Factors influencing earth resistance

Earth resistance is the resistance between the earth electrode and the general mass of earth. It must be low so that fault and surge currents flow easily to earth. The main factors influencing it are:

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.

Example: a pipe electrode giving 4 Ω in the monsoon may give 10 Ω or more in the dry winter months, because the soil loses moisture. Watering the pit and treating it with charcoal and salt keeps the value low.