2. Safety Precautions in Handling Electrical
Appliances
Following safety precautions must be taken while working with electrical
installations or while handling electrical appliances:
Make sure that all metallic parts of the electrical equipment are effectively
earthed. Broken switches, plugs, etc., should be replaced immediately.
Use a ‘line tester’ to check whether a terminal is live. Still better is to use a
‘test lamp’, as the line tester can show a glow even with a small voltage.
Before replacing a broken switch, plug or blown fuse, always put off the main
supply.
Never use equipments and appliances with damaged or frayed lead wires.
Never insert bare wires in the holes of a socket, for taking a connection. Always
use a proper plug.
Use rubber- sole shoes while repairing/testing electrical equipments. If this is
not possible, use some dry-wooden support under your feet, so that your body
has no direct contact with earth.
Use rubber gloves while touching any terminal or while removing insulation
layer from a conductor.
Always use well insulated tools (such as screw- drivers, pliers, cutters, etc.).
3. Safety Precautions in Handling Electrical
Appliances contd..
Never touch two different terminals at the same time.
Be careful that your body does not touch the wall or any other metallic frame
having contact with earth.
While repairing an electrical appliance (such as table fan, iron, heater, geyser,
etc.), be sure that its plug has been taken out from the socket. Switching off may
not be sufficient, since leaky insulation can given serious shock.
Strictly follow all the precautions and instructions given on the ‘name plate’ of
the machine you are working.
In case of electric fire, use only ‘soda- acid’ fire – extinguished. Do not throw
water on live conductors or equipments. Best remedy is to first disconnect to the
electric supply and then throw sand on fire.
While working on an electric pole or tower, use safely – belt and a rubber
padded ladder.
It is preferable to work in the presence of an ‘assistant’, so that he can
immediately disconnect the supply whenever needed.
4. What is Earthing?
By earthing, we generally mean an electrical
connection to the general mass of earth, the latter
being a volume of soil/rock etc., whose dimensions
are very large in comparison to the electricity
system being considered.
An electrical equipment or appliance is said to be
earthed, if its outer frame and its other parts not
carrying any current are connected to the earth so
as to attain as nearly zero potential as possible.
5. Objectives of Good Earthing System
• To limit voltage in electrical distribution system to definite
fixed values. To ensure that no part of equipments other than
the live parts should assume a potential that is dangerously
different from that of the surroundings.
• To limit voltage to within insulation ratings.
• To provide a more stable system with a minimum of transient
over voltage and electrical noise.
• To provide a path to ground in fault conditions for quick
isolation of equipment with operation of ground fault
protection.
• To provide grounding of all conductive enclosures that may be
touched by personnel, thereby eliminating shock hazards.
6. • To provide protection from large electrical
disturbances (such as lightning) by creating a low
resistive path to earth.
• To reduce static electricity that may be generated
within facilities. In some industrial (petrochemical,
refineries, where explosives or volatile chemicals are
present) premises the earthing system is required to
continuously discharge the build up of static charge,
and thus prevent a fire or explosion risk.
• Many power supplies now include a connection to
earth, through which residual and harmonic currents
are dispersed to ground.
7. The circumstances that make electric shock accidents possible are :
a) Relatively high fault current to ground in relation to the area of
ground system and its resistance to remote earth.
b) Soil resistivity and distribution of ground currents such that high
potential gradients may occur at points at the earth’s surface.
c) Presence of an individual at such a point, time, and position that the
body is bridging two points of high potential difference.
d) Absence of sufficient contact resistance or other series resistance to
limit current through the body to a safe value under circumstances
mentioned at a to c.
e) Duration of the fault and body contact, and hence, of the flow of
current through a human body for a sufficient time to cause harm at
the given current intensity.
8. Effect of current thro’ human body
Effect of electric current through the vital parts of human
body depend on the duration, magnitude and frequency of
this current. Most dangerous consequence could be
ventricular fibrillation, a condition of incoordinate action of
main chambers of the heart, resulting in immediate arrest of
blood circulation.
Currents at 50 Hz about 0.1 A can be lethal. But human can
sustain larger currents at 25 Hz and five times higher DC or at
frequencies in the range of 3,000 - 10,000 Hz.
Current depends on voltage applied and body resistance.
Resistance is mainly offered by skin. Skin resistance increases
with thickness and diminishes with moisture / perspiration.
Except for skin; blood vessels, intravascular spaces etc. offer
conduction system.
9. Tolerable current for human body
As per studies by Dalziel, 99.5% of all persons can safely withstand without
ventricular fibrillation, the passage of current (IB) for duration ranging
from 0.03 to 3.0 sec and is related to energy absorbed by the body as per
formula: SB = (IB)2 x ts
Value of SB = 0.0135 for person weighing 50 kg
i.e. IB = 116 mA for 1 sec.&
SB = 0.0246 for person weighing 70 kg
i.e. IB = 157 mA for 1 sec.
Ts IB (50 kg) IB (70 kg)
0.2 sec 259 mA 351 mA
0.5 sec 164 mA 222 mA
1.0 sec 116 mA 157 mA
10. Human Body Resistance
Body resistance including skin ranges from 500 to
3000 Ω which reduces by damage or puncture of
skin at contact point.
For earthing design, resistance of human body from
hand-to-feet (1100 Ω), foot-to-foot and also hand-to-
hand(2300 Ω) is considered as 1000 ohm as per IEEE
80-2000.
11. Components of Earthing system
Earthmat
Earth Electrodes
Risers for equipment to Earthmat connections
Bonding
12. Electrical bonding is the practice of intentionally electrically
connecting all exposed metallic items not designed to carry
electricity in a room or building as protection from electric shock. If
a failure of electrical insulation occurs, all bonded metal objects in
the room will have substantially the same electrical potential, so
that an occupant of the room cannot touch two objects with
significantly different potentials. Even if the connection to a distant
earth ground is lost, the occupant will be protected from
dangerous potential differences. Any exposed conductive metal
work which can be touched is connected together via bonding
conductors. In industries, bonding of exposed metalwork would
normally ensure that an electrical fault to the frame of one machine
did not create a potential difference between that and earthed
metalwork on an adjacent machine.
Bonding
13. 7
Grounding Cable
Grounding Bus
or Electrode
Bonding Cable
Proper grounding and bonding is used to address the dangers of static electricity.
In order for grounding to protect, all surfaces must be bonded together and grounded
to earth.
Static electricity is thereby released to earth as it is generated, preventing the
accumulation of dangerous charges that may ignite flammable / hazardous substances.
Container Bonding and Grounding
(Static Electricity)
14. Earth Electrode
The earth electrode is the component of the
earthing system which is in direct contact with
the ground and thus provides a means of
releasing or collecting any earth leakage
currents.
The material should have good electrical
conductivity and should not corrode in a wide
range of soil conditions. Materials used include
copper, copper bonded mild steel rod,
galvanised steel, stainless steel and cast iron.
16. Rod and Pipe Electrodes
Rod electrodes shall be at least 16mm in diameter of steel or
12.5mm in diameter of copper.
Pipe electrodes shall be larger than 38mm in diameter of
galvanized iron or steel and 100mm in internal diameter of CI
or mild steel.
The length of rod or pipe electrode not less than 2.5m, which
shall be driven to a minimum depth of 2.5 m.
Where rock is encountered at depth less than 2.5m, it can be
tilted by an angle of 35o to vertical.
*The distance between two adjacent electrodes should not be
less than twice the length of electrodes.
17. Plate Electrode
The size of copper plate shall not be less than
600mm x 600mm x 3.15mm and that of iron and
steel plates not less than 600mm x 600mm x 12mm.
The top edge of the plate shall be at a depth not less
than 1.5m from surface of ground.
When two plates are connected in parallel the
minimum distance of 8m shall be kept between the
two plates.
20. Main Terms
IS 3043 defines
Earth Grid: A system of grounding electrodes consisting of
inter-connected connectors buried in the earth to provide
a common ground for electrical devices and metallic
structures.
