A plant comprising: - an automatically operating machine situated in a working zone that extends upwards from a base surface along one side, and - a security monitoring system, which comprises a radar apparatus for forming a linear barrier protecting a secure area, said radar apparatus comprising a plurality of sensors arranged along a straight security line at a distance b from each other along said security line, wherein - each sensor of the plurality of sensors is a monostatic radar sensor configured to generate an electromagnetic signal having an observation field with an aperture angle α and a detection depth that is equal to a longitudinal distance p along a maximum radiation axis, each radar sensor being disposed with its maximum radiation axis perpendicular to said security line, - the observation fields of two adjacent radar sensors overlap over a portion thereof and cross each other at a longitudinal distance d, as measured from the security line in the direction of the axes and at a transverse distance s, as measured in a direction perpendicular to the axes and parallel to said security line between the two respective adjacent radar sensors, - the succession of observation fields along said security line defines a linear barrier which extends parallel to the security line at a distance from the security line which is equal to the longitudinal distance d and a transition area which extends from the linear barrier to a transition depth equal to a longitudinal distance r along the axis, the transition depth being defined by the difference between the detection depth p and the distance d, characterized in that: - the straight security line on which the plurality of sensors are arranged is located along said side at a height ranging from 0 to 30 cm from the base surface, - the longitudinal distance r of the transition area defined by the radar apparatus is equal to or smaller than 80 cm. - an automatically operating machine situated in a working zone that extends upwards from a base surface along one side, and - a security monitoring system, which comprises a radar apparatus for forming a linear barrier protecting a secure area, said radar apparatus comprising a plurality of sensors arranged along a straight security line at a distance b from each other along said security line, wherein - each sensor of the plurality of sensors is a monostatic radar sensor configured to generate an electromagnetic signal having an observation field with an aperture angle α and a detection depth that is equal to a longitudinal distance p along a maximum radiation axis, each radar sensor being disposed with its maximum radiation axis perpendicular to said security line, - the observation fields of two adjacent radar sensors overlap over a portion thereof and cross each other at a longitudinal distance d, as measured from the security line in the direction of the axes and at a transverse distance s, as measured in a direction perpendicular to the axes and parallel to said security line between the two respective adjacent radar sensors, - the succession of observation fields along said security line defines a linear barrier which extends parallel to the security line at a distance from the security line which is equal to the longitudinal distance d and a transition area which extends from the linear barrier to a transition depth equal to a longitudinal distance r along the axis, the transition depth being defined by the difference between the detection depth p and the distance d, - the straight security line on which the plurality of sensors are arranged is located along said side at a height ranging from 0 to 30 cm from the base surface, - the longitudinal distance r of the transition area defined by the radar apparatus is equal to or smaller than 80 cm.