Equipment earthing objective and earth resistance factors

Main objective of equipment earthing

Equipment earthing connects the non-current-carrying metal parts of equipment (frames, enclosures, conduits) to earth through a low-resistance conductor. Its objectives are:

1. Protect persons from electric shock: when insulation fails, the metal body would become live. Earthing keeps its potential near zero, and the fault current flows to earth, not through a person.
2. Quick operation of protective devices: the low-resistance earth path lets a large fault current flow, so fuses, MCBs or earth-fault relays trip quickly.
3. Protect equipment and buildings from fire by preventing long-lasting leakage and arcing.
4. Safely discharge static electricity and lightning/switching surges.

Factors affecting 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 (below about 15% moisture) has very high resistivity. This is why earth pits are watered and resistance is higher in the dry season.
3. Salt (chemical) content: dissolved salts make the soil water conductive. Adding salt, charcoal or bentonite lowers resistance.
4. Temperature: resistivity rises as temperature falls, very sharply when soil water freezes.
5. Size and shape of electrode: a larger plate or longer rod has more contact area and lower resistance. Rod length matters more than diameter.
6. Depth of burial: deeper electrodes reach moist, stable soil and have lower and steadier resistance.
7. Number and spacing of electrodes: several electrodes in parallel lower resistance, if spaced at least about twice their length apart.
8. Contact between electrode and soil: loose or dry backfill raises resistance; well-compacted, treated backfill lowers it.
9. Condition of electrode and connections: corrosion of the electrode or loose joints raises resistance over time.