This measurement device monitors the potentials of the connected grounds and evaluates the occurring voltages (see technical specifications in the datasheet).
While operating DC railways with the running rail as return conductor various ground resistances generate voltages that might be dangerous for humans or buildings. One of the measures to protect humans from impermissible contact voltages is the temporary short-circuit of the various grounding systems according to DIN EN 50122-1.
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: