Solid-State Devices Used in Industrial Logic Circuits: Relay Logic: Terms for Relays

Before one can begin to understand relay logic concepts, one must first become familiar with several terms used to describe the action of a relay. Contacts will be described as normally open (NO) or normally closed (NC). The term normal for relays means the position you find the contacts in when no power is applied to the coil. The word normal is sometimes referred to as the condition of the contacts when the relay is pulled out of the box that it's shipped in. Relay contacts are always shown in a diagram in their normal condition, which means they are drawn in the condition you would find them in when no power is applied to the coil.

When the relay coil is energized, the contacts will move to the opposite condition of their normal condition. For this reason, it's always important to verify whether the coil is energized or de-energized. When the coil is energized, it will be referred to as being picked up, and if it's dc-energized, it will be referred to as being dropped out. These terms may also be used to describe the condition of the contacts.