Equipment Earthing: What It Is & Why It’s Important

What is Earthing?

Equipment earthing, also called protective earthing, connects exposed conductive parts that are not normally live to the installation’s main earthing terminal through a protective conductor. During an insulation fault, this path and the bonding network help limit dangerous potential differences and allow the protective device to disconnect the supply. Earthing a transformer or generator neutral is system earthing, a separate design function. Earthing work must be designed, installed and tested by qualified personnel for the actual supply system and fault conditions.
Equipment earthing in India should follow the current applicable requirements, including IS 3043:2018 and the Central Electricity Authority safety regulations in force.

1. IS 3043:2018, Code of practice for earthing, is the current published second revision identified by the Bureau of Indian Standards.
2. The Central Electricity Authority (Measures relating to Safety and Electric Supply) Regulations, 2023 replaced reliance on the old Indian Electricity Rules, 1956 for current national safety requirements.
3. Protection must account for the effects of current through the body, fault duration, touch conditions and the automatic-disconnection requirements of the applicable standard.
4. Lightning protection uses the applicable IS/IEC 62305 series and must be coordinated with the building earthing and bonding design.
5. Earth: The conductive mass of the ground, used as a reference whose remote electric potential is conventionally taken as zero.
6. Earth Electrode: A Conductor or conductive assembly in electrical contact with the soil, concrete or another conductive medium that provides a connection to earth.
7. Earth Electrode Resistance: The electrical resistance between an electrode system and remote earth at power frequency under the stated test conditions.
8. Earthing Conductor: A conductor that connects the main earthing terminal to an earth electrode or another approved means of earthing. A protective conductor connects exposed conductive parts into the fault-protection system.
9. Equipotential Bonding: An electrical connection that keeps exposed conductive parts and extraneous conductive parts at substantially equal potential.
   Examples include bonding equipment frames, structural metal and other conductive services to the installation’s protective-conductor network where required.
10. Potential gradient: The change in potential difference per unit distance in a specified direction at the earth’s surface or within the soil.
11. Touch Voltage: The voltage a person may experience between simultaneously accessible conductive parts, commonly a grounded structure and the surface at the person’s feet during a fault.
12. Step Voltage: The voltage between two points on the earth’s surface one pace apart, conventionally assessed over a one-metre step for substation studies.
13. Earth Grid: A network of interconnected buried conductors and electrodes that provides a common earthing system for equipment and conductive structures.
14. Earth Mat: A grid of buried horizontal conductors, often combined with vertical electrodes, designed to carry fault current and control ground-potential rise, touch voltage and step voltage.

Why Earthing Is Important

Correct protective earthing and bonding help provide the following outcomes:

1. Reduced electric-shock risk by limiting touch voltage and supporting fast automatic disconnection of a faulty circuit.
2. A defined fault-current path that coordinates with fuses, circuit breakers and residual-current devices.
3. Reduced fire and equipment-damage risk when a fault is cleared within the required time.
4. A controlled reference and bonding network that supports reliable protection and system operation.

Classification of Earthing

Earthing functions are broadly divided into the following two groups.

1. System earthing connects a current-carrying point, such as the LV neutral of a power transformer, to an earthing system directly or through a designed impedance. It controls system voltage to earth and earth-fault behaviour.
2. Equipment earthing connects exposed conductive parts, such as an electric motor frame, transformer tank or switchgear enclosure, to the protective-conductor and bonding network. These parts are not intended to carry current in normal service.

How Earth Resistance Limits Are Set

No single resistance value proves that an earthing system is safe. The design must meet the applicable touch-voltage, step-voltage, earth-potential-rise, fault-loop and protective-device requirements. The figures below are historical rules of thumb, not universal acceptance limits.

