IT earthing system: a distribution system in which all live parts of a power source are isolated from the earth or one live part of the power source is earthed through an impedance, exposed-conductive-parts of an electrical installation are connected to earth electrodes of the electrical installation by protective earthing conductors (PE) [defined in the IEC 60364-1].
BS7671 provides the following definition: a system having no direct connection between live parts and Earth, the exposed-conductive-parts of the electrical installation being earthed.
The Meaning of the Letters I, T
The letter codes used for designations of the types of system earthing have the following meanings.
The first letter determines presence or absence of earthing of live parts of the power source:
- I – all live parts of the power source are isolated from the earth or one live part is earthed through a high impedance.
Conductors of the distribution network (if any), as a rule, should be isolated from the earth.
The second letter specifies the earthing of exposed-conductive-parts of the electrical installation or electrical connection presence between the exposed-conductive-parts and the earthed live part of the power source:
- Т – the exposed-conductive-parts are earthed irrespective of presence or absence of the earthing of any live part of the power source.
IT Earthing System Explained
In the IT type of system earthing (see Fig. 1), all live parts of the power supply are insulated from Earth, or some live part is earthed through a high impedance. The exposed-conductive-parts of the electrical installation of a building are earthed.
With the IT type of system earthing, the protective conductors of the electrical installation of a building do not have the same electrical connection to the earthed neutral of the power supply as in the TN-S, TN-C-S systems.
The IT system is generally used in the electrical installation of buildings for special applications. For example, a part of the electrical installation of a medical centre containing electrical medical life-support equipment should be performed with the IT type of system earthing. The IT system should be operated by instructed and skilled persons.
The IT system, in contrast to the TN-S, TN-C-S, TN-C and TT systems, has extremely low earth-fault currents. IEC 60364-4-41 therefore requires that the first earth fault must be detected by means of insulation control devices in the electrical installations of buildings which conform to the IT type of system earthing.
If a second earth fault has occurred in the electrical installation of a building before the first earth fault has been eliminated, it must be disconnected by means of overcurrent protection devices or residual current devices. However, RCDs may trip during the first earth fault due to capacitive leakage currents.
At the same time, switching from TN systems to IT systems is not justified. For example, even if a disadvantage of TN systems in normal conditions exists, such as the potential on earthed exposed-conductive-parts of class 1 electrical equipment, this can be eliminated without switching to the IT system.
Exposed-conductive-parts of class I electrical equipment in the electrical installations of buildings which correspond to the TT type of system earthing are earthed by attaching them to the earthing arrangements of the electrical installations of buildings. These exposed-conductive-parts are therefore under the electric potential of the earth in normal conditions.
If, therefore, it is not possible to ensure an adequate level of electrical safety in the electrical installation of buildings which conforms to the TN types of system earthing, the TT system should be used. Moreover, the changeover to the TT system does not require any modifications to existing low-voltage distribution networks.
Examples of IT Earthing System Diagrams
Diagrams of the IT earthing systems are shown below.
Diagram 1 shows:
- 1- earthing at the power source
- 2 – earthing arrangement of the electrical installation of a building
- 3 – exposed-conductive-parts
- 4 – protective contact of the plug socket
- 5 – impedance through which the live part of the power supply is earthed
- TS – transformer substation
- UC – underground cable
- OL – overhead line
Note 1: The system may be connected to earth via a sufficiently high impedance. This connection may be made, for example, at the neutral, artificial neutral, or a phase conductor.
Note 2: Additional earthing of the protective conductor in the electrical installation may be provided.
Note 3: The neutral conductor may or may not be distributed.
NOTE: All earth electrodes associated with the electrical installation shall be interconnected, see IEC 60364-4-44, 444.5.1.
What Are the Advantages of IT Earthing?
The main advantage of an IT earthing system is continuity of service. It also offers advantages related to low fault currents.
IT systems have the following advantages over other earthing systems:
- relative safety when touching live parts of electrical installations that are live
- earth-fault current is low in cases of single-phase faults to the earthed enclosure
- possibility of continuing to operate the equipment until the complete elimination of the fault, as such a short circuit is not considered an emergency
Applications of IT Earthing Systems
IT earthing systems, due to their construction and functional features, are used in electrical installations and buildings with high requirements for fire and electrical safety. Also, such schemes are used at facilities where uninterrupted power supply is required.
There are many applications which vary from country to country.
- Operating theatres and certain medical quarters in hospitals
- Cold storage
- Nuclear power stations
- Airports: control tower, runway lighting
- Official buildings: parliaments, stock exchanges, government departments, etc.
- Scientific laboratories (such institutions use electrical equipment, which is characterized by high sensitivity to voltage fluctuations)
- Oil rigs
- Industrial applications with continuous or sensitive processes.
Apart from mandatory situations (hospitals, the navy), the IT earthing system is typically used in instances where the unavailability of power would result in lost production or incur a significant non-production cost.
Other applications concern minimising the risk of fire and explosion. Lastly, the IT earthing system is chosen in certain cases because it can help facilitate preventive and corrective maintenance operations.
- IEC 60364-1
- IEC 60364-4-41