The Type PVD21 relay is a single-phase, high-speed, high-impedance, voltage-operated relay designed to provide protection in bus differential schemes. The Type PVD21 utilizes the same operating principle (high impedance voltage) as the earlier PVD models, but provides faster operating speeds and higher seismic capabilities. Typical operating speed is 20 ms at 4X pickup. The Type PVD21 is mechanically interchangeable with the earlier PVD21 models, and provides additional output contacts.