Earthing

Main objective of equipment earthing

Equipment earthing means connecting the non-current-carrying metal parts (motor frames, panel bodies, conduits, transformer tanks) to the general mass of earth through a low-resistance path. Its objectives are:

1. Safety from electric shock: if insulation fails and a live wire touches the body, the fault current flows to earth through the low-resistance earth path, not through a person. The body stays at a low touch voltage.
2. Fast operation of protection: the low-resistance path allows a large fault current, so the fuse/MCB/relay trips quickly and isolates the faulty equipment.
3. Protection of equipment and fire prevention: prevents sustained arcing and overheating at the fault.
4. Discharge of static charges and lightning surges safely to earth.

Size of plate electrode

For a plate electrode (IS 3043):

R = (ρ/4) × √(π/A)

where R = earth resistance (Ω), ρ = soil resistivity (Ω-m), A = area of one face of the plate (m²).

The resistivity is taken as 60 Ω-m (the unit is printed as Ω). Solving for A:

√(π/A) = 4R/ρ
A = π × (ρ / 4R)²
  = π × (60 / (4×8))²
  = π × (1.875)²
  = π × 3.5156
  = 11.04 m²

For a square plate:

Side = √11.04 = 3.32 m

Answer: Area of plate ≈ 11.04 m², i.e. a square plate of about 3.32 m × 3.32 m.

Such a large single plate is not practical. In practice a standard plate (for example 60 cm × 60 cm × 6.3 mm GI or 3.15 mm copper) is used with several electrodes in parallel, soil treatment with charcoal and salt (or bentonite) to lower ρ, or a deeper rod electrode, to reach 8 Ω.

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.

Earthing

Earthing is the connection of the neutral point of a supply system or the non-current-carrying metal parts of electrical equipment to the general mass of earth through a conductor of very low resistance, so that the earth connection is always at earth (zero) potential.

Purpose, with example

1. Safety from shock: keeps exposed metal parts at earth potential.
2. Fast fault clearing: gives a low-resistance path so the fuse/MCB trips.
3. Stable system voltage: earthing the transformer neutral fixes the phase-to-earth voltage (230 V) and prevents it rising during faults.
4. Lightning and surge protection: surge currents are discharged to earth.
5. Discharge of static charge in places with fuel, dust or belts.

Example: a 230 V washing machine has its live wire touching the metal body because of worn insulation.

• If the body is earthed (earth resistance about 1 Ω), a large fault current (about 230 A) flows to earth. The MCB/RCCB trips at once, and the body is dead before anyone touches it.
• If the body is not earthed, the body stays at 230 V. A person touching it completes the circuit to earth through the body and gets a dangerous shock.

Points to be earthed in an industrial plant

1. Neutral point of each supply transformer and generator (system earthing).
2. Transformer tank, generator and motor frames.
3. Metal enclosures of switchgear, panels, MCCs, distribution boards and bus ducts.
4. Metal conduits, cable trays, trunking and the armour and sheath of cables.
5. Lightning arresters and the lightning protection system (down conductors).
6. Metal bodies of all portable and fixed appliances through the earth pin of sockets.
7. Structural steel, cranes and crane rails, metal pipes and tanks (equipotential bonding).
8. Capacitor bank frames and the secondary of CTs and PTs.
9. Handles of operating mechanisms of switches and isolators.
10. Fences and gates of outdoor substations, and stay wires of overhead lines.
11. Equipment in hazardous areas (fuel storage, paint shops) to discharge static.

Methods to reduce earth resistance

1. Increase electrode length/depth: a longer rod or pipe driven deeper reaches moist soil of lower resistivity.
2. Use several electrodes in parallel: spaced at least about twice their length apart, so their resistance areas do not overlap.
3. Chemical treatment of soil: fill the pit with alternate layers of charcoal and common salt, or use bentonite or a chemical earthing compound, which holds moisture and adds ions.
4. Keep the soil moist: provide a watering funnel/pipe and water the pit regularly, mainly in the dry season.
5. Increase electrode size: use a larger plate or a bigger diameter pipe (a smaller effect than length).
6. Use a counterpoise or earth mat: buried strips or a grid in substations lower the total resistance.
7. Choose a good location: moist, low-lying clay soil away from rock and sandy areas.

