Previous Year Question

Correct answer: C — 0.5 Ω

Earth resistance and fault-current discharge

The expected maximum earth resistance for a large power station is 0.5 Ω. The earthing grid connects equipment frames, structures and other bonded metalwork to a common network of buried conductors and electrodes. It provides a path for earth-fault current and helps control dangerous potential differences during a fault.

When current enters the soil through the grid, the grid potential rises relative to remote earth. For a simplified resistive model, this rise is:

Here I_g is the current actually discharged from the grid into earth, and R_g is the grid resistance. For the same discharged current, a lower grid resistance produces a smaller potential rise. Large stations can experience substantial fault currents, which explains the low resistance value used in this question.

The 0.5 Ω figure is the answer to this examination item. In an actual earthing design, resistance alone does not establish safety: grid geometry, soil resistivity, current division, fault-clearing time, and permissible touch and step voltages must also be considered. A low measured resistance and safe surface-potential gradients are related but distinct requirements.