Littelfuse 601 Three-Phase Voltage MonitorLittelfuse 601 Voltage Monitor Description The model 601 is a fully programmable voltage monitor designed to protect 3 phase motors from loss of any phase (single phasing), phase reversal, low or high voltage, voltage unbalance, low or high frequency, and rapid cycling. It can be used as a stand alone product or networked with an RM1000, RM2000, PLC, computer or SCADA system. When a harmful condition is detected, the 601s output relay is deactivated