Last month, we covered the basics of classifying switches and relays based on poles and throws. Once you learn these, it’s easy to apply switches and relays to create “hard-wired” control logic. The term “hard-wired” means that the logic is based on how the contacts of switches and relays (and the coils of relays) are wired together.
When any two or more contacts from a switch or a relay are wired in series they represent an “AND” logic state. Contact #1 AND contact #2 both have to be closed at the same time for a signal to pass through. An example from hydronics would be that the boiler disconnect switch, AND a contact in the boiler’s low water cutoff both have to be closed to allow power to reach the boiler’s burner.