Industrial Engineer · phase 2 of 16
Fail-Safe State
Predetermined output or process condition entered after a detected fault, communication loss, or power loss.
The Explain card
- Plain English
- The fail safe state is what an output goes to when the controller can no longer control it. It is chosen ahead of time, per point, for a fault, a lost connection or a power loss.
- Example
- A remote I/O block loses its network link. Its valve outputs are configured to drop to closed, so the process shuts rather than holding whatever was last commanded.
- Why it matters
- The right fail state depends on the machine, and there is no universal safe answer. A cooling water valve should probably fail open, a steam valve should probably fail closed, and the person who knows which is the process engineer.
- Hook
- Decide what off means before the day it happens.
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.
