IT = a supply with the neutral earthed via an impedance at the source, or with the neutral isolated from earth.
The 'T' is from the Latin word 'Terra' meaning 'earth'. 'I' represents either impedance or isolated. I don't like the duality of the term 'IT' but I didn't make the rules. I don't recall seeing it used much in relation to MV and HV systems, and low voltage IT systems are pretty unusual in the UK.
IT systems are used where there are no line-neutral loads because they provide a means of keeping the prospective earth fault level within reasonable bounds in the case of an impedance-earthed source or reducing it to virtually zero in the case of an isolated system, thus limiting the scale of damage in the event of an earth fault. Impedance-earthed systems are common in industrial plants with a lot of rotating plant because a stator earth fault with a solidly grounded system will usually mean a write-off or major rebuild whereas a similar fault with an IT system usually results in a salvageable motor.
I'll let the utility guys talk about ungrounded systems and reasons why they are used or avoided.
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If we learn from our mistakes I'm getting a great education!
All of our HV is solidly grounded, wouldn't dream of anything else. Except for an extremely limited number of very specialized applications, all of our MV is also solidly grounded. There is no neutral run with any of the HV. Neutrals are run with all of the MV and are multi-point grounded.
So, in answer to the original question, generally they aren't used except for industrial MV systems.
IT Systems are illegal in the UK Electricity Distribution network under the ESQC Regs. With one exception -on power plants, so long as precautions are taken to prevent danger.
The low voltage 415/240V system is solidly earthed. The 11kV system is either solid or low impedance earthed via a resistor,reactor or petersen coil, the 33kV system is low impedance earthed, and the 132kV (and above)system is solidly earthed.
I'm not sure therefore why oconn thinks IT earthing is the chosen method.
Regards
Marmite
In North America, a significant proponent of impedance grounding for medium-voltage systems in the petrochem industry is John P Nelson/Nelson Electrical Engineering. See
Resistance earthing of MV systems with motor feeders is recommended by IEEE141.
This is to minimise stator core damage possibility during motor earth faults.
Shrotly: HV, above 110kV is solidly grounded, MV ( according to SEL document), but in industries MV is (99% of cases) low resistance grounding.
term IT, I see first time for the HV, MV application.
As Marmite wrote, 400/230V is usually also solidly grounded , except a (for example) medical application.
BTW, if I remember right, 132kV with low-impedance grounding (zig-zag trafo) used in Brazil.
Just for your information, in our grid (Chesf, Northeast of Brazil), all 500kV, 230kV, 138kV systems are solidly grounded. But our 69kV system is supplyed from a delta winding conneted tranformer and there is a zig-zag reactor in each 69kV busbar for grounding reference.
Remember that the "guard cable" or "earth wire" which is installed at the top of the trasmission line towers for lightning protection, connects the grounding mesh of system substations and seems to work as a (almost) "neutral wire" for HV grounded systems.
Just for information - in Russia MV networks are 6kV, 10kV and 35kV (50Hz) networks, and almost all of them are isolated from earth.
Networks above 35kV (that is 110kV, 220kV, 330kV etc) are so-called "effectively-grounded" - i.e. they are grounded via small impedance, almost solidly.
Networks below 1kV (that is 690/400V and 400V/230V) are normally solidly grounded.
