Earthing for shock protection and methods

How earthing protects a person from shock

When the insulation of a live conductor fails and it touches the metal body of a machine, the body becomes live. Earthing gives the fault current a path to earth of much lower resistance than a human body, and makes the protective device trip.

   Phase ──[MCB]──────┐
                      │ insulation fault
                 ┌────┴────┐
                 │  metal  │ ← body
                 │  body   │
                 └────┬────┘
   person ──┐         │ earth wire (≈1 Ω)
   (≈1000 Ω)│         │
  ══════════╧═════════╧════ earth
   Neutral earthed at transformer

Example: a 230 V motor body becomes live.

• Without earthing: if a person (body resistance about 1000 Ω) touches it, current through the person is about 230/1000 = 230 mA, which can cause heart fibrillation and death. The fuse does not blow because the current is small.
• With earthing: the earth wire (about 1 Ω, with the transformer neutral earth) carries a large fault current. The voltage of the body is held low, the person and the earth wire are in parallel so very little current passes through the person, and the large current blows the fuse or trips the MCB in a fraction of a second. An RCCB (30 mA) gives even faster protection.

Methods of equipment earthing used in industrial buildings

1. Plate earthing: a GI plate (60 cm × 60 cm × 6.3 mm) or copper plate (60 cm × 60 cm × 3.15 mm) is buried vertically at least 3 m deep. It is surrounded by alternate layers of charcoal and salt, and a watering pipe is provided. The earth lead is bolted to the plate. Used for substations and large equipment; costly.
2. Pipe earthing: a GI pipe of 38 mm diameter and 2.5 m long with 12 mm holes is placed vertically about 3.75 m deep, surrounded by charcoal and salt, with a funnel for watering. It is the most common and economical method.
3. Rod earthing: a copper, copper-bonded steel or GI rod (12.5–25 mm diameter, about 1.2–3 m long) is driven directly into the soil. Used in sandy or soft soil where digging is difficult; cheap and quick.
4. Strip or wire earthing: GI or copper strips (e.g. 25 mm × 1.6 mm) or wires are buried horizontally in trenches at least 0.5 m deep. Used in rocky areas where vertical electrodes cannot be driven, and as a counterpoise for lines.
5. Earth mat / grid: a mesh of buried conductors connected to several rods, used in substations to control step and touch voltages.
6. Chemical (maintenance-free) earthing: an electrode in a backfill compound (bentonite/graphite) that keeps resistance low without regular watering.

Inside the building, an earth continuity conductor runs with every circuit and connects all equipment bodies, conduits and socket earth pins to an earth bus bar, which is connected to two or more earth electrodes.