990–91278–001

IT Systems

Characteristics

In IT systems the installation is insulated from earth or connected to earth through a sufficiently high impedance. Exposed conductive parts are earthed individually, in groups, or collectively.

Reference to IEC/EN 60364-4-41 413.1.5

In IT systems the installation must be insulated from earth or connected to earth through a sufficiently high impedance. This connection must be made either at the neutral point of the system or at an artificial neutral point. The latter may be connected directly to earth if the resulting zero-sequence impedance is sufficiently high. In installations where no neutral point exists, a phase conductor can be connected to earth through an impedance. In case of a single fault to an exposed conductive part or to earth, the fault current will be low and disconnection will not be imperative.

Exposed conductive parts must be earthed individually, in groups or collectively and the condition “RA x Id ≤ 50 V” must be fulfilled.

In the condition:

  • RA is the resistance of the earth electrode for exposed conductive parts

  • Id is the fault current of the first fault of negligible impedance between a phase conductor and an exposed conductive part. The Id value takes the leakage currents and the total earthing impedance of the electrical installation into account

In systems where an IT system is used for continuity of supply, an insulation monitoring device must be provided to indicate the occurrence of a first fault from a live part to the exposed conductive parts or to the earth. It is recommended to eliminate a first fault as soon as possible.

Depending on whether all exposed conductive parts are interconnected by a protective conductor (collectively earthed) or are earthed in groups or individually, after a first fault, the disconnection conditions of the supply for a second fault must be as follows:

  1. In installations where the exposed conductive parts are earthed in groups or individually, the protection conditions for TT systems apply (see 413.1.4)

  2. In installations where the exposed conductive parts are interconnected by a protective conductor collectively earthed, the conditions for TN systems apply

In installations where the neutral is not distributed, the following conditions must be fulfilled:

Zs √3 x U0
2 x Ia

In installations where the neutral is distributed, the following conditions must be fulfilled:

Zs U0
2 x Ia

In the conditions:

  • U0 is the nominal AC RMS voltage between phase and neutral

  • Zs is the impedance of the fault loop comprising the phase conductor and the protective conductor of the circuit

  • Z’s is the impedance of the fault loop comprising the neutral conductor and the protective conductor of the circuit

  • Ia is the operating current of the protective device

Protective devices in IT systems

The following protective devices are recognized in IT systems:

  • Earth fault monitoring devices

  • Overcurrent protective devices

  • Residual current protective devices

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