In the case of this video, we have a double-pole double-throw (D. P. D. T) relay. The throw is what moves, and the pole is the connecting part. So, a DPDT relay may have either normally open (NO) or normally closed (NC) contacts, switches, or sets of points.

This particular 90-340 relay has a diagram printed near the actual terminals, which can help you quite a bit in the field.

The coil is electromagnetic and has a rating. In this case, the rating is 24v for 50 or 60 Hz. When you apply 24v to the coil of the relay, the electromagnetic force switches the relay. The two bottom contacts connect directly to the coil; even though those contacts aren’t labeled, it’s quite easy to see where they connect to the coil.

However, the contact ratings are based on ampacity. For example, at 120v, a contact may have 82.8 locked rotor amps (LRA), which is the instantaneous power when a motor starts up or is locked. It may also have 13.8 full load amps (FLA) at 120v. At 240v, those ratings drop.

If you use your meter to read continuity (ohms to determine if you have a path or no path), you’ll notice that there’s continuity between two NC contacts and no continuity between two NO contacts.

When you add power and a switch to the relay, you can hear the relay switching whenever you flip the switch and energize the coil. When the coil is energized, those paths would switch (NC contacts would have no path, and NO contacts would have a path).