The Model 21 is a microcontroller-based universal 3-phase monitor for protecting 3-phase motors from abnormal power conditions. It can detect under voltage, over voltage, voltage unbalance, reverse phase, and single-phasing even when regenerated voltage is present. The Model 21 includes adjustable trip and restart delays to prevent nuisance tripping during power fluctuations and short cycling in compressor applications.

The LED status indicators and adjustment pots for Voltage Adjustment, % Unbalance, Restart Delay and Trip Delay are mounted on the front of the unit for easy access. Automatic or manual restart versions are available. The Model 21 does not require a neutral connection and can be used on either Wye or Delta systems.