Pipe earthing (as per IS 3043 practice)

   Funnel with wire mesh
        ___
        \ /     Cement concrete chamber
   _____| |__________  ← ground level
  |  [ ]---- GI earth wire to equipment
  |  | |  (via 12.7 mm GI pipe)
  |  | |                 Pit filled with
  |  | |  ##  alternate layers of
  |  | |  ..  charcoal and salt
  |  | |  ##  (about 15 cm around pipe)
  |  | |  ..
  |  |o|  ← 38 mm dia GI pipe, 2.5 m long,
  |  |o|    with 12 mm holes along length
  |  \_/  ← tapered bottom end
  |________________
   about 3.75 m deep below ground

Process:

1. Pipe: a galvanised iron (GI) pipe of 38 mm internal diameter and 2.5 m length (longer in dry soil) is used. It has 12 mm holes drilled along its length so water can reach the soil, and its lower end is tapered for driving.
2. Pit: a pit is dug and the pipe is placed vertically so that its top is about 3.75 m below ground in normal soil (deeper in dry soil).
3. Backfill: the space around the pipe, about 15 cm wide, is filled with alternate layers of charcoal and salt. This keeps the soil moist and conductive.
4. Watering: a 19 mm pipe with a funnel and wire mesh is connected to the top of the earth pipe so water can be poured in during dry weather.
5. Earth lead: the earth wire (GI wire or strip) is connected to the pipe top by a reducing socket/clamp, and taken to the equipment through a 12.7 mm GI pipe for protection, at least 60 cm below ground.
6. Chamber: a cement-concrete chamber with a cast-iron cover is built at the top for inspection and watering.
7. Testing: the earth resistance is measured with an earth tester (fall-of-potential method). It should normally be below about 1–5 Ω depending on the installation; if higher, more pipes are added in parallel or the soil is treated further.

Pipe earthing is cheaper and easier to keep moist than plate earthing and is the most common type for buildings and small installations.

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 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), usually near the arrester. For a distribution transformer, typically two separate LA earth pits are provided, besides the neutral and body earths.
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).

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 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.

Diameter of rod electrode

For a rod (pipe) electrode driven vertically (IS 3043, written in SI units):

R = (ρ / 2πL) × ln(4L / d)

R = earth resistance (Ω), ρ = soil resistivity (Ω-m), L = length of rod (m), d = diameter of rod (m).

Given R = 10 Ω, ρ = 65 Ω-m, L = 5 m:

ln(4L/d) = R × 2πL / ρ
         = 10 × 2π × 5 / 65
         = 4.833
4L/d     = e^4.833 = 125.61
d        = 4L / 125.61
         = 20 / 125.61
         = 0.1592 m

Check: R = (65 / (2π×5)) × ln(20/0.1592) = 2.069 × 4.833 = 10.0 Ω.

Answer: Diameter of rod ≈ 0.159 m (about 15.9 cm).

Such a thick rod is not practical. Since resistance depends only weakly (logarithmically) on diameter, in practice a standard rod (16–25 mm) is made longer, or several rods are used in parallel, or the soil is treated, to get 10 Ω.

Factors influencing 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 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.

Size of plate electrode

For a plate electrode buried vertically (IS 3043):

R = (ρ/4) × √(π/A)

R = earth resistance (Ω), ρ = soil resistivity (Ω-m), A = area of one face of the plate (m²).

Given ρ = 50 Ω-m, R = 10 Ω:

√(π/A) = 4R/ρ
A = π × (ρ / 4R)²
  = π × (50 / 40)²
  = π × 1.5625
  = 4.909 m²
Side of square plate = √4.909 = 2.216 m

Check: R = (50/4) × √(π/4.909) = 12.5 × 0.8 = 10.0 Ω.

Answer: Plate area ≈ 4.91 m², i.e. a square plate of about 2.22 m × 2.22 m.

In practice a single plate this big is not used. Standard plates (60 cm × 60 cm) are used in parallel, with charcoal and salt treatment of the pit, to reach the required resistance.

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.

