Relays are electromechanical switches that play a major role in railway signaling systems. They allow one electrical circuit to control another electrical circuit.
A relay can use a small electrical current to control a larger current or to switch multiple circuits on and off. This allows a low-power control circuit to operate higher-power field equipment safely.
A basic electromechanical relay contains a coil, an electromagnet, an armature, a spring, and electrical contacts. These parts work together to open or close circuits.
When current flows through the relay coil, the coil creates a magnetic field. This magnetic field attracts the armature and causes it to move.
As the armature moves, it changes the position of the relay contacts. The contacts may open a circuit, close a circuit, or transfer a circuit from one path to another.
When current is removed from the coil, the magnetic field collapses. The spring returns the armature to its normal position and the contacts return to their normal state.
In railway signaling, relays are used for signal control, track circuit indication, switch control, interlocking logic, route locking, approach locking, and other safety-related functions.
Relays are also used in vital safety circuits. These circuits are designed so that signals, switches, and train movements only operate when the proper safe conditions are proven.
Fail-safe operation is extremely important. In many railroad signal circuits, a loss of power or relay failure is designed to result in a safer condition, such as a signal displaying Stop.