• Power stations – a stated 0.5-ohm target does not replace a fault-current and potential-rise study.
• EHT stations – a stated 1.0-ohm target does not establish safe touch and step voltages by itself.
• 33 kV substations – a stated 2-ohm target must not be treated as a general permission or design rule.
• Distribution-transformer structures – a stated 5-ohm target still requires protection coordination and compliance checks.
• Tower footing resistance – the required value depends on line design, lightning performance, soil and the governing utility standard.

Basis for an Acceptable Earthing Design

An acceptable design starts with the current regulations and standards, prospective earth-fault current, fault-clearing time, soil model, conductor thermal capacity, corrosion allowance, protective-device operation and tolerable touch and step voltages.

1. A fixed value of 523 volts is not a universal safe touch-voltage limit. Tolerable voltage depends on exposure duration, body path, surface conditions and the method defined by the applicable standard.
2. I_fault is the prospective current for the fault and system under study. The displayed relation is only a simplified illustration; a complete design must also include fault duration, current split, loop impedance and potential distribution.
   

   

3. A 100 kVA transformer does not imply one universal fault current. The value depends on rated secondary voltage, vector group, source and conductor impedance, transformer percentage impedance and the fault location.
   

   

4. The following historical 100 kVA example depends on assumptions that are not stated completely and must not be reused as a site design.
   

   

   
   Its calculated 0.26-ohm result is not a universal target. Electrode resistance, fault-loop impedance, protective-device clearing time and touch and step voltages must be calculated and tested for the actual installation.
   Urban and rural locations do not automatically receive different permissible values; the authority having jurisdiction and the current technical standard determine acceptance.
5. The earthing connection for lightning arrestors must provide a short, direct surge-current path and coordinate with the site’s bonding and lightning-protection system.
   The displayed resistance-only formula is an incomplete surge model because conductor inductance, current waveform, lead length and earth impedance affect the result.
   

   

   
   The stated 11 kV flashover voltage and 40 kA discharge current are example assumptions, not universal equipment ratings.

Type of Earthing

Plate Type Earthing

A plate electrode is a buried conductive plate connected to the main earthing terminal. Its material, dimensions, burial depth, joints and protective-conductor size must follow the current standard and project design. Selection depends on soil resistivity, corrosion, prospective fault current, clearing time and the required life of the installation. Historical dimensions such as 600 mm × 600 mm × 6.3 mm must not be copied without confirming that they apply to the site and electrode material.

The backfill and inspection arrangement must use an approved design that preserves electrode life and permits testing. Do not assume that adding charcoal, salt or water is an acceptable way to obtain a reading; these measures can change corrosion and soil conditions. Use the backfill, treatment and maintenance method specified by the latest standard, electrode manufacturer and responsible engineer.

Pipe Type Earthing

A pipe or rod electrode is installed vertically to make contact with suitable soil layers. Diameter, length, wall thickness, coating, coupling, depth and spacing must be selected from the current standard and site design. A historical 40 mm by 2.5 metre perforated pipe arrangement is not a universal specification. The connection to the earthing conductor must remain mechanically secure, electrically continuous and protected against corrosion.

Use native soil or an approved engineered backfill as the design specifies, compacted and installed in accordance with the product and project instructions. Do not improvise alternating salt and charcoal layers. Electrode performance must be demonstrated by measurement rather than inferred from pit dimensions or backfill appearance.

Provide a durable inspection and test point where required, while protecting the connection from mechanical damage, moisture ingress and unauthorised access. Record the as-built electrode location and test result. Retest at the intervals required by the authority and standard; the 2023 CEA regulations require specified supplier systems and systems above 650 V to be tested on a dry day in the dry season at least once each year.

Other Types of Earthing: The following are system-neutral earthing methods used to control earth-fault current and system overvoltage. They do not replace the protective earthing of exposed equipment parts. Selection requires a power-system study and protection coordination.

1. Resistance earthing.
2. Reactance earthing.
3. Petersen coil, or resonant, earthing.
4. Neutral creation and earthing through a grounding transformer.