Major purpose of earthing

Earthing connects the system neutral and the exposed metal parts of equipment to earth through a low-resistance path. Its main purposes are:

• Safety of persons: keeps metal bodies near earth potential so a fault does not cause a dangerous shock.
• Quick fault clearance: a large earth-fault current makes fuses, MCBs and relays trip quickly.
• Stable system voltage: the earthed neutral fixes phase-to-earth voltage and limits overvoltages.
• Safe discharge of lightning and static charges.

Consequences of earthed and unearthed systems

Summary: an earthed system is safer and is used for almost all LV industrial and building installations. An unearthed system gives continuity during a single fault, but causes overvoltage, insulation stress, arcing grounds and hidden faults, so it is used only in special cases (some mines and process plants with insulation monitoring).

Main objectives of equipment earthing

1. Protect people from electric shock: keeps exposed metal parts near earth potential when insulation fails.
2. Ensure quick operation of protection: the low-resistance path allows a large fault current, so fuses/MCBs/relays clear the fault fast.
3. Protect equipment and buildings from fire caused by leakage and arcing.
4. Discharge lightning surges and static charges safely.

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 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.

Size of plate electrode

The required earth resistance is not given. It is assumed to be 5 Ω, a common limit for building installations.

Using IS 3043:

R = (ρ/4) × √(π/A)
A = π × (ρ / 4R)²
  = π × (40 / (4×5))²
  = π × 2²
  = 12.57 m²
Side = √12.57 = 3.54 m

Answer: Plate area ≈ 12.57 m² (a square plate of about 3.54 m × 3.54 m) for R = 5 Ω. For R = 10 Ω, the same formula gives A = π × (40/40)² = 3.14 m² (about 1.77 m × 1.77 m).

Because such large plates are impractical, standard 60 cm × 60 cm plates are used in parallel, with charcoal and salt treatment, to reach the required value.

A lightning arrester (surge arrester) is a protective device connected between line and earth that sends high-voltage surges (from lightning or switching) to earth and so protects transformers, switchgear and cables in an industrial distribution system. At normal voltage it behaves as an insulator; above a set voltage it conducts and limits the voltage.

Working

 Line ──┬────────── to transformer
        │
      [LA]  non-linear resistor (ZnO)
        │
      [E]  earth (short, straight lead)

• At normal voltage the arrester draws only a tiny leakage current.
• When a surge arrives, its resistance falls sharply and it conducts the surge current to earth, holding the voltage at the equipment to its residual voltage, which is below the insulation level (BIL) of the equipment.
• After the surge passes, it returns to the insulating state and the normal power-frequency current does not continue.

Types used

• Metal oxide (ZnO) gapless arrester: most common today; fast, no series gaps, good energy capacity.
• Valve type (SiC with series gaps): older type.
• Rod gap / horn gap: simple, cheap backup protection on lines.
• Low-voltage surge protective devices (SPD) in LV panels to protect electronics and drives.

Use in industrial power distribution

• At the incoming HV line and at both sides of the distribution transformer (mounted close to the bushings).
• At cable terminations where overhead lines change to underground cable.
• SPDs in main LV panels and in control/PLC panels.

Requirements

• Rated voltage and discharge current (typically 5 kA or 10 kA) suited to the system.
• Short, straight earth lead to a separate low-resistance earth pit, bonded to the equipment body and station earth.
• Periodic testing of insulation and leakage current.

Assume a 230 V supply whose neutral is earthed at the transformer, and that the live wire inside the oven has touched its metal body because of insulation failure.

 L ─[MCB]──┐
           │ fault
       ┌───┴───┐
       │ oven  │──── earth wire (if present)
       │ body  │        │
       └───┬───┘        │
         person         │
           │            │
 ══════════╧════════════╧══ earth
 N earthed at transformer

Unearthed condition (no earth wire)

• The oven body rises to about 230 V with respect to earth.
• The person touching it becomes the only path to earth. With body plus contact resistance of about 1000 Ω, current ≈ 230/1000 = 230 mA.
• Currents above about 30 mA through the body can cause muscle spasm, inability to let go, and ventricular fibrillation, leading to death.
• The current is too small to blow the fuse or trip the MCB, so the body stays live and the danger continues for anyone who touches it.

