Earth resistance factors and lightning arrester earthing

Factors affecting earth resistance

1. Soil type: clay and loam have low resistivity; sand, gravel and rock have high resistivity.
2. Moisture content: more moisture means much lower resistivity. Dry soil can have very high resistance, so resistance is worst in the dry season.
3. Dissolved salts: salts in the soil water make it conductive. Salt, charcoal or bentonite added around the electrode lower resistance.
4. Temperature: resistivity rises as temperature falls, very sharply below freezing.
5. Electrode size and shape: larger plates and longer rods have lower resistance; length is more effective than diameter.
6. Depth of electrode: deeper electrodes reach moist and stable soil.
7. Number and spacing of electrodes: parallel electrodes reduce resistance if spaced well apart (≥ about twice their length).
8. Contact with soil and condition of electrode: poor compaction, corrosion and loose joints raise resistance.

Earthing of a lightning arrester

A lightning arrester (surge arrester) must send a very high, very fast surge current (several kA) to earth with the smallest possible voltage rise. Its earthing is therefore done as follows:

 Line ──┬───────────── to transformer
        |
      [LA]
        |  short, straight earth lead
        |  (copper / GI strip)
        └───┐
  ground ───┼──────────────
           [E] separate LA earth pit
            |   (pipe or rod, low R)
            └── bonded to station earth
                  grid / transformer body

1. Short and straight earth lead: the lead from the arrester's earth terminal to the electrode is as short and straight as possible, without sharp bends or loops. The inductance of a long lead adds an L·di/dt voltage to the arrester's residual voltage and reduces protection.
2. Separate earth electrode: each arrester (or set of three) has its own earth pit (pipe/rod electrode) near the arrester, separate from the transformer neutral and body earth pits. Some utilities provide two LA earth pits at a distribution transformer.
3. Low earth resistance: the LA earth resistance is kept low (in the order of 1–10 Ω, lower in substations), using chemical treatment or several electrodes where needed.
4. Interconnection (bonding): the arrester earth is bonded to the transformer tank and the station earth grid. This keeps the equipment and the arrester at the same potential during a surge, so the insulation sees only the arrester's residual voltage.
5. Conductor size: copper or GI strip/wire of adequate cross-section (for example 25 × 3 mm GI strip or 8 SWG GI wire, as per practice) to carry the surge and fault current, protected from mechanical damage.
6. Placement: the arrester is mounted as close as possible to the equipment it protects (transformer bushings).
7. Testing: the earth resistance of LA pits is checked periodically, especially before the monsoon (lightning season).