Collective protection
Primary fall arrest systems are preventive safety measures designed to stop falls from happening in the first place - as opposed to secondary fall arrest systems which are designed to arrest a fall after it has happened.
Primary fall arrest systems are always to be given preference over secondary fall arrest systems as they prevent - possibly serious - injury as a result of a fall. Guard rails are typical primary fall arrest devices as they are installed in such a way as to ensure that the falling edge is inaccessible and that anyone walking over a roof surface, for example, can move about freely within the secured zone.