n GENERAL PROVISIONS:
The IT earthing system does not require the power supply to be automatically disconnected on occurrence of an insulation fault, referred to as the "first fault".
Indeed, we have seen that for this system the touch voltages which develop on occurrence of a first insulation fault are not dangerous for persons.
Thus, operation can continue in spite of the "first fault" thus increasing service continuity. This system requires the following means to be implemented:
- Installation of a permanent insulation monitor ( PIM ) which must signal the "first fault" (sound or visual signal or both).
- Search for the first insulation fault by an efficient maintenance service so that all the advantages of this earthing system are exploited. Automatic fault location equipment may be used to facilitate the process.
- Tripping on occurrence of any new fault, referred to as the "second fault".
- Checking of phase-to-phase fault protective device tripping conditions with the setting up of specific means if these conditions cannot be guaranteed.
- Installation at the transformer secondary terminals of an over voltage limiting device connected between the neutral and earth or between one phase and earth.