The vital relays NS1, NS2 and NCO use automatic connections to assume the security functions of uncontrolled railway signalling installations.. The NS1 vital relays are approved by SNCF and compliant with NF-F-70-030 referential. They are specifically designed to operate in signalling centres. They are classified under « NS1 unified material » label. The references NS2 and NCO are particularly used to manage train headways functions. They are mainly « block » units located along the track.