Earthing techniques and substation safety

Earthing techniques commonly practised in industry

1. Pipe earthing: perforated GI pipe (about 38 mm, 2–3 m long) in a pit with charcoal and salt; most common for buildings and small installations.
2. Plate earthing: copper or GI plate (600 × 600 mm) buried about 3 m deep with charcoal and salt.
3. Rod earthing: copper-bonded steel rods driven into the ground; easy where soil is soft.
4. Strip/wire earthing: horizontal copper or GI strips buried 0.5 m deep; used in rocky soil.
5. Chemical (maintenance-free) earthing: electrode with conductive backfill compound such as bentonite.
6. Earth mat/grid: horizontal mesh with vertical rods for substations and power plants.
7. Counterpoise and tower footing: buried wires/rods for transmission towers.
8. Neutral earthing of the system: solid, resistance, reactance or resonant earthing of transformer and generator neutrals.
9. Equipment earthing and bonding: frames, enclosures, cable sheaths and structures bonded to the earth bus; double earthing of major equipment.
10. Lightning protection earthing: separate earth pits for arresters and lightning conductors, bonded to the main grid.

How a properly designed substation earthing protects people

During an earth fault, fault current $I_g$ flows into the soil and the whole grid rises to $GPR = I_g R_g$. The danger to a person is not the GPR itself but the difference in potential across the body (step, touch, mesh and transferred potentials). A good design (IEEE Std 80) keeps these differences below safe limits:

   ____  ____  ____  earth mat (mesh)
  |    ||    ||    |  0.5-1 m deep
  |____||____||____|  + vertical rods
  |    ||    ||    |
  |____||____||____|
   gravel layer on top, fence bonded

1. Low grid resistance: a large grid with many rods lowers $R_g$ and thus the GPR.
2. Equipotential area: the closely spaced mesh makes the ground surface nearly the same potential as the equipment, so touch and mesh voltages are small. Finer mesh is used near equipment operated by people.
3. Bonding of all metal: frames, structures, fences, cable sheaths and gates are bonded to the grid, so a person touching them stands on ground at nearly the same potential.
4. High-resistivity surface layer: 10–15 cm of crushed rock (2000–3000 Ω-m) greatly increases foot contact resistance and reduces body current.
5. Grading rings and perimeter conductors outside the fence reduce steep gradients at the edge and step voltage outside.
6. Fast fault clearing: low $R_g$ gives enough fault current for relays to trip quickly; shorter $t_s$ allows larger safe voltages:

$$E_{touch} = (1000 + 1.5C_s\rho_s)\frac{0.116}{\sqrt{t_s}}$$

7. Control of transferred potential: isolation of communication circuits and care with fences and rails leaving the yard.
8. Low earth-conductor impedance so that lightning and switching surges are discharged safely.

Thus a well-designed earth mat makes the substation an equipotential zone where actual step and touch voltages remain below the tolerable values, so personnel do not receive a dangerous shock.