What is an IT system? How does it differ from TN and TT?

What does the earthing arrangement change?

In a TN system, one source point is directly earthed and the exposed conductive parts of the installation are connected to that point by protective conductors. In a TT system, one source point is directly earthed, while exposed conductive parts use earth electrodes electrically independent of the source earth electrode. In an IT system, live conductors are isolated from earth or one system point is connected to earth through a sufficiently high impedance. “Unearthed IT” does not remove protective earthing: exposed conductive parts are earthed individually, in groups or collectively through protective conductors.

Why does the first insulation fault matter?

The first fault current in an IT system depends on the system's insulation resistance and leakage capacitance to earth. Under suitable design conditions, the first fault can be indicated without immediately disconnecting the supply. A Bender ISOMETER insulation monitoring device tracks the condition of the electrically connected system and alerts the operating team when resistance falls below the configured alarm level. The first fault should be investigated and cleared before a second insulation fault occurs on a different live conductor. The conditions for automatic disconnection after the second fault must be met by the protective and exposed-conductive-part earthing arrangements.

What do IMD and EDS provide together?

The IMD reports the overall insulation condition; it does not always identify the faulty outgoing circuit by itself. A system with a compatible locating current injector, EDS evaluator and current transformers can narrow the search to the relevant circuit. Fixed or portable fault location supports maintenance planning. In healthcare, industry, marine and energy applications, system configuration is assessed alongside loads, system size and operating requirements. Continued operation after the first fault is not an unconditional guarantee of safety or uninterrupted supply for every IT installation.

Select the product function for the actual earthing arrangement

The possibility of earthing one point through a high impedance in the IT definition does not make any IMD suitable for every HRG or LRG system. In HRG systems, NGRM monitoring of the neutral grounding resistor and ground faults differs from locating the faulty outgoing circuit with suitable RCMS solutions. Verify the monitoring method and product compatibility against the actual earthing arrangement and manufacturer documentation.

• In an IT system, a suitable IMD monitors insulation resistance; EDS helps identify the faulty circuit using a compatible locating current source and sensors.
• In a suitable TN-S/TT arrangement, RCM/RCMS monitors residual current. An RCD or suitable MRCD arrangement provides protection; the residual current value is not a direct insulation resistance measurement.
• For resistance-earthed systems, NGRM selection follows the HRG/LRG application scope. A device’s supply voltage or ability to detect AC/DC earth fault components does not establish suitability for every type of system.