Domestic Wiring

A network of wires drawn connecting the meter board to the various energy consuming loads (lamps, fans, motors etc) through control and protective devices for efficient distribution of power is known as electrical wiring.
Electrical wiring done in residential and commercial buildings to provide power for lights, fans, pumps and other domestic appliances is known as domestic wiring.
There are several wiring systems in practice. They can be classified into:

Conduit Wiring:

In this system PVC or VIR cables are run through metallic or PVC pipes, providing good protection against mechanical injury and fire due to short circuit.
This is most desirable for workshops and public Buildings.
Depending on whether the conduits are laid inside the walls or supported on the walls, there are two types of conduit wiring which are :
Surface conduit wiring: In this method conduits are mounted or supported on the walls with the help of pipe books or saddles.
In damp situations, the conduits are spaced apart from the wall by means of wooden blocks.
Concealed conduit wiring: In this method, the conduit are buried under the wall at the some of plastering. This is also called recessed conduit wiring.
The beauty of the premises is maintained due to conduit wiring.
It is durable and has long life.
It requires very less maintenance.
But the repairs are very difficult in case of concealed conduit wiring.
This method is most costly and erection requires highly skilled labor.
In concealed conduit wiring, keeping conduit at earth potential is must.

FACTORS AFFECTING THE CHOICE OF WIRING SYSTEM

The choice of wiring system for a particular installation depends on technical factors and economic viability.
Durability: Type of wiring selected should conform to standard specifications, so that it is durable i.e. without being affected by the weather conditions, fumes etc.
Safety: The wiring must provide safety against leakage, shock and fire hazards for the operating personnel.
Appearance: Electrical wiring should give an aesthetic appeal to the interiors.
Cost: It should not be prohibitively expensive.
Accessibility: The switches and plug points provided should be easily accessible. There must be provision for further extension of the wiring system, if necessary.
Maintenance Cost: The maintenance cost should be a minimum.
Mechanical safety: The wiring must be protected against any mechanical damage.

Specification of Wires:

The conductor material, insulation, size and the number of cores, specifies the electrical wires. These are important parameters as they determine the current and voltage handling capability of the wires.
The conductors are usually of either copper or aluminum. Various insulating materials like PVC, TRS, and VIR are used.
The wires may be of single strand or multi strand.
Wires with combination of different diameters and the number of cores or strands are also available.
Ex: 1/20 or 3/22
The numerator indicates the number of strands while the denominator corresponds to the diameter of the wire in SWG (Standard Wire Gauge).
SWG 20 corresponds to a wire of diameter 0.914mm, while SWG 22 corresponds to a wire of diameter 0.737 mm.
A 7/0 wire means, it is a 7-cored wire of diameter 12.7mm (0.5 inch).
The selection of the wire is made depending on the requirement considering factors like current and voltage ratings, cost and application.
Example: Application: domestic wiring

Lighting – 3/20 copper wire
Heating – 7/20 copper wire

The enamel coating (on the individual strands) mutually insulates the strands and the wire on the whole is provided with PVC insulation.
The current carrying capacity depends on the total area of the wire.
If cost is the criteria then aluminum conductors are preferred.
In that case, for the same current rating much larger diameter of wire is to be used.

EARTHING:

The potential of the earth is considered to be at zero for all practical purposes as the generator (supply) neutral is always earthed.
The body of any electrical equipment is connected to the earth by means of a wire of negligible resistance to safely discharge electric energy, which may be due to failure of the insulation, line coming in contact with the casing etc.
Earthing brings the potential of body of equipment to zero, thus protecting operating personnel against electrical shock.
The body of the electrical equipment is not connected to the supply neutral because due to long transmission lines and intermediate substations, the same neutral wire of the generator will not be available at the load end.
Even if the same neutral wire is running it will have a self resistance, which is higher than the human body resistance.
Hence, the body of the electrical equipment is connected to earth only.
Thus earthing is to connect any electrical equipment to earth with a very low resistance wire, making it to attain earth potential.
The wire is connected to copper plate placed at depth of 2.5 to 3meters from the ground level.
The earth resistance for copper wire is 1 ohm and that of G I wire less than 3 ohms.
The earth resistance should be kept as low as possible so that the neutral of any electrical system, which is earthed, is maintained almost at the earth potential.
The typical value of the earth resistance at powerhouse is 0. 5 ohm and that at substation is 1 ohm.

