Facilities Support Services

Constructing and Maintaining an Earth-Pit

Now a days, earth-pits are the preferred method for earthing, especially for electrical networks. Electricity always follows the path of least resistance, and to divert the maximum current away from a circuit, earthing pits are designed to reduce ground resistance, ideally to 1 ohm.

To achieve this: An area of 1.5m X 1.5m is excavated to a depth of 3m. The pit is half filled with a mixture of wood coal powder, sand and salt. A 500mm X 500mm X10mm GI plate (earth plate) is placed in the middle Connections between the earth plate and the surface are installed for system earthing.

The rest of the pit is filled with the coal/sand/salt mixture. To connect the earth plate to the surface, two GI strips with a cross section of 30mm X 10mm can be used, but a 2.5” GI pipe with a flange at the top is preferable. Additionally, the top of the pipe can be covered with a T-section, to prevent mud and dust from entering and clogging the pipe. In summer, the pit should be watered to keep it from drying out. 

Advantages of the earth-pit method include: Wood coal powder is a great conductor and prevents corrosion of the metal parts The salt dissolves in water easily, increasing conductivity significantly Sand allows water to percolate through the entire pit To test the effectiveness of the pit, check that the voltage difference between the pit and the neutral of the mains supply is less than 2 volts. The resistance of the pit should be maintained at under 1 ohm, up to a distance of 15m from the conductor.

Earth Resistivity: The resistance of the earthing system is affected by a variety of factors: Soil Resistance – The composition of the soil, grain size and distribution.

Moisture – Up to 15% water content significantly changes resistivity. Beyond that, it has little effect. Dissolved Salts – Pure water has very low conductivity. Salt is an electrolyte that reduces the resistance when it’s dissolved in water. Obstructions – Nearby concrete buildings nearby or rocks in the soil underneath the earthing system can increase its resistance.  Current Magnitude – Long periods of exposure or higher currents flowing through the earth can dry the soil in the surrounding area and increase the system’s resistance.

Measuring Earth Resistance Earth resistance is measured with an earth tester, also called a ‘megger’, which can test the resistance across a range of currents and distances. It consists of a voltage source, ohm-meter to measure resistance and spikes that are staked into the ground for measuring. You can measure the soil resistance using either the 3-point method or the 4-point method.