Earth Mat: A grounding system formed by a grid of
horizontally buried conductors and which serves to
dissipate the earth fault current to earth and also as an
equipotential bonding conductor system.
Ground Potential Rise (GPR): The maximum electrical
potential that a substation grounding grid may attain
relative to a distant grounding point assumed to be at the
potential of remote earth. This voltage, GPR, is equal to the
maximum grid current times the grid resistance.
21. Touch potential is the difference in voltage between the object
touched and the ground point just below the person touching the
object when ground currents are flowing.
Step Potential is the difference in voltage between two feet, which
are one metre apart along the earth when ground currents are
flowing.
Mesh Voltage: The maximum touch voltage within a mesh of a
ground grid.
Metal-to-Metal Touch Voltage: The difference in potential between
metallic objects or structures within the substation site that may
be bridged by direct hand-to-hand or hand-to-feet contact.
26. As per the Indian Electricity Rule no. 67 (1) in
every E.H.V./ H.V. installations :
(a) Touch voltage and step voltage shall be kept
within limits.
(b) The ground potential shall be limited to a
tolerable value.
27. RESULT: Higher Tolerable Step and Touch Voltages
High Resistivity Surface Material:
High Resistance
Surface Material
Laying of High Resistivity Material
28. The black metal is used to provide high resistivity
layer. The resistivity of the black metal is taken as 3000
Ohm-m for calculation of the tolerable touch voltages
in most of the designs of earth mat of sub-station.
Crushed stone, i.e. the black metal, of the size of 30 to
40 mm for a layer of 100 mm is recommended by the
CBIP.
29. Granite, Gneiss - 25000 Ohm-metre
Bolder Gravel - 15000 Ohm-metre
Lime Stone - 5000 Ohm-metre
Moran Gravel - 3000 Ohm-metre
Base Rock Hard - 1190 Ohm-metre
Rock, Hard - 1150 Ohm-metre
Boulders - 477 Ohm-metre
The range of the values of the resistivity is wide. It is, therefore,
essential to know the source of the rock from which the black
metal is obtained so that the idea of the resistivity of the black
metal can be had prior to laying of the metal.
The values of resistivity of the different
types of rocks
30. Black metal is spread in the substations to provide high
resistivity layer :
To avoid formation of pools of oil in case of leakages
from
Transformers and
Circuit Breakers
to eliminate spreading of fire
to keep reptiles away
to control the growth of grass and weeds
to maintain moisture in the soil
to discourages persons running in the switch-yard and
saves them of the risk of being subjected to possible
high step voltage
Importance of gravel metal layer in substation
switchyards
31. Parameters Influencing The Earthing
Design
The earthing resistance of an electrode is made up of:
Resistance of the (metal) electrode
Contact resistance between the electrode and the
soil, and
resistance of the soil from the electrode surface
outward in the geometry set up from the flow of
current outward from the electrode to infinite earth.
The most important factor influencing the impedance
of the earthing system is the impedance of the
medium in which the earth electrodes are situated,
i.e. the soil.
32. Earth is a poor Conductor of Electricity.
Typical Resistivity (ρ) of soil is 100 ohm-metre, and
for copper is 1700 micro ohm-metre,
Two main constituents of soil are silicon oxide and
Aluminium oxide which are insulators,
Soil becomes conductive due to salts and moisture
embedded in between them,
Surface of soil layers-clay and moisture with
decayed vegetable material. When dry this does
not conduct. With moisture contain, it conducts.
Cont..
Soil Properties
33. Soil under the surface of earth is non-
homogenous, hence resistivity values in wide
range between 1 ohm metre to 1,00,000 ohm
metres depending on type, nature of soil &
physical and chemical properties.
Sandy soil drains faster, solid rock does not retain
water and have high ρ.
Soil resistivity measurement is important for
design of earthing system.
Soil Properties
34. Moisture in the soil is the most important
element determining its conductivity / resistivity.
