Low earth resistance and equipment earthing methods

Why earth resistance is kept low

1. Fast operation of protection: fault current = voltage / (earth loop resistance). A low resistance gives a large fault current, so the fuse/MCB/relay trips quickly.
2. Low touch and step voltage: the voltage of a faulty metal body = fault current × earth resistance. A low resistance keeps this voltage safe for people.
3. Effective lightning protection: surge arresters and lightning conductors need a low-resistance path so that the voltage rise during a surge is small.
4. Stable neutral voltage: keeps the system neutral near earth potential.
5. Fire prevention: avoids long-lasting leakage currents and heating at the fault.

Typical limits: about 1 Ω for large power stations and substations, a few ohms for small substations and industrial installations.

Methods of equipment earthing

1. Plate earthing
2. Pipe earthing
3. Rod earthing
4. Strip or wire earthing
5. Earth mat (grid) earthing in substations
6. Chemical (maintenance-free) earthing

Plate earthing (explained)

  ground level
 ══╤═══════════════════╤══
   │ funnel+watering   │ GI pipe protecting
   │ pipe (19 mm)      │ earth lead
   │                   │
   │   charcoal + salt │
   │   layers around   │
   │   ┌───────────┐   │
   │   │  plate    │◄──┘ earth lead
   │   │ 60×60 cm  │     bolted to plate
   │   └───────────┘
   │   at least 3 m deep

1. Electrode: a GI plate of 60 cm × 60 cm × 6.3 mm or a copper plate of 60 cm × 60 cm × 3.15 mm.
2. Pit: dug so the top of the plate is at least 3 m below ground. The plate is placed vertically.
3. Backfill: the plate is surrounded by alternate layers of charcoal and salt (about 15 cm thick), which keep the soil moist and conductive.
4. Earth lead: a GI wire/strip (or copper for a copper plate) is bolted to the plate with nuts and washers, and taken up through a GI pipe for mechanical protection.
5. Watering pipe: a 19 mm GI pipe with a funnel and wire mesh is placed near the plate so the pit can be watered in dry weather.
6. Chamber: a masonry chamber with a cover is built at the top for inspection.
7. Testing: earth resistance is measured with an earth tester; if too high, more plates are added in parallel or the soil is treated further.

Plate earthing gives low resistance and is used for substations, transformers and large machines, though it costs more than pipe earthing.