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Understanding LV Earthing Systems: TT, TN, and IT Explained

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Understanding LV Earthing Systems: TT, TN, and IT Explained

Earthing systems are crucial for electrical safety, protecting people and equipment from faults and shocks. In low-voltage (LV) installations, the IEC 60364 standard defines three main earthing systems: TT, TN, and IT, each with distinct characteristics and applications.

This blog post explains these systems, their subtypes, and key safety considerations, referencing the attached technical screenshots for clarity.

1. Classification of LV Earthing Systems

Earthing systems are identified by a two-letter code:

First Letter: Neutral Connection

  • T (Terra) – Neutral is directly earthed at the transformer.

  • I (Isolated) – Neutral is not earthed or connected via high impedance (≥1000 Ω).

Second Letter: Equipment Earthing

  • T – Frames are locally earthed, independent of the neutral.

  • N – Frames are connected to the neutral, which is earthed at the transformer.

2. Types of Earthing Systems


A. TT System

Definition:

  • Neutral is earthed at the transformer (T).

  • Equipment frames are earthed locally (T).

Key Features:

  • Fault currents are small (~20 A), so standard circuit breakers (e.g., 80 A) won’t trip.

  • Residual Current Devices (RCDs) are mandatory to detect leakage






B. TN System

Frames are connected to the earthed neutral (N). Three subtypes exist:

1. TN-C (Combined Neutral & Earth)

  • Single PEN conductor carries both neutral and protective earth.

  • RCDs cannot be used (PEN bypasses detection).

  • Only 3-pole breakers allowed (neutral must not be switched).

  • Risk: Stray currents can cause voltage on exposed metal parts.


2. TN-S (Separate Neutral & Earth)

  • PE (Protective Earth) and N (Neutral) are separate.

  • RCDs can be used.

  • 4-pole breakers allow neutral switching.

  • Safer than TN-C, used in modern installations.


3. TN-C-S (Hybrid System)

  • Upstream: TN-C (PEN conductor).

  • Downstream: Splits into separate PE and N (TN-S).

  • Critical Rule: TN-C must never be used downstream of TN-S (safety hazard).

Applications: Common in urban power distribution.








C. IT System (Isolated Neutral)

Definition:

  • Neutral is isolated or impedance-earthed (I).

  • Frames are locally earthed (T).

Key Features:

  • First fault:

    • No tripping (current is negligible).

    • Touch voltage remains safe (~0 V).

    • Insulation Monitoring Devices (IMDs) detect faults.

  • Second fault: Acts like a TN fault, requiring fast tripping.

Applications: Hospitals, industries where power continuity is critical.




Conclusion

  • TT: Relies on RCDs; used where local earthing is feasible.

  • TN-S: Safest for modern installations; allows RCDs.

  • TN-C: Cost-effective but limited safety; avoid in new installations.

  • IT: Best for critical systems (hospitals, data centers); requires active monitoring.

For electrical designers and engineers, selecting the right earthing system ensures compliance with safety standards (IEC 60364, NEC) and minimizes risks.

Need further details on a specific system? Let us know in the comments!

References: Attached technical screenshots from IEC standards and practical examples.

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