Earth resistance factors and plate electrode size

Factors influencing earth resistance

1. Type of soil: clay, loam and garden soil have low resistivity; sand, gravel and rock have very high resistivity.
2. Moisture content: resistivity falls sharply as moisture rises. Dry soil has very high resistivity, so earth resistance is highest in the dry season.
3. Dissolved salts (chemical content): salts make soil water conductive. Adding salt, charcoal or bentonite lowers the resistance.
4. Temperature: resistivity rises as the soil cools, very sharply when soil water freezes.
5. Size and shape of electrode: a larger plate or longer rod has more contact with soil and lower resistance. Rod length matters much more than diameter.
6. Depth of electrode: deeper electrodes reach moist, stable soil, so resistance is lower and changes less with season.
7. Number and spacing of electrodes: electrodes in parallel lower the resistance, provided they are spaced at least about twice their length apart.
8. Contact with soil: loose, dry backfill increases resistance; compacted, treated backfill reduces it.
9. Condition of electrode and joints: corrosion and loose connections increase resistance with time.

Size of plate electrode

For a plate electrode buried vertically (IS 3043):

R = (ρ/4) × √(π/A)

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

Given ρ = 50 Ω-m, R = 10 Ω:

√(π/A) = 4R/ρ
A = π × (ρ / 4R)²
  = π × (50 / 40)²
  = π × 1.5625
  = 4.909 m²
Side of square plate = √4.909 = 2.216 m

Check: R = (50/4) × √(π/4.909) = 12.5 × 0.8 = 10.0 Ω.

Answer: Plate area ≈ 4.91 m², i.e. a square plate of about 2.22 m × 2.22 m.

In practice a single plate this big is not used. Standard plates (60 cm × 60 cm) are used in parallel, with charcoal and salt treatment of the pit, to reach the required resistance.