Minimum Voltage Protection
The minimum (under) voltage protection function is based on phase-to-phase voltage measurement.
For circuit configuration 1 (Circuit Configuration 1), 2 (Circuit Configuration 2) or 3 (Circuit Configuration 3), if a fuse opens the trip unit will inject voltage on the open phase. Therefore the trip unit will meter the voltage being injected on the open phase accurately. The phase-to-phase voltage (VLL) measurement will be higher than when the open phase is at zero volts. The trip unit will also accurately meter the phase-to-neutral voltage (VLN) injected on the open phase and display a value greater than zero.
For circuit configuration 4 (Circuit Configuration 4), the trip unit has a return path through the transformer and the injected voltage on the open phase will be zero. In this configuration the trip unit will accurately meter zero volts on VLN.
To ensure the MicroLogic trip system will perform as expected regardless of system configuration the user should limit the undervoltage pickup range to 80%–100% of the nominal phase-to-phase system voltage.