BACC is a train protection system used in Italy on railway lines using 3kV DC electrification. The term is an abbreviation of Blocco automatico a correnti codificate (automatic block system with codified currents).
It is an extension of RS4 Codici. While RS4 Codici is a simple cab signalling system that only requires the driver to acknowledge any change in the aspect of the next signal, SSB (On board sub system) RS9 equipment also continuously monitors the train speed and computes braking curves according to the train's length, mass, and braking ability.
The information is conveyed by superposition of two amplitude-modulated alternating currents in the rails (with a carrier frequency of 50 Hz and 178 Hz, respectively). Receiver coils in front of the first axle of a locomotive or control car are used to detect the signal.
The frequency of the modulating signal encodes the signal aspect:
RS4 Codici uses a single 50 Hz alternating current. All codes that do not use the 178 Hz carrier are used identically for both BACC and RS4 Codici. Thus, BACC is backward-compatible to RS4 Codici.
An alarm is sounded in the cab if the train exceeds the speed setpoint by more than 3 km/h. If the train exceeds the speed setpoint by more than 5 km/h or misses the designated stopping place, the system applies emergency brakes.