Conditions which increase / decrease distribution
of moisture content in the soil result
corresponding changes
• Resistivity goes seasonal changes as per moisture
in the soil, due to climate conditions,
• Values of resistivity are minimum in rainy season
and maximum in summer / dry season,
• For safe design of earth mat, measurements in
dry season are adopted.
cont..
Soil Properties
35. Resistivity for Different Soils
Type Resistivity (Ohm metre)
Sea water 0.1 - 1
Garden soil/alluvial clay 5 - 50
clay 5 - 100
Clay, sand and gravel 40 - 250
Porous chalk 30 - 100
Quartzite/crystalline limestone 300+
Rock 1,000 - 10,000
Gneiss/igneous rock 2,000+
Dry concrete 2,000 - 10,000
Wet concrete 30 - 100
Ice 10,000 - 100,000
39. Conventional:
Salt,
Charcoal,
Water
Disadvantage: electrode corrosion.
Bentonite:
High moisture,
Swelling to High volume,
Moisture retains to long time,
No maintenance required.
Marconite:
Gypsum:
Low Resistivity Materials
40. Electronic Equipment Earthing
The single point earthing (earth connection of all cabinets are
connected to the power system earth electrode at one point)
is used for earthing electronic equipment operating at low
frequencies, say up to 300 kHz. This method is effective in
preventing circulating earth currents which can produce
common mode noise.
For equipment operating at high frequencies, multiple-
point earthing system should be used. The signal common of
electronic equipment is tied to the metallic cabinet of the
equipment. Each cabinet is further connected to earth at the
nearest point. At times, a signal reference grid (SRG) is
installed beneath the area where cabinets are placed for
facilitating implementation of multiple –point earthing
system. SRG is a local closely meshed grid tied to main
earthing system.
41. Measuring the impedance of Earth
Electrode Systems
Measurement of the ohmic value of a buried
electrode is carried out for two reasons:-
• To check the value, following installation and prior to
connection to the equipment, against the design
specification.
• As part of routine maintenance, to confirm that the
value has not increased substantially from its design
or original measured value.
The most common method of measuring the earth
resistance is the “fall of potential” method (section
37.1 of IS 3043:1987).
42. Measurement of Earth Resistance
The measurement of earth resistance is done using three
terminal earth meggars or four terminal earth meggars.
Four Terminal: Four spikes are driven in straight line into the
ground at equal intervals. The two outer spikes are connected
to current terminals of earth meggar and the two inner spikes
to potential terminals of the meggar. Then the earth resistance
is measured by rotating the meggar till a steady value is
obtained.
Three Terminal: Two temporary electrodes are spikes are
driven in straight line one for current and the other voltage at a
distance of 150 feet and 75 feet from the earth electrode under
test and ohmic values of earth electrode is read in the meggar.
43. Combined earth resistance shall be the same at every earth
pit unless it gets disconnected from the earth mat
Measurement of Earth Resistivity
44. a) Power stations 0.5 ohms
b) EHT Stations 1.0 ohms
c) 33KV SS 2 ohms
d) DTR(distributed transformer) 5 ohms
Structures
e) Tower foot resistance 10 ohms
Permissible values of earth resistance
45. Testing for Earth Continuity Path
This test is conducted to ensure proper earthing of all the metallic parts used in the
installation. For this test, we need an instrument called earth continuity tester, which is
capable of measuring low- value resistances. Before the test is conducted, we must ensure
that
the main switch is in OFF position,
the main fuse is taken out, or main MCB is put OFF,
all other fuses are in their position, or all other MCBs are ON,
all switches are in ON position, and
all lamps are in their holders.
Now, one terminal of the earth continuity tester is connected to an independent earth
(the one which is not used in the installation) and the other terminal to a metallic part of
the installation (say, the conduit or the main switch – board). The resistance as indicated
by the tester under these conditions should not be more than 1 . A high value of the
resistance measured is indicative of improper or poor earthing of the installation.
46. Colour coding of wires:
Three phases, L1, L2, L3: Red, Yellow, Blue
Neutral: Black
Ground/Protective earth: Green/yellow striped or
green
