The types of fault that can possibly occur in a three-phase, four-wire distribution system with neutral grounded are as follows, in the order of the frequency of occurrence:

L-G: One line shorting to the ground (not a fault in an ungrounded system) L-L: Two lines shorting together but not to the ground L-L-G: Two lines shorting together and to the ground L-L-L: All three lines shorting together but not to ground (three-phase sym-

metrical fault) L-L-L-G: All three lines shorting together and to ground (three-phase sym-

metrical fault to ground)

The L-L-L and L-L-L-G faults are called symmetrical faults because they involve all three lines symmetrically, resulting in fault currents that are balanced symmetrical three-phase currents. The other three types are called unsymmetrical faults, where the fault currents are not symmetrical in all three lines. The frequency of occurrence of electrical faults in practical power systems is shown in Figure 8.1. About 70% of faults in three-phase systems start as single-line-to-ground (L-G) fault. However, the subsequent heat generation breaks down the insulation between other lines in the cable as well, soon leading to three-phase symmetrical L-L-L-G fault. Similarly, faults that start as L-L (15%) and L-L-G (4%) also soon lead to symmetrical fault. For this reason, the fault current analysis presented below is focused on the three-phase symmetrical fault.