BACC is a train protection system used in Italy

Italy

Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

 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

RS4 Codici

RS4 Codici is a train protection system used in Italy. The term is an abbreviation of Ripetizione Segnali a 4 codici ....

. While RS4 Codici is a simple cab signalling

Cab signalling

Cab signalling is a railway safety system that communicates track status information to the cab, crew compartment or driver's compartment of a locomotive, railcar or multiple unit, where the train driver or engine driver can see the information....

 system that only requires the driver to acknowledge any change in the aspect of the next signal

Railway signal

A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers. The driver interprets the signal's indication and acts accordingly...

, 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

Superposition principle

In physics and systems theory, the superposition principle , also known as superposition property, states that, for all linear systems, the net response at a given place and time caused by two or more stimuli is the sum of the responses which would have been caused by each stimulus individually...

 of two amplitude-modulated

Amplitude modulation

Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...

 alternating current

Alternating current

In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

s in the rails (with a carrier frequency of 50 Hz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

 and 178 Hz, respectively). Receiver coils in front of the first axle of a locomotive

Locomotive

A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

 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
(none)	(none)	AC (Assenza di Codice)	transmission failure, e.g. on a siding not equipped with the required track circuitry
75 / min	(none)	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	(none)	120	the next signal displays a speed restriction (to 30 km/h, 60 km/h or 100 km/h)
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 (this code is currently not being used)
180 / min	(none)	180	the next signal is at "proceed", but the signal after that is at "stop" or displays a speed restriction (this code is also being used to announce the end of the BACC-equipped line)
180 / min	75 / min	180*	the next signal is at "proceed", but the signal after that displays a speed restriction (to 100 km/h or 130 km/h), or the train is approaching a slow zone Slow Zone A slow zone, in America, is an area where a train is forced to slow down for either structural, construction, power, signal, or track problems. Slow zones limit a train to about 10-35 MPH...  with a speed limit of 150 km/h or less
270 / min	(none)	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

Backward compatibility

In the context of telecommunications and computing, a device or technology is said to be backward or downward compatible if it can work with input generated by an older device...

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.