Equipment earthing objective and plate electrode size

Main objective of equipment earthing

Equipment earthing means connecting the non-current-carrying metal parts (motor frames, panel bodies, conduits, transformer tanks) to the general mass of earth through a low-resistance path. Its objectives are:

1. Safety from electric shock: if insulation fails and a live wire touches the body, the fault current flows to earth through the low-resistance earth path, not through a person. The body stays at a low touch voltage.
2. Fast operation of protection: the low-resistance path allows a large fault current, so the fuse/MCB/relay trips quickly and isolates the faulty equipment.
3. Protection of equipment and fire prevention: prevents sustained arcing and overheating at the fault.
4. Discharge of static charges and lightning surges safely to earth.

Size of plate electrode

For a plate electrode (IS 3043):

R = (ρ/4) × √(π/A)

where R = earth resistance (Ω), ρ = soil resistivity (Ω-m), A = area of one face of the plate (m²).

The resistivity is taken as 60 Ω-m (the unit is printed as Ω). Solving for A:

√(π/A) = 4R/ρ
A = π × (ρ / 4R)²
  = π × (60 / (4×8))²
  = π × (1.875)²
  = π × 3.5156
  = 11.04 m²

For a square plate:

Side = √11.04 = 3.32 m

Answer: Area of plate ≈ 11.04 m², i.e. a square plate of about 3.32 m × 3.32 m.

Such a large single plate is not practical. In practice a standard plate (for example 60 cm × 60 cm × 6.3 mm GI or 3.15 mm copper) is used with several electrodes in parallel, soil treatment with charcoal and salt (or bentonite) to lower ρ, or a deeper rod electrode, to reach 8 Ω.