Introduction:
The earthing grid is an essential component of electrical systems that provides a low-resistance path for fault currents to flow into the ground, ensuring the safety of equipment and personnel. Ground rods are commonly used in the earthing grid to improve its effectiveness. This response will explain why the addition of ground rods in the earthing grid slightly decreases the earth resistance.

Explanation:
1. Function of ground rods: Ground rods are metal rods made of copper or steel that are installed vertically into the ground. They help to dissipate fault currents into the earth by providing a low-resistance path. The presence of ground rods increases the surface area in contact with the ground, allowing for better electrical conductivity.
2. Earth resistance: Earth resistance is a measure of the opposition offered by the earth to the flow of electric current. It depends on factors such as soil resistivity, moisture content, and the geometry of the earthing system. The lower the earth resistance, the better the earthing system's performance.
3. Effect of ground rods on earth resistance: When ground rods are added to the earthing grid, they increase the number of grounding points and the surface area in contact with the ground. This leads to a decrease in the overall resistance of the earthing system.
4. Improved conductivity: The additional ground rods create multiple paths for fault currents to flow into the ground, reducing the concentration of current at a single point. This helps to distribute the fault current more evenly, reducing the resistance encountered.
5. Reduced soil resistivity: The presence of ground rods also helps to improve the conductivity of the surrounding soil. The rods provide a direct connection between the earthing system and the soil, allowing for better dissipation of fault currents. As a result, the effective resistance of the system decreases.
6. Slight decrease in earth resistance: Although the addition of ground rods in the earthing grid improves its performance, the decrease in earth resistance is usually slight. The overall resistance still depends on factors such as soil resistivity and the design of the earthing system. Therefore, it is important to consider other factors and design the earthing system appropriately to achieve the desired level of earth resistance.
7. Conclusion: The addition of ground rods in the earthing grid slightly decreases the earth resistance by improving conductivity and providing multiple grounding points. However, the overall resistance depends on various factors, and proper design considerations are necessary to ensure an effective earthing system.