Industrial Automation

What is an Earth Pit?

What is an Earth Pit? 1

In electrical systems, an “Earth pit” is a hole dug in the ground and filled with standard materials. Most of the time, earth pits are used to protect against short circuits. It also acts as a point of reference for different types of electric power sources. Since Earth Pits are easy to make and keep up, they have been used as reference points since the beginning of commercialized electricity. 

What is the importance of Earthing?

The main objective of earthing is to create a surface with zero potential around and under the area where electrical equipment is set up or installed. To reach this goal, parts of the electrical equipment that don’t carry current are connected to the earth. This keeps dangerous voltage from showing up on the enclosures and keeps workers and the public safe.

The earthing is provided as follows:

  • Keeping people safe
  • Prevent or at least reduce damage to equipment caused by heavy fault currents.
  • Make sure the power supply is more reliable

The earthing is mainly split into 

  • System earthing, which is the connection between a part of a working system (like the LV neutral of a Power Transformer winding) and the ground.
  • Equipment earthing (Safety grounding): Connecting the frames of equipment (like the motor body, transformer tank, switch gear box, operating rods of air break switches, etc.) to earth.

How does earthing keep you safe?

The fundamental of this safety theory is that “current flows from high voltage to low voltage.”

As we know, electricity flows along the path with the least resistance. And the resistance of the human body is about 300 ohms (depending on several things), while the resistance of the earth is very low. So the current will go through the grounding wire and into the earth.

How does earthing protect equipment?

If a machine or piece of equipment is connected to the earth in the right way, it is less likely to break.

In the event of a short circuit or other accident, the current will rush to the path with the least amount of resistance, which is the earth. And an OCR (over-current relay) will sense the high current and cut off the supply.

What are the types of earthing?

The following kinds of earthing are the ones that are most often used:

1. Plate earthing.

2. Pipe earthing.

3. Rod earthing.

4. Strip earthing.

 Plate earthing

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  • Earthing plates made of copper or galvanized iron are buried into the ground at a depth of at least three meters below the ground level in this sort of earthing system.
  • The earth plate is covered in alternating layers of coke and salts, with the total thickness of these layers being at least 15 centimeters.
  • With the assistance of a copper bolt nut and washer in the case of copper plate earthing and a G.I. bolt nut and washer in the case of G.I. plate earthing, the earth wire (copper wire for copper plate earthing and G.I. wire for G.I. plate earthing) is securely bolted to an earth plate. This is done for both copper plate earthing and G.I. plate earthing.

Pipe earthing

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  • Pipe earthing is the best and cheapest way to connect to the earth. In this method, a GI pipe with a diameter of 38 mm and a length of 2 meters is buried vertically in the ground to act as an earth electrode. The depth of the pipe depends on the soil conditions, so there is no hard and fast rule.
  • However, the wire is buried deep inside the moist earth.
  • With nuts and bolts, the earth wire is secured to the upper portion of the pipe.
  • The area around the GI pipe in the pit was filled with a combination of salt and coal in order to improve the soil conditions and the effectiveness of the earthing system.
  • In comparison to plate earthing, it is capable of withstanding a significant amount of leakage current while having the same electrode size.
  • When the earth wire connection is above ground level, as it is with GI pipes, a continuity test can be done at any time. This is hard to do with plate earthing.
  • During the warm summer months, three or four buckets of water are poured down the funnel in order to improve the continuity of the earthing process and provide a more successful earthing.

Rod earthing

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  • In this earthing system, solid copper rods with a diameter of 12.5 mm, solid rods with a diameter of 16 mm made of GI or steel, or hollow sections of 25 mm GI pipe that are at least 3 meters long are driven vertically into the ground.
  • More than one rod section is hammered on top of the other to increase the length of electrode that is buried in the ground. This is sometimes needed to get the earth resistance down to the level that is wanted.
  • This earthing method works well in areas with a sandy texture.
  • This method of earthing doesn’t cost much.

Strip earthing

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  • In this method of earthing, copper or steel strips with a cross section of at least 25 mm x 1.6 mm or 25 mm x 4 mm are buried in horizontal trenches at least 0.5 m deep. If round conductors are used, their cross-sectional area must be at least 3 Sqmm for copper and 6 Sqmm for GI or steel.
  • The length of the buried conductor must be long enough to give the required earth resistance, which is between 0.5 and 1.5 ohms.
  • Despite this, it must have a minimum distance of 15 meters.
  • The electrodes must be spread out as much as possible in straight or circular trenches that radiate from a single point.
  • This type of earthing is used on earth beds with rocky soil, where it is hard to dig holes for plate earthing.

