NO and NC stand for Normally Open and Normally Closed. They are terms used in various fields, especially common in:

• Electrical Engineering: They describe the default state of contacts in relays and switches.
• PLC Programming: They represent instructions within a program for a Programmable Logic Controller.

In both contexts, NO and NC refer to the initial state of a connection (current flow) when no external signal is applied.

• Normally Open (NO): In its natural state, the connection is open, and current does not flow. When a signal is applied (like flipping a switch on), the contact closes, allowing current to pass.
• Normally Closed (NC): Here, the connection is closed by default, and current flows freely. Applying a signal opens the contact, interrupting the current flow.

Here's an analogy to visualize it:

• NO (Normally Open): Imagine a light switch on a wall. When off (normal state), there's no connection (NO), and the light is off. Flipping the switch (applying a signal) completes the circuit and turns on the light.
• NC (Normally Closed): Think of a button where pressing it turns off a light (already on). The button acts like an NC contact - normally allowing current (light on), and pressing it (applying a signal) breaks the connection and cuts power.

Understanding NO and NC is essential for designing circuits and writing PLC programs that control various functions. They provide the foundation for building logic by allowing you to specify how the system behaves under different conditions.