Earthing means connecting any non-current carrying conductor part of an electrical system with general mass of earth in such a manner that there is an immediate discharge of electrical energy to the earth in the event of electrical potential developed at that part of the system. For example, metallic frame work of electrical appliances, metallic covering of electrical cables, the earth terminal of three pin socket outlets, stay wires and also neutral point of single phase and three phase supply systems must be properly earthed. Earthing is done to ensure that no current carrying part of the system rises to be potential beyond its normal value, no non-current carrying conducting part of a system rises to a potential beyond earth potential that is zero. Proper earthing also helps to avoid electrical shock to the human beings also to avoid the chance of fire hazard due to leakage current through unwanted path.
Purpose of Earthing
1. Safety for Human life / Building /Equipment
To save human life from danger of electrical shock or death by blowing a fuse i.e. To provide an alternative path for the fault current to flow so that it will not endanger the user
To protect buildings, machinery & appliances under fault conditions.
To ensure that all exposed conductive parts do not reach a dangerous potential.
To provide safe path to dissipate lightning and short circuit currents.
To provide stable platform for operation of sensitive electronic equipment i.e. To maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment .
2. Over voltage protection
Lightning, line surges or unintentional contact with higher voltage lines can cause dangerously high voltages to the electrical distribution system. Earthing provides an alternative path around the electrical system to minimize damages in the System.
3. Voltage stabilization
There are many sources of electricity. Every transformer can be considered a separate source. If there were not a common reference point for all these voltage sources it would be extremely difficult to calculate their relationships to each other.
What are the different methods of earthing of electrical installations?
There are different methods used for earthing of electrical installations depending upon the requirements. Such as strip or wire earthing, rod earthing, pipe earthing and plate earthing etc.
What is strip earthing or wire earthing?
In this system of earthing, a copper strip of minimum cross-section 25 mm × 1.6 mm is buried horizontally inside the ground at minimum depth 0.5 m and alternatively a galvanised iron strip of minimum of cross-section 25 mm × 4 mm can be buried horizontally at a same depth inside the ground. For this purpose around conductor can also be used and at that case the minimum cross-sectional area for copper conductor would be 3 mm² and for galvanized iron conductor it would be 6 mm². The buried portion of the electrode that is either script or round conductor should be long enough to provide required minimum resistance to the earth path. Generally the length of the conductor inside the ground is maintained more than 15 m. The buried conductor should be widely distributed as possible preferably in a single straight trench or in a circular trench or in a number of trenches radiating from a point. This type of earthing is mainly used in rocky area where excavation work is quite difficult.
What is rod earthing?
In this type of earthing, a metallic rod of sufficient length is driven vertically into the ground normally by hammering on the top. Normally galvanized iron rod of 16mm diameter or hollow galvanized iron pipe of 25 mm diameter of minimum length 2.5 m are used for this purpose. The electrical installation which to be earthed, is connected to the top of the earth rod or pipe by means of copper or aluminium earth continuity conductor of sufficient cross-section. The rod earthing system is mainly used where soil has sandy characters and also it is often used for temporary earthing purpose. This is cheapest and easiest method of earthing as this method does not require any excavation work.
What is pipe earthing?
Pipe earthing system is most commonly used and reliable system. In this method of earthing, a galvanized steel pipe of suitable length and diameter is buried vertically in the permanent wet soil under the ground. The length and diameter of the pipe are determined by the conditions of soil and the current to be carried. Normally minimum diameter and length of the pipe is maintained 40 mm and 2.5 m respectively for ordinary condition of soil and greater length is used for rocky and dry soil conditions. The depth under ground level at which the pipe is buried, depends upon the moisture condition of soil but it should not be less than 3.75 m under the ground. The earthing pipe is surrounded by alternative layers of charcoal and salt to keep moisture and thereby reduces the earth resistance. Another galvanized iron pipe of lesser diameter (19 mm) is fitted vertically on the top of the earthing pipe by means of reducing socket. The top of this pipe is projected in a cement concrete work on the ground. One or more GI plates are welded on this pipe by keeping the pipe openings clear to facilitate the connections of earth continuity conductors from different electrical installations. The cement concrete work is done to keep the water arrangement accessible and in dry season to keep the earth resistance minimum, 3 to 4 buckets of water are put in the concrete work or through the funnel if it is fitted to the top of the 19 mm diameter pipe.
What is plate earthing?
