NAMUR is a (German: Normenarbeitsgemeinschaft fuer Mess- und Regeltechnik) standardization working group, dealing with all issues around C&I. A NAMUR-sensor can detect different status of sensor via analog signal. It is used e.g. in fire alarm buttons to ensure functioning even if it is not used for long time, as there is a zero signal current. See e.g. sample for a NAMUR locker: