Voltage Unbalance Protection

The voltage unbalance 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 (V_LL) 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 (V_LN) 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 V_LN.

To ensure the MicroLogic trip system will perform as expected regardless of system configuration the user should limit the voltage unbalance protection settings to 0–20%.