Electromechanical relay (EMR).
An electromechanical relay (EMR) is a device that uses electromagnetic force to close (or open) switch contacts—in other words, an electrically powered switch.
Electromechanical relays are used as interface modules between the process peripherals and the control, signaling and regulating devices for level and power adaptation.
Functional difference between relay and switch.
Switches and relays became widely used in industry for controlling motors, machines, and processes. A switch can turn a single machine on and off, but a relay logic network can control an entire process—turning on one machine, waiting until that operation is done, then turning on the next operation. Decisions can be made by the logic—for example, if a part is too tall, it goes in one bin; otherwise, it goes in the other bin.
Relay contacts configurations
Relay contacts come in two basic configurations—normally open contacts (NO), which are open in the unenergized state (and close when the relay coil is energized), and normally closed contacts (NC), which are closed in the unenergized state (and open when the relay is energized). By convention, the relay symbol always shows the contacts in the unenergized state.
Image – basic circuit of relay: