Industrial Engineer · phase 2 of 16
Normally Open / Normally Closed
Contact state defined with the field device in its normal, unactuated, and de-energized condition.
The Explain card
- Plain English
- Normally open and normally closed describe a contact with nothing acting on it. Normally open is open until something pushes it closed, and normally closed is closed until something pushes it open.
- Example
- A stop button is wired normally closed. The circuit is live while nobody touches it, and pressing the button breaks it.
- Why it matters
- A stop wired normally closed also stops the machine if the wire breaks, which is why safety circuits use it. Normal always means de-energized and unpressed, never the state the machine spends most of its time in.
- Hook
- Normal is what the contact does when nobody is home.
Where it sits in the deck
Phase 2: PLC I/O and Field Signals
Where the controller meets the machine: discrete and analog signals, the wiring conventions behind them, and the field devices that send them.
