Comparison of power supply connection schemes

Electrical systems are classified according to the following basic criteria:

• Type of current: alternating current (AC), direct current (DC) or three-phase current with neutral (3(N)AC).
• Type and number of active conductors in the system: e.g. L1, L2, L3, N or L+, L- in DC systems.
• System grounding scheme: IT, TT or TN.

The selection of the type of grounding is a crucial aspect, as it defines to a large extent the operating behavior and technical characteristics of the power system. In addition, it has a direct impact on the following factors:

• Reliability of power supply and availability.
• Installation costs.
• Maintenance and downtime.
• Electromagnetic compatibility with other equipment and installations.

The right choice of connection scheme optimizes power system performance, improves operational safety and ensures regulatory compliance in industrial, commercial and critical infrastructure applications.

In TT systems, one of the system points (usually the transformer neutral) is directly connected to ground by means of a functional ground. In turn, the masses or exposed conductive parts of the electrical installation are connected to one or more independent grounding electrodes, which are electrically isolated from the electrode used for system grounding.

This type of scheme is common in installations where a direct equipotential bonding with the transformer grounding point cannot be guaranteed, and its behavior depends largely on the resistance of the electrodes used.

Suitable protective devices:

• Overcurrent protection devices.
• Residual Current Differential or Residual Current Devices (RCD).

In TN systems, one point of the system, usually the neutral, is directly connected to ground. Exposed metallic parts of the electrical installation are connected to this grounding point by protective earth (PE) conductors.

This scheme offers a low return impedance for ground fault currents, allowing a quick disconnection of the power supply in case of a fault.

Three variants of the TN system are distinguished, according to the configuration of the neutral (N) and protective (PE) conductors:

• TN-S: The neutral (N) and protective (PE) conductors are completely independent throughout the system.
• TN-C: The neutral and protective conductors are combined into a single conductor (PEN) throughout the system.
• TN-C-S: A combined conductor (PEN) is used in a part of the system, which is then separated into separate neutral (N) and protection (PE) conductors.

The IT system is characterized by keeping all active conductors completely isolated from ground or, failing that, connected to ground through an impedance. This particular configuration allows that, in the event of an insulation fault, the leakage current generated is minimal, mainly originated by the capacitances distributed in the system.

Since the fault current is very low, conventional protection devices (fuses or circuit breakers) are not activated in the event of a first fault. In addition, the system continues to operate without interruption even if a single-pole ground fault occurs, resulting in high availability of the power supply.

Grounding scheme of the earths

The exposed conductive parts (grounds) can be grounded in different ways, depending on the design of the installation:

• Through individual grounding connections.
• Through a single shared grounding point.
• Connected to the same grounding system as the one used by the reference point of the IT system.

Protective equipment used

To ensure operational safety, the following devices are permitted and recommended in IT systems:

• IMD (Insulation Monitoring Device): Detects insulation degradation and triggers an alarm.
• Overcurrent protection devices: Limit the current in case of short circuits or overloads.
• Residual current devices (RCD/RCCB): Complement protection, especially against multiple faults.

Fault behavior

• The first insulation fault does not cause automatic disconnection. However, it must be located and corrected promptly.
• If a second fault occurs on another active conductor, the system becomes equivalent to a phase-to-ground short circuit, which can lead to serious service interruption and safety hazards.
• Constant insulation monitoring is critical to maintaining system integrity.

Signs and symbols

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