Primary Protection Against Electric Shock

This is the paramount reason for earthing. In the event of an insulation fault (e.g., a live conductor accidentally touches the metallic casing of an appliance, or insulation breaks down), the metallic enclosure becomes "live" (at a high electrical potential relative to earth). If a person touches this live enclosure, current would flow through their body to the earth, causing a severe electric shock, potentially fatal. The earthing system provides an alternative, much lower resistance path for this fault current to flow directly to the earth. Because the resistance of the earthing path is very low, a large fault current flows. This large current immediately causes the protective device (fuse or circuit breaker, especially an RCD) to operate very quickly, disconnecting the power supply and making the faulty equipment safe before a person can be shocked or for a duration too short to cause significant harm.

Prevention of Electrical Fires

High fault currents that are not quickly cleared by an effective earthing system can cause arcing, sparking, and excessive localized heating, potentially igniting combustible materials and leading to devastating electrical fires. Earthing ensures rapid fault clearance, mitigating this risk.

Stabilizes System Voltage

Earthing helps to maintain the voltage of the system at a known reference potential (zero potential of the earth). This prevents undesirable voltage fluctuations or transient overvoltages (e.g., from lightning strikes or switching surges) from building up on equipment frames, protecting both equipment and personnel.

Provides a Return Path for Fault Currents

In various three-phase and single-phase distribution systems, the earth can serve as an integral part of the fault current return path, ensuring that protective devices effectively detect and clear faults.

Protects Equipment from Overcurrents/Overvoltages

By facilitating rapid fault current discharge, earthing prevents sustained overcurrents and overvoltages on equipment, thus safeguarding sensitive electronics and extending the lifespan of electrical apparatus.

1. Plate Earthing:

• Method: A copper or galvanized iron (GI) plate (typically 60cm x 60cm, with thickness of 3.18mm for copper or 6.35mm for GI) is buried vertically at a significant depth (at least 3 meters or below the permanent moisture level) in the ground.
• Details: The main earthing lead from the electrical installation is securely bolted to the plate. To reduce the earth resistance and keep the soil moist, alternating layers of charcoal (which acts as a moisture absorber and conductive agent) and salt (to increase soil conductivity) are typically placed around the plate in the excavation pit. A watering pipe is often installed to periodically moisten the earth pit, especially in dry seasons.
• Advantages: Provides a large surface area for current dissipation, relatively long lifespan if properly maintained.
• Disadvantages: Requires a large excavation, maintenance (watering) might be needed, inspection can be difficult.

2. Pipe Earthing:

• Method: A galvanized iron (GI) pipe (commonly 38mm to 75mm diameter, 2.5 to 4 meters long) with numerous holes drilled along its length is buried vertically into the ground.
• Details: The earthing lead is clamped securely to the top of the pipe. A funneled arrangement is usually provided for pouring water to maintain soil moisture. The holes in the pipe allow the water to seep into the surrounding soil, further reducing resistance. Sometimes, a mixture of salt and charcoal is also used around the pipe.
• Advantages: More effective and easier to install and maintain than plate earthing. The pipe acts as a conduit for periodic watering, ensuring stable resistance. It provides a large contact surface area.
• Disadvantages: Can be challenging in rocky terrain.

Definition:

The combined resistance offered by the earth electrode system, the connection leads, and the surrounding soil to the flow of fault current into the general mass of the earth.

Crucial Importance:

For an earthing system to be effective, its total resistance must be as low as possible. Regulatory standards specify maximum permissible earthing resistance values (e.g., 1 Ohm for major power stations, 2-5 Ohms for general installations, 8 Ohms for small installations), but lower is always better. A low resistance ensures that in case of an earth fault, a large fault current flows, triggering protective devices quickly to clear the fault and limit the touch potential (voltage on the faulty equipment's casing) to a safe level.

Factors Affecting Earthing Resistance:

1. Soil Resistivity:
2. Varies with soil type: Clayey soils generally have low resistivity; sandy or rocky soils have high resistivity.
3. Moisture Content:
4. Higher moisture leads to lower soil resistivity; dry soil acts as an insulator.
5. Temperature:
6. Soil resistivity declines with increasing temperature, but freezing temperatures can increase it.
7. Chemical Composition:
8. Salts and minerals in soil can also affect conductivity.
9. Size and Shape of Earth Electrode:
10. Larger electrodes provide better contact and lower resistance.
11. Depth of Burial:
12. Burying electrodes deeper usually leads to lower resistance.
13. Number of Electrodes (Parallel Connection):
14. Using multiple electrodes in parallel can significantly reduce overall resistance.
15. Quality of Connections:
16. All connections must be strong and corrosion-free to maintain low resistance.