Earthing (grounding) provides a low-resistance path for fault current to flow safely into the earth, protecting personnel and equipment. On the railway, earthing appears in two related but distinct contexts: general electrical installation earthing, and traction earthing/bonding of masts and structures (covered in the Traction & OHE chapter).
Types of Earthing
- Plate earthing — a metal plate buried vertically in the ground, surrounded by charcoal/salt to reduce soil resistivity.
- Pipe earthing — a galvanised iron pipe driven vertically into the ground; more common for smaller installations.
- Strip/wire earthing — a conductor laid horizontally in trenches, often used where rocky soil prevents deep vertical electrodes.
Earth Electrode Resistance (Rod Type, Approx.)
R = (ρ / 2πL) × [ln(4L/d) − 1]
ρ = soil resistivity, L = electrode length, d = electrode diameter. In practice: resistance falls as electrode length increases and as soil resistivity decreases (moist soil, chemical treatment).
Key Points to Remember
- Earth resistance should be tested periodically with an earth tester (megger-type instrument), not assumed constant.
- Equipment earthing (body earthing) and system/neutral earthing serve different purposes — one protects personnel from a faulty casing, the other stabilizes system voltage.