Designed to remove power from equipment loads when these loads have a potentially lethal ground current in excess of six milliamperes.  Normal loads will draw current from the line conductor (black wire) and return it to the power source through the neutral conductor (white wire).  Faulty loads can return some of the current to the power source through a ground path such as a water pipe, gas line, wet floor, third conductor (green wire), or worse, through a person who is in contact with an extrinsic ground.  Protection is also provided against voltage transients or surges.