Short circuits occurs mostly to power system because of different factors like , equipment failure , lightning strikes , falling of branches or trees on the transmission lines , switching surges , insulation failures and other electrical or mechanical causes . All of these are collectively referred to as faults in power systems . Symmetrical fault refers to those problems through which all of the three phases of a power system are grounded at the same point . For this reason the symmetrical faults sometimes are also called three-line-to-ground ( 3LG ) faults .