We’ll be using the most common type of relay in residential service, a 24v relay. A relay is an electronically operated switch that opens and closes using an electromagnet. The electromagnet is a coil of wire that when powered acts as a magnet to close/open a switch.
Terminal 1 of this relay is the common terminal. It’s “common” to terminal 2 and 3. Terminal 1 will be our terminal that will pass on power to and/or complete the circuit between terminal 2 or 3 when 24v is applied to the electromagnet.
Terminal 2 of the relay is a NC (normally closed) contact which you will usually see written on diagrams and equipment as NC. So when no 24v is powering this relay, the switch between terminals 1+2 is closed completing that side of the circuit.
Now when you power the coil with 24v, the switch between terminals 1+2 open and break the NC connection.
Example: If you had 120v to terminal 1 and terminal 2 was connected to a light, that light would be on as that circuit is NC. As soon as the relay is powered by 24v, the switch opens and the NC connection is now open breaking the circuit and the light goes out.
Terminal 3 of the relay is a NO (normally open) contact which you will usually see written on diagrams and equipment as NO. So when no 24v is powering this relay, the switch between terminals 1+3 is open with no completion of that circuit.
Now when you power the coil with 24v, the switch between terminals 1+3 closes and completes the NO circuit making it closed allowing for power to pass.
Example: You have no 24v powering the relay. You do have 120v to terminal 1. Terminal 3 is connected to a light. So this light would be off as that circuit between 1+3 is NO. As soon as the relay is powered with 24v, the switch between 1+3 closes and the NO connection is now closed making the circuit, and the light goes on.
As you can see with a powered relay, the NO changes to NC and the NC changes to NO.
Terminals 4, 5, and 6 are controlled the same as terminals 1, 2, and 3. It just gives you the option to control another piece of equipment at the same time with one relay instead of two.