GROUNDINGAND

TECHNIQUES

BY:M.SAMATHA

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WHAT IS A

GROUND?

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GROUND: A conducting connection, whether intentional or

accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of earth.

Ground is a connection to Earth made either intentionally or accidentally.

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NEED OF GROUNDING:

To protect people and equipment from dissipating stray energy from:

1. Electrical faults (fuses, breakers etc.)

2. Lightning strikes3. Radio Frequency4. Static discharges

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Electrical installations which need grounding:The frames, tanks and enclosures of electric

machines, transformers and apparatus, lightning fittings and so on.

Operating mechanism of the switchgear.

Framework of the switchboards, control boards, individual panel boards.

Structural steelwork of indoor & outdoor substations, metal cable jointing boxes, metal sheaths of cables & similar metal works .

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Losses due to improper grounding: Estimation of at least 15% of power quality

problems are related to grounding. Lightning strikes on equipment with poorly

maintained protection systems destroy millions of dollars of equipment and lost production every year.

Electric shocks, vibrations & noise from electronic devices due to improper wiring, also leads to mankind loss also.

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Types of grounding systems:

Different methods are available but the choice depends on local conditions and required function.

Thus, we can see basically four types:

1. Single stake

2. Ground rod group

3. Ground plate

4. Ground mesh

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1. Single stake:

The simplest form of grounding element is the ground stake, this can take many forms with a variety of lengths from a few feet to many feet long made of materials such as brass, galvanised or stainless steel, the size and material as required locally

The simple ground rod can be used for lightning protection on stand-alone structures such as pole mounted transformers or radio towers, it can also be used as a back up to a utility ground.

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Single stake

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2. Ground rod group: A slightly more complicated version of the rod

system is the ground rod group, this is typically for lightning protection on larger structures or protection around potential hotspots such as substations.

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3. Ground plate:Ground plates are used widely in telecoms

applciations. They are particularly good where the deeper ground has high resistivity.

For areas where there is rock (or other poor conducting material) fairly close to the surface ground plates are preferred as they are more effective

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4. Ground mesh: A ground mesh consists of network of bars connected

together, this system is often used at larger sites such as electrical substations.

At substation site an area of ground could be reserved at the start of the life of the substation with a ground mesh under the whole of the site. As the site grows over a period of years new equipment can easily be installed and grounded by the mesh.

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Ground mesh

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Measurements and

Techniques

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Steps involved:1. Before grounding one has to know the resistivity

strength of the land or soil of the place.2. Grounding tests are performed to know the

influence of spheres of influences.3. Selection for type of grounding needed at the

place.4. Grounding tests to know the depth and wide range

grounding needed for the site based on type of devices or equipments used as loads.

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Ground testing methods:1. Resistivity method( Wenner method):

Used to determine which type of earthing is to be used by knowing the resistance(RE)

E = 2 . a . RE 

E ...... mean value of soil resistivity (W.m)

RE ...... measured resistance (W)a ...... probe distance (m)

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2. Fall of potential testing( Three or Four pole method):

Most commonly used method of testing. Three or four pole method, this refers to the number of

connections made to the ground tester. The forth pole of the connection is made if the wire to

connect to the system under test is particularly long >> 4 meters. The additional wire cancels out an error due to the extended length of wire used.

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3. Selective measurement method:

The selective method is based on the Fall of Potential test.

A current clamp is used to isolate the test current injected into the electrodes under test, the current will flow to earth by any path.

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4. The stake less method: Eliminates the need for temporary ground stakes. This is useful in a wide range of situations.

Eg: Inside buildingsAirportsUrban locationsChemical and industrial plant

The temporary ground stakes are replaced by two current clamps. The first clamp generates a voltage on the ground condutor, the second clamp measures the current flowing due to the generated voltage.

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5. Two pole method:

Used where other methods are not available.

Uses nearby metal structures as a temporary spike.

Metal water pipes are typically used

Drawbacks:

The resistance of the metal pipe should be significantly less than the electrode under test.

Metal pipes are being replaced with plastic.

Some metal pipes use plastic couplings.

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SUMMARY:

Following Proper grounding methods decreases the electrical faults.

Improper grounding, no grounding for a plant or substation or house are to be avoided.

Get known resistivity of the land suitable method of grounding is to be employed.

Proper grounding is the sign of effective working of equipment.

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Queries

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Thank you