is meant by jointing of cables does not at all happen in this system. Its main disadvantage, however, is that the length of cable required for wiring is somewhat more.