The earth resistance is affected by the following factors:

Material properties of the earth wire and the electrode
Temperature and moisture content of the soil
Depth of the pit
Quantity of the charcoal used

Effect of earthing for protection of equipment and protection from electric shock can be understood from the following figures:

The above case describes when insulation of device is healthy, but apparatus is not earthed
In this case the supply current cannot flows through the human body due to proper apparatus insulation resistance.
Hence no current passes to human body and the person gets saved from electric shock even if earthing is not done properly.

The above case describes when insulation of device is not healthy, and apparatus is not earthed
In this case the supply current cannot flows through the human body as the apparatus insulation resistance is not high.
Hence part of the current passes to human body and the person gets electric shock.

The above case describes when insulation of device is defective, and apparatus earthed properly.
In this case the supply current is divided and flows through the human body as well as the earthing part.
As the earth resistance is very very low, majority part of current passes through earthing and a very negligible part of the current passes to human body and the person doesn’t gets electric shock.

Necessity of Earthing:

To protect the operating personnel from danger of shock in case they come in contact with the charged frame due to defective insulation.
To maintain the line voltage constant under unbalanced load condition.
Protection of the equipment.
Protection of large buildings and all machines fed from overhead lines against lightning.

Methods of Earthing:

Plate Earthing:

In this method a copper plate of 60cm x 60cm x 3.18cm or a GI plate of the size 60cm x 60cm x 6.35cm is used for earthing.
The plate is placed vertically down inside the ground at a depth of 3m and is embedded in alternate layers of coal and salt for a thickness of 15 cm.
In addition, water is poured for keeping the earth electrode resistance value well below a maximum of 5 ohms.
The earth wire is securely bolted to the earth plate.
A cement masonry chamber is built with a cast iron cover for easy regular maintenance.

Pipe Earthing:

Earth electrode made of a GI (galvanized) iron pipe of 38mm in diameter and length of 2m (depending on the current) with 12mm holes on the surface is placed upright at a depth of 4.75m in a permanently wet ground.
To keep the value of the earth resistance at the desired level, the area (15 cms) surrounding the GI pipe is filled with a mixture of salt and coal.
The efficiency of the earthing system is improved by pouring water through the funnel periodically.
The GI earth wires of sufficient cross- sectional area are run through a 12.7mm diameter pipe (at 60cms below) from the 19mm diameter pipe and secured tightly at the top as shown in the following figure.

When compared to the plate earth system the pipe earth system can carry larger leakage currents as a much larger surface area is in contact with the soil for a given electrode size.
The system also enables easy maintenance as the earth wire connection is housed at the ground level.

PROTECTIVE DEVICES:

Protection for electrical installation must be provided in the event of faults such as short circuit, overload and earth faults.
The protective circuit or device must be fast acting and must isolate the faulty part of the circuit immediately.
It also helps in isolating only required part of the circuit without affecting the remaining circuit during maintenance.
The following devices are usually used to provide the necessary protection:

Fuses
Relays
Miniature circuit breakers (MCB)
Earth leakage circuit breakers (ELCB)

FUSE:

Fuse is a safety device used in any electrical installation, which forms the weakest link between the supply and the load.
It is a short length of wire made of lead / tin /alloy of lead and tin/ zinc having a low melting point and low ohmic losses.
Under normal operating conditions it is designed to carry the full load current.
If the current increases beyond this designed value due any of the reasons mentioned above, the fuse melts (said to be blown)
and isolates the power supply from the load as shown in the following figures.

The material used for fuse wires must have the following characteristics:

Low melting point
Low ohmic losses
High conductivity
Lower rate of deterioration

Important terms related to fuse are:
Rated current: It is the maximum current, which a fuse can carry without undue heating or melting.
It depends on the following factors:

Permissible temperature rise of the contacts of the fuse holder and the fuse material
Degree of deterioration due to oxidation.

Fusing current: The minimum current at which the fuse melts is known as the fusing current.
It depends on the material characteristics, length, diameter, cross-sectional area of the fuse element and the type of enclosure used.
Fusing Factor: It is the ratio of the minimum fusing current to the rated current. It is always greater than unity