In ladder diagrams, what do normally open (NO) and normally closed (NC) contacts represent?

• A. NO closes when energized; NC opens when energized
• B. NO opens when energized; NC closes when energized
• C. NO and NC are always closed
• D. NO and NC are only used for signaling circuits

Answer: (A)

In ladder diagrams, the state of a contact depends on whether the controlling input is energized. A normally open contact is open when there is no energizing signal and closes as soon as the input energizes, allowing current to flow. A normally closed contact is closed when there is no energizing signal and opens when the input energizes, interrupting current flow. So the correct idea is that a normally open contact closes when energized, while a normally closed contact opens when energized. This helps you model real switches: a NO contact behaves like a pushbutton that conducts only after pressing (energizing), and a NC contact behaves like a switch that is normally conducting but releases (opens) when energized.