Completando un poco....
ENS = Selbsttätig wirkende Freischaltstelle mit zwei voneinander unabhängigen Einrichtungen zur Netzüberwachung mit zugeordneten allpoligen Schaltern in Reihe.
Other description for such kind of protection is:
MSD = Mains Monitoring Units with Allocated All-pole Switching Devices.
Vamos, si no he entendido mal es la protección ante el funcionamiento en isla. Concretando un poco más...
Islanding and line disconnect
Islanding operation can be detected or monitored by passive or active islanding detection method. Passive method includes detecting rate of change of frequency, voltage phase jump and three-phase voltage drop monitoring. With active islanding operation detection method frequency shift, active frequency drift - AFD, ENS (impedance measurement), and reactive power fluctuation are detected and monitored. ENS disconnect unit is a standalone unit which disconnects inverter from the grid.
Si queréis investigar algo más está sacado de http://www.pvresources.com/en/inverter.php