Implementation of the IT system
The basic feature of the IT earthing system is that, in the event of a short-circuit to earth fault, the system can continue to operate without interruption. Such a fault is referred to as a “first fault”.
In this system, all exposed conductive parts of an installation are connected via PE conductors to an earth electrode at the installation, while the neutral point of the supply transformer is:
- Either isolated from earth
- Or connected to earth through a high resistance (commonly 1,000 ohms or more)
This means that the current through an earth fault will be measured in milli-amps, which will not cause serious damage at the fault position, or give rise to dangerous touch voltages, or present a fire hazard. The system may therefore be allowed to operate normally until it is convenient to isolate the faulty section for repair work. This enhances continuity of service.
In practice, the system earthing 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)(1).
The second fault results in a short-circuit through the earth and/or through PE bonding conductors.
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