IT system - Principle

In this type of system:

• The installation is isolated from earth, or the neutral point of its power-supply source is connected to earth through a high impedance (commonly 1500 Ω or more)
• All exposed and extraneous-conductive-parts are earthed via an installation earth electrode.

The basic feature of IT earthing system is that, in the event of a fault between phases and earth, the system can continue to operate without interruption. Such a fault is referred to as a “first fault”.

The first fault current value depends on the neutral impedance (if any) and on downstream network capacitances (cables, filtering, leakage…). The first fault current Id should be low enough to meet the rule Id. R_A ≤ 50 V, so that no dangerous fault voltages can occur.

The system may be allowed to operate normally until it is convenient to isolate the faulty section for repair work. This enhances continuity of service.

In practice, IT system requires certain specific measures for its satisfactory exploitation:

• Permanent monitoring of the insulation with respect to earth, which must signal (audibly or visually) the occurrence of the first fault
• A device for limiting the voltage which the neutral point of the supply transformer can reach with respect to earth
• A “first-fault” location routine by an efficient maintenance staff. Fault location is greatly facilitated by automatic devices which are currently available.
• Automatic high-speed tripping of appropriate circuit breakers must take place in the event of a “second fault” occurring before the first fault is repaired. The second fault (by definition) is an earth fault affecting a different live conductor than that of the first fault (can be a phase or neutral conductor on systems where the neutral is distributed, see Figure F39 ).