How do you Build Earthing?

The procedure should be followed in order to fill up the new earth pit

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  1. Dig a hole in the ground that is 2000 mm by 2000 mm by 2500 mm deep. Sprinkle enough water on the bottom and the walls around it so that it becomes wet.
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  1. Fill the bottom 250 mm of the pit with a mixture of black soft soil, salt, and charcoal pieces made from wood.
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  1. Prepare the electrode assembly and put the whole assembly in the pit.
  2. Gather thin C.R.C. sheets that are between 18 and 20 SWG and 500 mm wide and about 3.5 meters long. Please join three or four pieces together to meet the length requirement of 3.5 meters.
  3. Make the Cylindrical Ring by bending both ends of the above sheet and putting them together. The cylinder’s diameter should be about 1000 mm, and its height should be 500 mm. Get two pieces of scrap G.I. wires that are about 8-SWG and make two round lifting handles (Hooks) on the top of the cylindrical ring to make it easier to move.
  4. Now, put this cylindrical ring on the electrode pipe of the electrode assembly so that the pipe stays in the middle of the ring.
  5. Fill the inside of the cylindrical ring with Mixture I, which is a uniform mix of black soft soil.
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  1. The other part, which is the space between the walls of the pit and the outside edge of the cylindrical ring, is filled by a mixture-II. After both mixtures have been filled to a height of 500 mm, they need to be rammed and watered in the right way.
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  1. Then, with the help of two lifting handles (hooks), lift the cylindrical ring and put it back on the layer so that the second layer can be filled.
  2. Again, use Mixture I to fill the inner cylindrical part of the ring. Then, use Mixture II to fill the space between the walls of the pit and the outer cylindrical edge up to the height of the cylinder (i.e. 500 mm)
  3. After the right amount of ramming and watering, you can lift the cylinder-shaped ring by its lifting handles (hooks). Now, put the cylindrical ring back on the top layer and set up the third cycle. Then, fill the whole pit. Make sure that at least 20% of it is water.
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  1. Fill the top layer of the pit with crushed rock pieces 50 x 35 mm in size. 1 CMT. approx. to provide a layer of insulation for people walking next to the pit and to keep reptiles from moving in ways that could be dangerous.
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Which type of earthing is best?

Pipe earthing is the most prevalent and best earthing technology when compared to other systems appropriate for the same ground and humidity conditions. Pipe earthing is the best way to ground something, and it’s also very cheap.

What are the factors influencing earth pit resistance?

  • Soil Resistivity: The soil’s resistance is an important consideration. Earth pit resistance increases with resistivity. Resistivity is impacted by saltiness, temperature, moisture content, and soil composition.
  • Electrode Material: Resistance is influenced by the type and size of the electrode material used in the earth pit. Commonly utilised materials include copper, galvanised steel, and steel with copper plating.
  • Length and Diameter of the Electrode: Resistance is influenced by the electrode’s length and diameter. Electrodes that are longer and thicker typically have lower resistance.
  • Electrode Depth: Resistance is affected by the electrode’s installation depth. Deeper electrodes can access soil layers with lower resistivity to minimise resistance.
  • Number of Electrodes: Resistance is influenced by the number of electrodes connected together. Resistance can be decreased by using more electrodes spread out across a bigger region.
  • Moisture Content: Resistance is impacted by soil moisture content. Soil that is moist increases conductivity and lowers resistance. Resistance rises in dry soil.
  • Seasonal Variation: Due to variations in soil moisture content, temperature, and other environmental conditions, resistance might change as the seasons change.
  • Temperature of the soil: Soil resistivity is influenced by temperature. Lower resistivity and decreased resistance are typically caused by higher temperatures.
  • Soil Chemical Composition: Low resistivity and low resistance may be found in soil with a high salt or chemical content.
  • Interference from neighbouring Structures: Due to mutual coupling and interference, the presence of neighbouring structures, underground utilities, and their grounding systems might affect earth pit resistance.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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