To understand how relays are used in railway signaling, it is important to understand the basic characteristics that control how a relay operates.
A relay coil requires the correct voltage and current to operate properly. If the voltage is too low, the relay may fail to pick up. If the voltage is too high, the relay coil may overheat or become damaged.
Relay contacts are the parts of the relay that open and close electrical circuits. These contacts may be normally open, normally closed, or part of a changeover contact arrangement.
A normally open contact is open when the relay is de-energized and closes when the relay picks up. This allows current to flow only after the relay is energized.
A normally closed contact is closed when the relay is de-energized and opens when the relay picks up. This allows current to flow until the relay is energized.
Some relays use changeover contacts. These contacts transfer a circuit from one path to another when the relay changes position.
Operating time is the amount of time it takes for a relay to respond after voltage is applied to the coil. In many relay circuits, this time is measured in milliseconds.
Contact rating is another important relay characteristic. It tells the maximum voltage and current that the relay contacts can safely switch. Exceeding this rating can damage or weld the contacts.
Fail-safe design is critical in railroad signaling. A relay circuit is often designed so that loss of power, broken wiring, or relay failure causes the system to move toward a safer condition, such as displaying a Stop signal.