Industrial Power Distribution and Illumination

​ arthing Process in commercial and Industrial complex.

Chapter 2​: E

2.1 Introduction​:

To connect the metallic Parts of an Electric appliance or installations to the earth is called
Earthing

It is the connection of the metallic parts of electric machinery and devices such as metallic
covering of metals, earth terminal of socket cables, stay wires that do not carry current to the
earth etc.

Earthing can be said as the connection of the neutral point of a power supply system to the
earth so as to avoid or minimize danger during discharge of electrical energy.

Below are the basic needs of Earthing.

A) To protect human lives as well as provide safety to electrical devices and appliances from
leakage current.

B) To keep voltage as constant in the healthy phase (If fault occurs on any one phase).

C) To Protect the Electric system and buildings from lightning.

D) To serve as a return conductor in electric traction systems and communication.

E) To avoid the risk of fire in electrical installation systems.

2.2 System Earthing and Equipment Earthing

A) System Earthing

In system earthing, the neutral of the system is directly connected to earth by the help of the GI
wire. Such type of earthing is mostly provided to the system which has star winding. For
example, the neutral of star connected winding of generator, transformer, motor are earthed.

B) Equipment Earthing

Such type of earthing is provided to the electrical equipment. The non-current carrying part of
the equipment like their metallic frame is connected to the earth by the help of the conducting
wire. If any fault occurs in the apparatus, leakage current comes to the body of the equipment
and may pass through the operator. That causes electric shock to the operator. But the earth
wire passes current to the earth. Thus, protect the equipment and the operator from damage.
2.2.1 point to be earthed

Earthing is not done anyhow. According to IE rules and IEE (Institute of Electrical Engineers)
regulations,

1) Earth pin of 3-pin lighting plug sockets and 4-pin power plug should be efficiently and
permanently earthed.

2) All metal casing, metallic coverings, apparatus such as GI pipes and conduits enclosing
VIR or PVC cables, iron clad switches, iron clad distribution fuse boards etc should be
earthed.

3) The frame of every generator, stationary motors and metallic parts of all transformers
used for controlling energy should be earthed by two separate and yet distinct
connections with the earth.

4) In a dc 3-wire system, the middle conductors should be earthed at the generating

station.

5) Stay wires that are for overhead lines should be connected to earth by connecting at
least one strand to the earth wires.

2.2.2 Factor influencing the earth resistance

The resistance of the earthing electrode is not concentrated at one point, but it is distributed
over the soil around the electrode. Mathematically, the earth resistance is given as the ratio of
the voltage and the current shown below.

Where V is a measured voltage between the voltage spike and I is the injected current during
the earth resistance measurement through the electrode.

The value of the earth resistance for different power stations is shown below
Large Power Station – 0.5 ohms

Major Power Station – 1.0 ohms

Small Substation – 2.0 ohms

In all other cases – 8.0 ohms

The fault current which is injected from the earth electrode is passing away from the electrode in
all directions as shown below.

The flow of current into the grounds depends on the resistivity of the soil in which the earth
electrode is placed. The resistivity of the soil may vary from 1 to 1000 ohm-m depending on the
nature of the soil. This depends upon mainly three factors

1. The resistance of the electrode itself,

2. The contact resistance between electrode surface and soil,
3. The resistivity of soil between the electrode and infinite earth.

The first two factors can be taken as negligible compared to the third factor. This is why we
generally consider resistivity of the soil only, when we deal with resistance of earth that depends
upon the following factors
 1. The chemical composition of the soil : the resistivity (or conductivity) of a soil is
mainly electrolytic in nature. So, concentration of water, salt and other chemical
components in the soil largely determines its resistivity.

2. The grain size: uniformity of grain distribution and packing of grains in the soil
control the moisture holding capacity of the soil.
3. The temperature of the soil: ​Below 0 degree C, the water contained in the soil
begins to freeze, which largely affects the electrolysis process in the soil. It is
found that, just below the freezing point the resistivity of soil or earth resistivity is
tremendously increased.
4. Depth of electric rod/plate: The lower layer of soil has more moisture and lower
resistivity. If the lower layer contains hard and rocky soil, then their resistivity
increases with depth.
5.

