3. Nature of ground faults, neutral systems and protection

The form that a ground fault takes depends on a number of factors. For example, the behavior of air or synthetic insulation is different (air regenerates fairly quickly, synthetic insulation hardly ever); another example is that a contact may be rigid or temporary (a wire may fall to the ground and a wire may touch vegetation in a rocking movement); finally, a contact may be frank or resistant (a conductor touches an earthed metallic mass and a conductor is in contact with vegetation). Nevertheless, it is possible to define three main categories of earth fault applicable to these different cases:

• stable faults: these are faults at mains frequency whose resistance does not exceed one or two hundred ohms;

• Resistive faults: these are faults which are stable at the mains frequency and whose resistance is greater...