SONET's SVD-04 is a 3-phase microprocessor controlled under / overvoltage& phase angle protection relay. 3-phase relay is used in systems exposed to over, under voltage & phase sequence are all normal the OUT light will turn on, relay contact will be energized and when there is default phase or phase sequence errors the OUT light will off, the relay will de-energizes its contact. It can be use as command, control or compensation systems.