2.2.3 Method of Reducing earth resistance

1. Lengthen the earth electrode in the earth

2. Use multiple rods
3. Treat the soil
 Water solution of Magnesium sulfate, copper sulfate, and ordinary rock salt are suitable
for treatment to reduce resistance of earth. Magnesium sulfate is the least corrosive, but
rock salt is cheaper and does the job if applied in a trench dug around the electrode.

Chemical treatment also has the advantage of reducing the seasonal variation on
resistance that results from periodical wetting and drying out of the soil.

2.3 Methods and Types of Electrical Earthing

Earthing can be done in many ways. The various methods employed in earthing (in house
wiring or factory and other connected electrical equipment and machines) are discussed as
follows.

Plate Earthing:
In plate earthing system, a plate made up of either copper with dimensions 60cm x 60cm x
3.18mm (i.e. 2ft x 2ft x 1/8 in) or galvanized iron (GI) of dimensions 60cm x 60cm x 6.35 mm (2ft
x 2ft x ¼ in) is buried vertical in the earth (earth pit) which should not be less than 3m (10ft) from
the ground level.

For a proper earthing system, follow the above mentioned steps in the (Earth Plate introduction)
to maintain the moisture condition around the earth electrode or earth plate.
Pipe Earthing:

A galvanized steel and a perforated pipe of approved length and diameter is placed vertically in
a wet soil in this kind of system of earthing. It is the most common system of earthing.

The size of pipe to use depends on the magnitude of current and the type of soil. The dimension
of the pipe is usually 40mm (1.5in) in diameter and 2.75m (9ft) in length for ordinary soil or
greater for dry and rocky soil. The moisture of the soil will determine the length of the pipe to be
buried but usually it should be 4.75m (15.5ft).

Rod Earthing

It is the same method as pipe earthing. A copper rod of 12.5mm (1/2 inch) diameter or 16mm
(0.6in) diameter of galvanized steel or hollow section 25mm (1inch) of GI pipe of length above
2.5m (8.2 ft) are buried upright in the earth manually or with the help of a pneumatic hammer.
The length of embedded electrodes in the soil reduces earth resistance to a desired value.

2.4 Lightning Protection Earthing

1. The lightning protection system consists of lightning earthing terminals, roof conductors,
down conductors, test links and earth electrodes.
2. Each down conductor has a separate earth electrode of the GI pipe.
3. The number of down conductors and earth terminals is determined from the Risk index
as per IS 2309.
4. All terminals and roof conductors are interconnected by welding or riveting to form a
continuous grid. Water, steam and other service pipes shall not be used for
interconnections.
Type A Earthing System :

earthing rods are used to form the earth electrode and usually each down conductor, such as
copper earthing tapes, are connected to an earth rod.

The type A earth termination arrangement is suitable for low structures (below 20 metres in
height)

Type B Earthing System:

The type B Earthing arrangement is most suitable for:

● Structures built on rocky ground

● Structures housing sensitive electronics/equipment
● Large structures

The type B earthing is recommended as either a ring conductor outside the perimeter of the
structure which it’s recommended should be in contact with the soil for at least 80% of its total
length.
The alternative is to use a foundation earth electrode which can be in a mesh form, ​where a
foundation is used as an earth-termination. The reinforcing bars must be clamped or welded
together to ensure electrical continuity.

Q.1 The soil resistivity of industrial premises has been found to be 60 Ohm-m. If the acceptable
earth resistance is not to exceed 8 Ohm. Find the area of the plate as an earth electrode in the
plate earthing system.

Here,

q=60 Ohm-m

R=8 Ohm

Wa have, R=(q/4) sqrt( pi/A)

Or, 8=(60/4)sqrt (3.14/A)

Solving, we find A=11.045 sq m