This is another popular method of earthing. In this method a metallic plate of sufficient size is buried in wet soil vertically under the ground. If copper plate is used for this purpose the minimum dimensional of the plates should be 60 cm × 60 cm × 3 mm and if it is GI plate, then minimum dimensional should be 60 cm × 60 cm × 6 mm. In case of copper plate, a copper earth continuity conductor is connected to the plate with the help of copper nuts bolts and washers whereas in the case of GI plate, GI earth continuity conductor is connected to the plate with help of GI nut bolts and washers. This earthing plate along with connected earth continuity conductor, is buried vertically at minimum 3 m depth under the ground. The surroundings of the plate are filled with alternative layers of charcoal and salt of minimum 15 cm thickness of each layer. From the buried plate, the earth continuity conductor is passed through a GI pipe of 12 mm diameter. These GI pipe is used to protect the earth continuity conductor from direct contact of soil. Now another GI pipe of 19 mm diameter is driven vertically to the GI plate. Top of this 19 mm diameter pipe should be projected vertically on the ground level. A concrete chamber is made around the projected 19 mm diameter pipe and this chamber is covered by cast iron shutter. The 19 mm diameter pipe is used to keep the water arrangement accessible to the earthing plate. In this type of earthing, 1 to 2 buckets of water is poured on every 3 to 4 days through a funnel at the top of the 19 mm diameter pipe to facilitate the moisture content of the surroundings of the earthing plate.
Touch and Step potential
when electricity is generated remotely and there are no return paths for earth faults other than the earth itself,then there is a risk that earth faults can cause dangerous voltage gradients in the earth around the site of the fault (called ground potential rises). this means that some one standing near the fault can receive a dangerous electrical shock due to:
Touch voltages - There is dangerous potential difference between the earth and a metallic object that a person is touching.
step voltage - There is a dangerous voltage gradient between the feet of a person standing on earth.
The earthing grid can be used to dissipate fault currents to remote and reduce the voltage gradients in earth .The touch and step potential calculation are performed in order to assess whether the earthing grids can dissipate the fault currents so that dangerous touch and step voltages cannot exist.
Design
Guide lines for Earthing calculations
Purpose:
The
purpose of this document is to lay down the guidelines and design procedure for
performing earthing calculation for safety earthing for plant area projects.
Reference
Standards:
IS
3043-Code of Practice for Earthing
IEEE 80 –
IEEE Guide for safety in AC Substation Grounding
General Features of Earthing
System:
Earthing
system shall include an earthing network with designed number of earth
electrodes, earth strips attached to meet the required value of earth
resistance.
The
following Equipment shall be earthed.
-
Metallic
non current carring parts of all electrical apparatus such as Transformers,
Switchgears, Bus ducts, UPS, Chargers, capacitor banks, NGR’s, Motors, lighting
power panels, cable trays, terminal boxes, control stations, lighting fixtures,
receptacles etc.,
-
Steel
Structures
-
Static
equipment like storage tanks, vessels, spheres, colomns and all other process
equipment
-
Cbale
shields and armour
-
Fence/gate
for transformer and capacitor banks
-
Any
other equipment required to be earthed/bonded
Calculation procedure:
Earthing
Calculation shall be as per IS3043-1987
As per IS 3043 the formula for
arriving the size of earthing conductor / inter connecting strip is as follows:
Isc x √t
The minimum cross section of earth
conductor in Sq.mm (A) = -------------
K
Where Isc is the system
maximum short circuit current = 50kA **
t is
the maximum fault clearing time = 1 Sec
K is
the constant for Steel Conductor = 80 (As per IS 3043)
for
Al. Conductor = 126
for
Cu. Conductor = 204
CALCULATION OF PIPE ELECTRODE
EARTH RESISTANCE
As per IS 3043 the formula for
arriving the resistance of earthing electrode is as follows:
100 x ρ 4 L
Resistance of Pipe Earth
Electrode in Ohms (Re) = ---------- log e ------
2
x п x L D
Where ρ is the soil
resistivity in Ω-m
= 16 Ω-m
L is
the length of the pipe electrode in cm
D is
the diameter of the pipe electrode in cm
COMBINED
RESISTANCE OF EARTH ELECTRODES:
Combined Resistance of Earth
Electrodes RE = Re
/ N
N
- Min. no. of electrodes installed per any Isolated grid with the plant
COMBINED RESISTANCE
OF INTER CONNECTING STRIP:
As per IS 3043 the formula for
arriving the resistance of earthing strip is as follows:
100 x ρ 4
L2
Resistance of
interconnecting strip in Ohms (Rs) = ----------- log e -------
2
x п x L w x t
Where ρ is the soil resistivity in Ω-m
L is the length of the strip in cm
W
is the depth of burial in cm
t
is the width of strip in cm
CALCULATION
OF EQUIVALENT EARTH RESISTANCE OF THE
GRID
Total
Earth resistance RT = Re // Rs
Where,
Rt
= Total Earth resistance(Ohms)
Rs
= Resistance of strip Electrode(Ohms)
Re
= Total Resistance of Earth Electrode(Ohms)
Re
= Re/N, Where N – Total no. of Earth Electrodes
CURRENT
DENSITY
Maximum
Permissible Current Density
7.57
x 103
I = ------------
Sqrt(ℓ xt)
Whre,
t = Duration of earth fault(in sec.)
ℓ = Soil resistivity(Ohm-m)
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