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A voltage swell is a state of increased voltage. The swell threshold is usually set at the level close to 110{b7e1b70d923112b2dce5c7f61317ee3c713200fca463085e7ae47b1e4ec3e7da} of Unom. The swell starts at the moment when the URMS(1/2) voltage increases above the swell threshold value, and ends at the moment when the URMS(1/2) voltage is equal or less than the swell threshold value minus the voltage hysteresis. During the interruption, the power quality analyser remembers the maximum recorded voltage and the average voltage value.