Relays are electrically operated switches. They allow one electrical circuit to control another circuit without the operator having to physically open or close the switch by hand.
A relay uses a small electrical current applied to a coil to control a separate circuit. This separate circuit may carry a higher voltage, higher current, or perform a safety-related function.
When current flows through the relay coil, the coil creates a magnetic field. This magnetic field moves the relay armature and changes the position of the contacts.
The contacts are the switching part of the relay. They may open a circuit, close a circuit, or transfer a circuit from one path to another depending on the relay design.
A simple way to think of a relay is as a remote-controlled switch. Instead of pushing the switch by hand, electricity energizes the coil and the relay performs the switching action.
Relays provide several advantages. They allow remote control, electrical isolation between circuits, control of larger loads with smaller control currents, and the ability to build logic circuits.
Electrical isolation is very important because the coil circuit and the contact circuit can remain separate. This helps protect equipment, personnel, and signal circuits.
In railway signaling, relays are used for signal aspect control, track circuit indication, switch control, route locking, approach locking, and interlocking logic.
Relays are important in safety circuits because their operation is predictable and easy to prove. Railroad signal systems often depend on relay contacts proving that the correct safe conditions exist.
Because railroad signaling is safety-critical, relays must be reliable, properly maintained, correctly wired, and designed so that failures move the system toward a safer condition whenever possible.