Protective relays and monitoring relays detect or monitor for abnormal power system conditions. Protective relays detect defective lines, defective apparatus, or other power system conditions of an abnormal or dangerous nature, and initiate appropriate control circuit actions. Monitoring relays are used to verify conditions in the power system or in the protective system. Monitoring relay functions include fault detection, voltage checking, and direction-sensing that confirms power system conditions, but does not directly sense the fault or problem. Both protective relays and monitoring relays may be sensitive to voltages, power or phase, current, or frequency.