Installation of Metal-Sheathed Wiring | Wiring | Electrical Engineering

In this article we will discuss about the procedure for installation of metal-sheathed wiring.

At first wooden plugs or rawl plugs or phil plugs are grouted in the wall at intervals not exceeding 75 cm (2 ½’). On to each plug suitably wide, polished, hard wood battens of thickness at least 10 mm (⅜”) are screwed. The lead-covered cable is then fixed on the batten by means of brass or aluminium link clips spaced 10 cm (4″) horizontally and 15 cm (6″) vertically.

The rawl plug is a sort of narrow lengths like soft skin of a tree. Narrow holes are made into a wall along a row or a column and rawl plugs are pushed into them so as to make them flush with the wall. There is a central co-axial opening in the rawl plug through which a screw may be driven in order to fix a link clip to the wall.

In the opinion of the inventors of this type of wiring, planting the rawl plugs into the wall and fixing wiring thereon makes the wiring sufficiently strong and durable. But from practical experience it may be said that this method does not always yield good results. It is convenient to work with rawl plugs on concrete walls.

At some place it is found that cables fixed by rawl plugs come out of the wall along with the plug. Cables, so fixed, touch the wall also. As a result, on a damp wall, the lead covering soon wears out probably due to contact with lime, and the cable becomes unserviceable much earlier than its expected life.

Now-a-days for this type of work, deto-fix or phil plug is preferred over rawl plug. Besides, if the building is old and the bricks are set with mortar of soorki, as a general rule, the wooden plugs are grouted in the wall with inside out and the wooden battens are screwed to these plugs. Later cables are drawn and fixed to the battens by means of link clips.

The wooden plugs used for lead-covered wiring may be a bit narrower in size than those used for other systems of wiring. But in practice wooden plugs of general size are mostly used. If rawl plug is used, sizes of screws must be different with different sizes of rawl plug. The specialty of such screws used with rawl plugs is that, these must be threaded upto the neck.

In case the cable is to be drawn under an iron beam, the wooden batten should at first be fixed at the bottom of the beam by means of hoop iron clamps; the cable is the fixed with the batten as usual.

In case it becomes necessary to protect the cable from external injury, it may be covered by a channelling. The channel may be made of mild steel sheet or other suitable material. Alternatively the cable may be drawn through a heavy gauze conduit; but in that case all the rules applicable to conduit wiring are to be observed.

After wooden batten is fixed to the wall and the clips are fixed on the batten, wiring is to be started from the farthest point of the load circuit as in case of other wiring systems. In lead- sheathed wiring system it is always convenient to fix the cable first to the wall near the beam and then to switch etc.

As the lead-covered cable is comparatively heavy, there is inconvenience to raise it upwards when it is first fixed to the switch. While installing the cables on the highest part of the wall near the ceiling, it is better to keep the entire reel of the cable hanging near the beam. Metal-sheathed cables are available in reels as well as in coils, but the wiring job can be carried on more conveniently with reel than with coil.

If it is necessary to paint the cable after wiring, the strategy given below may be resorted to so that only the upper surface of the cable is painted without an iota of paint on the narrow wooden strip under the cable or on the wall. The work then looks neat and clean. If there is wooden batten under the cable, long strips of paper is to be laid all along the batten at first and then cables is fixed on these paper strips.

The strips of paper should be a bit wider than the cable. This paper will prevent the paint to come in contact with the wall or any other thing. When the painting of cable is over the strips of paper may be torn off and thrown away.

Connectors:

Some special types of connector are used for jointing wires or for a T-joint to lead a cable to switch board etc. These types of connector are more or less the same for almost all types of wiring. As per requirement two, three or four holes are provided in small pieces of porcelain or plastic, and inside those holes there are connectors in the form of brass tubes.

At the two ends of the connector there are brass screws for fixing the wires. The porcelain or plastic portion acts as insulators. When only one piece of wire is to be joined with another piece, the smallest size connector with a single piece of brass tube is used. For jointing twin-wire (2-core) from a single cable, a connector with two pieces of brass tubes is needed. In place of junction cut-outs connector is used even in cleat and casing wirings.

Views of different sizes of connectors are given in fig. 158. From these views it can be seen that there are holes on the top of all connectors with screws to connect wires with the connector (the left hand one shows single-joint connector).

Sometimes the outer cover of a connector is made of P.V.C. or Bakelite in place of porcelain. But as an insulator the use of porcelain is better than P.V.C. or Bakelite.

Thimbles:

Now-a-days use of a different type of connector is found here and there. This is known as thimble. Thimble is made of porcelain or plastic and look like a cap. Fig. 159 shows its view.

