How to Design an Earthing System for Substations and Buildings

An earthing system (or grounding system) is a critical component in electrical installations for both substations and buildings. It provides a low-resistance path for fault currents to flow safely into the earth, protecting equipment, structures, and human life.

Proper earthing ensures:

• Safety – Preventing electric shock hazards.

• Equipment Protection – Limiting overvoltage from lightning or switching surges.

• System Stability – Ensuring fault currents are effectively cleared by protective devices.

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1. Understanding Earthing System Types

Before designing, you must choose the correct type of earthing system based on standards such as IEEE Std 80, IEC 60364, or IS 3043.

Types of Earthing:

1. TN System (TN-C, TN-S, TN-C-S) – Neutral directly earthed, with protective earth conductors.

   • TN-C System:  In a TN-C system, the neutral conductor and the protective earth conductor are combined into a single conductor known as the PEN (Protective Earth and Neutral) conductor. This PEN conductor links all exposed conductive parts of the installation to the source. However, as specified in the Electricity Safety, Quality, and Continuity Regulations 2002 (Section 8(4)), consumers are prohibited from combining the neutral and protective functions into a single conductor within their own installations.
   
   •  TN-S System: In the TN-S system, the neutral and protective earth conductors remain separate throughout the entire distribution network. The metallic sheath of the supply cable acts as the protective conductor, ensuring that all exposed conductive parts of the installation are connected either directly to this protective conductor or through the main earthing terminal.
   
   • TN-C-S System: The TN-C-S system combines neutral and protective earth conductors into a single PEN conductor at certain points. This multiple protective earthing system grounds the PEN conductor at two or more locations. Consumer installations may need an earth electrode. The PEN conductor connects all exposed conductive parts to the main earthing terminal, which links neutral and protective earth.
   

2. TT System – Neutral earthed at source, installation has its own earth electrode.

    
   In a TT system, both the supply source and the installation’s exposed conductive parts are directly connected to earth. For overhead distribution lines, the general mass of the earth serves as the return path for current. Within the installation, the neutral and protective earthing conductors must remain completely separate, since the power distributor supplies only the neutral conductor or, in some cases, a protective conductor. 

3. IT System – Isolated or impedance-earthed neutral.

   
   In an IT system, the neutral point is either completely isolated from earth or connected through a high impedance. Its defining characteristic is the ability to continue operating in the event of a single phase-to-earth fault, known as the “first fault.” This capability ensures continuity of supply even when such a fault occurs, allowing maintenance or corrective action without immediate shutdown. IT systems are commonly employed in power distribution from substations and generators where high service continuity is critical.

For substations, a mesh or grid earthing is generally used, while buildings often use rod or plate electrodes.

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2. Step-by-Step Earthing Design Procedure

Step 1: Define Design Standards and Requirements

• Standards: IEEE Std 80 (for substations), IEC 60364, IS 3043.

• Parameters: Soil resistivity, fault current, permissible touch and step voltages.

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Step 2: Soil Resistivity Measurement

The performance of an earthing system heavily depends on soil characteristics.

• Method: Wenner Four-Probe Test (as per IEEE Std 81).

  

• Formula:

$$\rho = 2 \pi a R$$

Where:

• $\rho$ = Soil resistivity (Ω·m)

• $a$ = Probe spacing (m)

• $R$ = Measured resistance (Ω)

Example:
If $a = 5 \, m$ and $R = 3 \, \Omega$,

$$\rho = 2 \pi (5) (3) = 94.25 \, \Omega\cdot m$$

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Step 3: Determine Maximum Fault Current and Duration

Obtain:

• Fault current magnitude (3-phase and single-line-to-ground fault).

• Clearing time from protection coordination studies.

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Step 4: Calculate Tolerable Touch and Step Voltages

IEEE Std 80 formulas:

$$E_{\text{touch}} = \frac{(1000 + 1.5 C_s \rho_s)}{1 + 0.116 \sqrt{t}}$$ $$E_{\text{step}} = \frac{(1000 + 6 C_s \rho_s)}{1 + 0.116 \sqrt{t}}$$

Where:

• $C_s$ = Surface layer derating factor

• $\rho_s$ = Soil resistivity (Ω·m)

• $t$ = Fault clearing time (s)

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Step 5: Design Earthing Grid (for Substations)

• Grid conductors are laid horizontally at 0.5–1 m depth.

• Mesh size: 3 m × 3 m or 5 m × 5 m.

• Include vertical rods for deep earth contact.

Grid Resistance Approximation:

$$R_g = \frac{\rho}{4L} \left( \ln\frac{8L}{\sqrt{A}} - 1 \right)$$

Where:

• $L$ = Total conductor length (m)

• $A$ = Grid area (m²)

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Step 6: Electrode Design for Buildings

Common electrodes:

• Copper-bonded rods (16–20 mm dia, 2–3 m length)

• GI pipes/plates (as per IS 3043)

• Ring earthing around the building for lightning protection integration.

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Step 7: Bonding and Interconnection

• All metallic structures, cable trays, transformer neutrals, and building steel must be bonded to the earthing system.

• Lightning protection earthing should be interconnected to avoid potential differences.

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Step 8: Verification and Testing

• Fall-of-Potential Method for resistance measurement.

• Target:

  • ≤ 1 Ω for substations.

  • ≤ 5 Ω for buildings.

• Annual maintenance and testing required.

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Example Design Calculation

Given:

• Soil resistivity $\rho = 100 \, \Omega\cdot m$

• Fault current $I_f = 10 \, kA$

• Clearing time $t = 0.5 \, s$

Step:
Design a grid for a 20 m × 20 m substation with mesh spacing 5 m.

Solution:

• Grid length $L =$ total perimeter + internal meshes = 4×20 + (4×(20/5 - 1) × 20) = calculated accordingly.

• Insert values in $R_g$ formula to get resistance (target < 1 Ω).

• Adjust rod depth and spacing if resistance is too high.

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Best Practices

• Use corrosion-resistant conductors (copper, copper-bonded steel).

• Minimize joint resistance by exothermic welding or compression clamps.

• Lay conductors below frost line for stability.

• Avoid sharp bends in conductors (min radius ≥ 8× conductor diameter).

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FAQs

Q1: Why is soil resistivity so important in earthing design?
Because it determines the conductor length and depth required to achieve low resistance. High resistivity soils require deeper or more electrodes.

Q2: Can the building earthing and lightning protection system share the same electrode?
Yes, but they must be bonded properly to avoid dangerous potential differences.

Q3: What happens if the earthing system resistance is too high?
Fault currents may not clear quickly, protective devices may fail, and dangerous voltages can appear on exposed parts.

Q4: Which is better for substations — rod earthing or grid earthing?
Grid earthing is preferred for substations because it provides uniform potential distribution and lower touch/step voltages.

Q5: How often should the earthing system be tested?
At least once a year, and after any major modifications or lightning strikes.

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