This measurement device monitors the potentials of the connected grounds and evaluates the occurring voltages (see technical specifications). This device does not need any auxiliary power supply; the electrical power is supplied by the monitored input voltage.
The voltage between the earths or return conductor respectively that are to be monitored is continuously measured. By exceeding the threshold setting a switching command is given via a potential-free changeover for the activation of further devices such as an earth short-circuiting device or an isolator.
You will find more information in the download of the datasheet: