What is Transformer Earthing Types?

Transformer earthing is a critical aspect of power system design. Proper earthing ensures human safety, equipment protection, and system stability by providing a safe path for fault currents and overvoltage surges. Without effective earthing, transformers and connected systems are vulnerable to insulation failure, electric shock, fire hazards, and severe equipment damage. This article explains what transformer earthing is, the main types of earthing used in transformers, and their purposes and applications.

Why Transformer Earthing Is Important

Transformer earthing serves several essential functions:

• Protects personnel from electric shock
• Limits damage during earthquakes
• Ensures fast and reliable fault detection
• Controls overvoltage due to lightning and switching surges
• Improves overall system reliability

Types of Transformer Earthing

Transformer earthing can be broadly classified into Neutral Earthing, Body (Tank) Earthing, Lightning Arrester Earthing, and Shield Earthing.

1. Neutral Earthing

Neutral earthing involves connecting the transformer’s neutral point to earth. This helps control earth-fault currents and stabilize system voltage.

a) Solid Earthing

• Neutral is directly connected to the earth
• High earth fault current flows
• Faults are detected and cleared quickly

Used in: Low Voltage (LV) systems
Advantage: Simple, reliable, and low cost

b) Resistance Earthing

• Neutral connected to earth through a resistor
• Limits the earth fault current
• Reduces thermal and mechanical stress on equipment

Used in: Medium Voltage (MV) and High Voltage (HV) systems
Advantage: Controlled fault current and reduced damage

c) Reactance Earthing

• Neutral connected to earth through a reactor
• Limits short-circuit current
• Lower power loss compared to resistance earthing

Used in: High voltage transmission networks. Advantage: Efficient fault current limitation

2. Body (Tank) Earthing

• The metal body or tank of the transformer is directly earthed
• Prevents dangerous touch voltage if insulation fails
• Ensures fault current flows safely to ground

Rule: Body earthing is mandatory for all transformers
Purpose: Human safety
Used in: All voltage levels, especially HV installations

3. Lightning Arrester Earthing

• Dedicated earthing provided for lightning arresters
• Safely discharges lightning and surge currents into the ground
• Prevents overvoltage from reaching transformer windings

Lightning arrester earthing must be separate from neutral and body earthing to avoid potential rise and interference.

4. Shield Earthing

• Used for cable shields, control cables, and signal cables
• Reduces electrical noise and electromagnetic interference (EMI)

Purpose: Signal protection, Cable safety, and Noise and interference control

Main Earthing Connections in Transformers (N–B–L Concept)

Transformers typically use three main earthing systems:

• N – Neutral Earthing
• B – Body (Tank) Earthing
• L – Lightning Arrester Earthing

Each earthing system has a distinct role and must be properly designed and maintained for safe and reliable operation.

Conclusion

Transformer earthing is not just a regulatory requirement—it is a fundamental safety and protection measure. Correct selection and separation of earthing systems ensure effective fault clearing, protection against lightning, reduced equipment damage, and, most importantly, human safety.