A digital voltage protection circuit (138) and a phase monitor circuit (154, 160, 162, 168) disconnect AC-powered user equipment (22,182) from the AC power supply (141) whenever the AC power line voltage is outside a predetermined operating voltage window and when any two phases of the power supply (141)...http://www.google.de/patents/US5319514?utm_source=gb-gplus-sharePatent US5319514 - Digital voltage and phase monitor for AC power line
