The Models 646-1 and 646-3 are portable instruments designed to monitor the quality of ac power being supplied to sensitive electronic equipment. These units operate by continually measuring the voltages at their inputs, and then reporting any unusual occurrences (such as drop-outs and high voltages which sometimes occur during severe thunderstorms). 1.3.1 Disturbance Reporting A disturbance is reported whenever the measured value falls outside "normal" (outside the programmable high or low limit). For example: A Series 646 may be programmed to report whenever the line voltage falls below 105 V ac or rises above 125 V ac, Each time a disturbance is detected, a detailed printout describing the event is generated. This printout includes the time, date, amplitude, and duration of the disturbance.