Grounding Systems: TN, TT, and IT Schemes and Applications

Grounding Systems: TN, TT, and IT Schemes and Applications

Grounding is one of the most crucial technical measures for ensuring electrical safety in power installations.

The purpose of grounding is to connect non-live metal parts of electrical equipment to grounding devices, preventing electric shock hazards for humans and animals.

In three-phase AC networks up to 1 kV, different grounding systems are used based on the neutral configuration and the handling of protective (PE) and neutral (N) conductors.

• TN Systems – Feature a solidly grounded transformer neutral.
• TT System – Uses a locally installed ground electrode for protective grounding.
• IT System – Has an isolated or resistance-grounded neutral to limit fault currents.

TN Grounding System: Types and Configurations

1. TN-C System

The TN-C (Terra Neutral Combined) system has a solidly grounded transformer neutral at the substation.

• Power is delivered via a four-wire system (three-phase conductors + a combined PEN conductor).
• PEN serves as both the neutral (N) and protective earth (PE) conductor.
• No additional grounding electrodes are required on the consumer’s side.

Identifying a TN-C System in a Home

• Three-phase connections use a four-wire setup.
• Single-phase connections use only two wires.
• Older homes have two-prong outlets with no grounding contacts.

Disadvantages of TN-C

• High risk of electric shock – If the PEN conductor is damaged, appliance casings may become live.
• GFCIs (Ground Fault Circuit Interrupters) do not work properly in this system.

Upgrading from TN-C

• Transition to TN-S requires installing a dedicated PE conductor from the substation.
• A simpler alternative is TN-C-S, which involves splitting PEN into PE and N before the electrical panel.
• Installing a GFCI or a circuit breaker with built-in ground fault protection can enhance safety.

2. TN-C-S System

The TN-C-S (Terra Neutral Combined-Separated) system is a hybrid solution.

• Initially, power is distributed via a TN-C (four-wire) system.
• Just before entering the building, the PEN conductor is split into separate PE (ground) and N (neutral) wires.
• The internal electrical distribution uses five wires (three-phase + N + PE).

Advantages of TN-C-S

• Provides basic ground fault protection.
• Allows the use of GFCIs (RCDs), which can trip when leakage currents are detected.

Disadvantages of TN-C-S

• PEN conductor is vulnerable to damage before the split, which can cause dangerous voltage surges.
• Additional grounding electrodes are required at regular intervals (100–200 meters) to maintain safety.

3. TN-S System

The TN-S (Terra Neutral Separated) system provides the highest level of safety.

• The PE and N conductors are completely separate from the substation to the consumer.
• The system uses a dedicated PE conductor throughout the power line, ensuring reliable grounding.

Benefits of TN-S

✅ Reliable grounding reduces electrical shock risks.
✅ No risk of losing ground protection due to PEN conductor failure.
✅ Better protection against voltage surges and faults.

Challenges of TN-S

• Expensive to implement due to the need for additional wiring.
• Difficult to retrofit in existing TN-C networks.

TT and IT Grounding Systems

1. TT Grounding System

The TT system (Terra-Terra) is used when a dedicated protective ground is required at the consumer’s location.

• The PEN conductor is grounded at the substation, but consumers must install their own ground electrode.
• The local PE conductor is isolated from the supply neutral (N).

When Is TT Grounding Used?

• In metallic buildings where the structure itself poses grounding risks.
• In areas where TN systems are impractical due to soil conditions.
• In older buildings that lack dedicated grounding conductors.

Safety Considerations in TT Systems

• GFCIs (RCDs) are mandatory with a trip current of ≤30mA.

• The grounding resistance (Rz) must be low enough to ensure proper fault protection:

  Rz × I_GFCI ≤ 50V

  Where:

  • Rz = total ground resistance
  • I_GFCI = residual current trip threshold

This formula ensures that touch voltage never exceeds 50V, reducing electrocution risks.

2. IT Grounding System

The IT (Isolated-Terra) system differs significantly from TN and TT systems.

• The transformer neutral is either completely isolated or connected through a high resistance.
• No protective earth (PE) conductor is supplied from the substation.
• Consumers must use a local grounding system for safety.

Where Is IT Grounding Used?

• Industrial plants, chemical plants, and oil refineries where equipment safety is critical.
• Hospitals – IT reduces the risk of ground faults affecting medical equipment.
• Explosive environments – IT grounding limits short-circuit currents, preventing sparks.

Advantages of IT Grounding

✅ Minimal fault currents prevent electrical fires.
✅ Systems can continue operating after the first ground fault.
✅ Ideal for high-risk environments.

Challenges of IT Grounding

• Complex and expensive to install.
• Requires regular insulation resistance monitoring to detect ground faults.
• Not suitable for standard residential buildings.

Comparison Table: TN, TT, and IT Systems

Conclusion: Choosing the Right Grounding System

• TN-S is the safest system but is costly to implement.
• TN-C-S is a good compromise for improving safety in older TN-C installations.
• TT is useful when a separate ground electrode is necessary.
• IT is ideal for critical infrastructure and industrial settings.

For residential and commercial buildings, TN-C-S or TN-S are the best options for ensuring safety and electrical reliability.

If upgrading from TN-C, consult a licensed electrician to transition to a safer grounding system!