Earthed condition (body connected to a good earth)

• As soon as the fault occurs, a large fault current flows through the low-resistance earth wire back to the transformer neutral (for example, with total loop resistance of about 2 Ω, the current is over 100 A).
• This current trips the MCB or blows the fuse within a fraction of a second, so the oven is disconnected, often before anyone touches it.
• If the person touches it during this short time, the body is in parallel with the very low earth path, so only a small current passes through the person. With an RCCB (30 mA) the supply is cut in about 30 ms.

Conclusion: the unearthed oven is a serious shock hazard; the earthed oven makes the fault safe by clearing it quickly. This is why every appliance with a metal body must use a 3-pin plug with the earth pin connected.

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.

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.

The types of earthing system (IEC 60364) describe how the supply source neutral and the exposed metal parts of the installation are connected to earth. They are named by two letters:

• First letter (source): T = neutral directly earthed; I = neutral isolated or earthed through a high impedance.
• Second letter (installation): T = exposed parts earthed by a local electrode; N = exposed parts connected to the earthed source neutral.
• Further letters: S = separate neutral (N) and protective (PE) conductors; C = combined (PEN).

1. TN-S system

Neutral earthed at the source; separate N and PE conductors run all the way to the equipment.

 Source         Installation
 L1 ─────────────────────
 L2 ─────────────────────
 L3 ─────────────────────
 N  ─────────────────────
 PE ──┬──────────────┬───
      │            [body]
     ⏚ source earth

Safe and low-noise; good for computers and sensitive loads. Costlier (5 wires).

2. TN-C system

N and PE combined as one PEN conductor throughout.

 L1 ─────────────────────
 L2 ─────────────────────
 L3 ─────────────────────
 PEN ─┬──────────────┬───
      │            [body]
     ⏚

Cheaper, but a broken PEN can make all bodies live; not allowed for small cables or portable equipment.

3. TN-C-S system (PME)

PEN conductor in the supply part, split into separate N and PE inside the installation.

 L1 ──────────────┬──────
 L2 ──────────────┼──────
 L3 ──────────────┼──────
 PEN ─┬───────────┼─┬─ N
      │           │ └─ PE ─[body]
     ⏚          ⏚ (re-earthed)

Most common in public LV distribution (as in Nepal's 400/230 V network).

4. TT system

Source neutral earthed at the transformer; equipment bodies earthed through a separate local electrode at the consumer.

 L1 ─────────────────────
 L2 ─────────────────────
 L3 ─────────────────────
 N  ─────────────────────
      │            [body]
     ⏚ source       │
                    ⏚ local earth

Fault current is limited by two earth resistances, so an RCCB is needed. Used where the supply PE is not available (rural areas).

5. IT system

Source neutral isolated (or earthed through high impedance); bodies earthed locally.

 L1 ─────────────────────
 L2 ─────────────────────
 L3 ─────────────────────
  (neutral not earthed)
                   [body]
                     │
                     ⏚

A first earth fault gives only a tiny current, so supply continues; an insulation monitoring device gives an alarm. Used in hospitals (operating theatres), mines and continuous process plants.

For a plate electrode buried vertically, IS 3043 gives:

R = (ρ/4) × √(π/A)

R = earth resistance (Ω), ρ = soil resistivity (Ω-m), A = area of one face of the plate (m²).

The soil resistivity is taken as 60 Ω-m (the unit is printed as Ω). Required R = 8 Ω.

√(π/A) = 4R/ρ
A = π × (ρ / 4R)²
  = π × (60 / 32)²
  = π × (1.875)²
  = π × 3.5156
  = 11.04 m²
Side of square plate = √11.04 = 3.32 m

Check: R = (60/4) × √(π/11.04) = 15 × 0.5333 = 8.0 Ω.

Answer: Area of plate ≈ 11.04 m², i.e. a square plate of about 3.32 m × 3.32 m.

A single plate of this size is not practical. Standard plates (60 cm × 60 cm × 6.3 mm GI) are used in parallel, with charcoal and salt treatment of the pit, or deeper rod/pipe electrodes, to obtain 8 Ω.