A thimble is threaded inside and it becomes pointed towards the upper end. Where two or more wires are to be connected together, about 0.64cm (¼”) of end insulation of each wire is taken off and all the ends are then twisted together. The combination is then put inside a thimble which is turned like a screw driver. As a result the thimble pulls the combination of twisted ends in by means of threads and thus holds it tightly.

T-Joint:

Where T-connection is taken for a point, connectors used there and the mode of connection are shown in fig. 160. A small box, called ‘joint-box’, is used here to cover the joint. The box may be made of metal or wood, but the wooden box is often preferred over metal box. Whether the box is made of metal or wood, it must be so made that at the time of white washing the wall, insects, dust or lime-water must not get access into it.

The advantage of a metal box is that the specialty of a lead-sheathed wiring is automatically retained in it, whereas in case of a wooden box it is not so. The specialty of a lead-sheathed wiring is to maintain electrical continuity of metal sheath of the cables everywhere beginning from the main board upto the farthest point of the load circuit.

Now, if a metal box is used, it is so skillfully constructed that as soon as its covering lid and the base are clamped together, the continuity of lead sheath is automatically maintained. But the same condition is not maintained with a wooden box. If a wooden box is to be used, an additional bonding clamp must be provided in the box.

The lead sheaths of all the cables taken in for connection shall remain fixed with this clamp so that electrical continuity is established among them. If metal sheath of the cable is to be used as an earth continuity conductor, then in case of non-metal box (e.g. wooden box) a strip of metal is to be used for maintaining continuity of metal sheath, and the resistance of such metal strip shall be negligible in comparison to that of the largest size of cable coming into the box.

Joint-box must not be installed in a damp place. Oftener than not, there is more leakage of current in the joint-box installed in a damp place.

Arrangement for maintaining continuity between wires near a ceiling rose is shown in fig. 162.

In this way, maintaining continuity and electrical connections among lead sheaths, finally the sheath is connected to earth at the main distribution board. If this is not done, the insulation of the cable gets damaged in a very short time. This is the specialty of a metal-sheathed wiring.

If two or more lead-covered wires are laid side by side and one wire has leakage and its sheath is not well-bonded, there will be spark between them. This will damage the cable. In metal- sheathed wiring, electrical continuity of sheath must be maintained, and this sheath must not only be well-earthed, but the earth connection must also be well-maintained. This instruction is so very essential that it is repeated time and again.

Lead-Sheathed Cables with Earth Wire:

Continuous earthing system has been introduced in a.c. circuits for the sake of safety. According to this system metallic covers of table fan, electric iron, electric heater, table lamp etc. are to be earthed. In case of cleat or casing wiring a single G.I. wire is to be drawn as the earth wire along with the wiring throughout the house.

To remove this difficulty a kind of cable is available in the market. In this cable a single earth wire is provided along with insulated copper wire or wires within the same lead sheath. While jointing two or more wires, a separate connector should also joint all related earth wires.

If the outer cover and inner lever is made of metal, the switch should also be earthed as per rule. In such cases lead-sheathed cables with earth wire inside is used. How earth wire of a circuit remains connected with earth in the distribution board is clearly shown in fig. 163.

There is nothing more to describe the wiring proper. Descriptions of different methods and systems of wiring (e.g. cleat or casing wiring) are also applicable to this metal-sheathed system. Looping-in-system of wiring may also be adopted with lead-sheathed cables if and when necessary.

Taking out Conductors from Metal-Sheathed Cables:

In order to take conductors out of metal-sheathed cables for connection by removing the sheath, a sharp-edged knife is at first pressed upon the sheath in such a way that only the sheath is somewhat pierced, but the rubber insulation over the conductors should remain intact.

The cable end is then bent this way and that way so that the sheath is loosened and it easily comes out from the surface of the cable. Later on the cloth tape is opened carefully and a little bit of rubber insulation is cut off so that only as much conductor is exposed as is required for connection. The cloth tape on the remaining part without sheath should remain unchanged.

Drawing of Cables through the Floor:

The lead-sheathed cable should be drawn through a heavy gauze conduit pipe when the cable is drawn from lower floor to upper floor. The conduit length shall remain extended upto a height of 1.5 m above the floor level in the upper floor, while the lower end of the conduit shall remain flush with the ceiling of the lower floor. Both ends of the conduit should be fitted with bushes made of wood or ebonite or some other insulating material (see fig. 143).

Drawing of Cables through Partition Wall between Two Adjacent Rooms:

Like other systems of wiring metal-sheathed cable also should be drawn through porcelain wall-tube or steel conduit or hard P.V.C. conduit as straight as possible.

Concealed Wiring through the Wall:

Lead-sheathed cable cannot be laid direct under plaster. For concealed wiring it should be either drawn through conduit pipe or some other means is to be adopted first after which the whole thing is covered with plaster.