Normally open, or NO, means the contacts are apart in that rest condition, the circuit is open, and nothing flows. This guide explains what normally open (NO) and normally closed (NC) contacts do and how to choose, wire, and test them so doors, drawers, and machines behave safely and predictably. Conversely, a. A standard wired alarm door contact is, in its uninstalled state, open. It is important for an installer to be aware of. Controlling processes safely and effectively in industrial automation, particularly inside PLC (Programmable Logic Controller) systems, is fundamentally based on the usage of Normally Open (NO) and Normally Closed (NC) contacts. Understanding what they mean and the difference between them can be difficult at first. In this article, we will explain what each term.