In an IT system the supply has no direct connection to earth, or one through a deliberately high impedance, while exposed metalwork is earthed locally. A first earth fault produces only a small capacitive current, so the system can keep running, which is why IT is used in operating theatres, ships, mines and continuous process plant. An insulation monitoring device must alarm on that first fault, because a second fault on another phase becomes a phase-to-phase short through the earth.
Theatre 3 is on an IT system and the insulation monitor is showing 85 kilohms and falling, find the leak before we get a second fault.
Also heard as
- isolated system
- unearthed system
- ungrounded system
Do not mix it up with
- TT system: In a TT system the source is solidly earthed; in an IT system it is not.
Related terms
Earthing system
Arrangement describing how a supply's neutral and an installation's exposed metalwork are connected to earth.
Earth fault
Fault in which a live conductor makes unintended contact with earth or with earthed metalwork.
DC ground
Unwanted earth fault on a substation battery or DC control supply, which normally floats free of earth.
Insulation resistance test
DC test that applies a high voltage between conductors and earth and reads leakage resistance in megohms.