Relays are an important part in your BAS and you will be running into these with almost every installation or troubleshooting situation. This is a must to understand while in the Automation industry.
Relays are used to isolate circuits from one another, parallel a binary signal or can be used to start equipment for your BMS. When you apply voltage across the contacts you will pull in the relay by inducing a electromagnetic field through the internal coil. After this occurs the internal magnetic armature connected to the common terminal (COM) will switch from its NC (normally closed) position to the NO (Normally Open) position.