What is the difference between normally open and normally closed contacts in a control circuit?

• A. NO contacts close when energized; NC contacts open when energized, providing opposite behavior for logic.
• B. NO contacts open when energized; NC contacts close when energized.
• C. NO contacts are always closed; NC contacts are always open.
• D. NO contacts are only used in power circuits; NC in control circuits.

Answer: (A)

The key idea is how each type of contact responds when the controlling device (like a relay coil or switch) is energized. A normally open contact is open when the circuit is at rest and closes when the control energizes. A normally closed contact is closed when the circuit is at rest and opens when the control energizes. This opposite behavior lets you design control logic so a load can be made to run only when a signal is present (using the normally open contact) or be held off or stopped when a signal is present (using the normally closed contact).

In practice, a normally open contact provides current flow only after activation, while a normally closed contact stops current flow when activation occurs. This is useful for safety and fail-safe designs: normally closed contacts are often used so that a loss of power or a fault will cause a circuit to open and equipment to stop. Meanwhile, normally open contacts are common when you want an action to occur only after a deliberate input.

So the correct description is that normally open contacts close when energized, and normally closed contacts open when energized. The other statements—such as one type being always closed or always open, or being restricted to power circuits—don’t fit how these contacts actually behave or where they’re used.