Earthing System (TNC, TNS, IT): The Key Role of the Isolation Transformer

Galvanic Isolation: Creating a "New Source"

The main advantage of an ABL Transfo isolation transformer lies in the galvanic isolation between the primary and secondary windings. Physically, there is no electrical connection between the input and the output. Energy is transferred solely by magnetic field.

This means that the transformer’s secondary is considered a new, autonomous power source.

The "Blank Slate" of Potential

At the output of an isolation transformer (if no terminal is earthed), the potential is floating. By default, you are in an IT system. It is up to the installer to decide which SLT they want to create on the secondary, regardless of the system present on the primary.

Use Case: Creating an IT System for Continuity of Service

This is the prime application for industries where production downtime is costly (or dangerous).

The Principle

To establish a local IT system (islanding), an isolation transformer is used whose secondary neutral point is not connected to earth (or is connected via a high impedance).

The Role of the Transformer

It contains the fault. If a phase touches the frame on the machine powered by the secondary:

• The fault current is negligible (a few mA), as there is no return path to earth on the secondary.
• No protection device trips. The machine continues to run.
• Galvanic isolation prevents the fault from propagating upstream and disturbing the primary network.

Essential Equipment: The IMD

Warning, "not tripping" does not mean "ignoring". The standard requires the installation of an Insulation Monitoring Device (IMD) on the transformer’s secondary. It signals the first fault so that a maintenance team can intervene before a second fault occurs (which would cause a trip).

Use Case: Switching from TN-C to TN-S (Combating Disturbances)

In many older industrial installations, the TN-C system (combined Earth and Neutral in the PEN conductor) is ubiquitous.

Problem: Unbalanced currents and harmonics flow through the building’s metal structures via the PEN conductor. This severely disrupts sensitive electronics, PLCs, and IT equipment.

The Transformer Solution

Inserting an isolation transformer (preferably with a Dyn11 winding connection) allows you to:

• Stop the propagation of the PEN conductor: The PEN stops at the primary.
• Recreate a clean neutral: On the secondary, the neutral point is connected to earth locally. This creates a "clean" TN-S (Separate Earth and Neutral) system.

The transformer here acts as a firewall against common-mode disturbances and stray currents from the upstream network.

Technical Selection Criteria for Your Isolation Transformer

To ensure the safety of your SLT, the transformer sizing must be rigorous. Here are the key points our ABL engineers monitor:

Why Choose ABL Transfo for Your SLT Architectures?

A transformer intended to manage an earthing system is not a standard catalogue product. It is a safety component.

At ABL Transfo, we manufacture dry-type solutions (single-phase or three-phase) specifically adapted to these constraints:

• Custom-made: Option to integrate the electrostatic screen as standard.
• Compliance: Strict adherence to IEC 61558-2-4 standards (Isolating transformers).
• Adaptability: We can bring out the neutral point to a specific terminal block or provide ±5% adjustment tappings to compensate for voltage drops and ensure the stability of your new earthing system.