Blocco Automatico a Correnti Codificate
BACC is a train protection system used in Italy on railway lines using 3 kV DC electrification. The term is an abbreviation of Blocco automatico a correnti codificate.
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 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. 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:
| modulation frequency of 50 or 83.3 Hz carrier | modulation frequency of 178 Hz carrier | code | meaning |
| AC | transmission failure, e.g. on a siding not equipped with the required track circuitry | ||
| 75 / min | 75 | the next signal is at "stop" or the train is about to enter a dead-end track, a partially occupied track or a track with particular short braking distance | |
| 120 / min | 120 | the next signal displays a speed restriction | |
| 120 / min | 75 / min | 120* | the next signal displays a speed restriction to 100 km/h |
| 120 / min | 120 / min | 120** | the next signal displays a speed restriction to 130 km/h |
| 180 / min | 180 | the next signal is at "proceed", but the signal after that is at "stop" or displays a speed restriction | |
| 180 / min | 75 / min | 180* | the next signal is at "proceed", but the signal after that displays a speed restriction, or the train is approaching a slow zone with a speed limit of 150 km/h or less |
| 270 / min | 270 | proceed with a maximum speed of 180 km/h | |
| 270 / min | 75 / min | 270* | proceed with a maximum speed of 220 km/h |
| 270 / min | 120 / min | 270** | proceed with a maximum speed of 250 km/h |
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.