1 Phase Failure (loss) - The relay monitors for voltage with the incorrect phase sequence, or one or more phases open. 2  Failure may occur because of a blown fuse, a mechanical failure of the switching equipment, or if one of the power lines opens. 3  Phase failure involves three phases where there are three wires.rotective relays and monitoring relays detect or monitor for abnormal power system conditions. Protective relays detect defective lines, defective apparatuses, or other power system conditions of an abnormal or dangerous nature.