There are four types of abnormal voltage conditions: sag, swell, surge, and interruption. A sag temporarily reduces the standard AC voltage, usually caused by a short circuit. A swell is the opposite of sag and refers to the increase of power frequency voltage, usually caused by a load reduction. A surge is a sub-cycle disturbance lasting less than half a cycle and mostly less than a millisecond, usually caused by lightning or switching. An interruption means the complete loss of voltage, usually caused by a line outage or a blown fuse.