Industrial Engineer · phase 2 of 16 · noun
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.
Word knowledge
How the word is built, where it came from, and what it sits beside in memory.
In a sentence
After the safety PLC loses its remote I/O link, the output module applies the configured Fail-Safe State to the motor starter coil.
Why these words
- fail Old French faillir, from Latin fallere, to deceive or disappoint identifies the fault or loss around which the phrase is organized
- safe Old French sauf, from Latin salvus, unharmed describes the intended safety quality
- state Old French estat, from Latin status, condition or standing names the particular operating condition
Where it came from
- Origin
- English compound fail-safe, formed from fail and safe, followed by state
- Entered the language
- 1940s
- What changed
- Fail-safe first described designs that preserved safety during failure, then became a modifier for specified conditions and behavior in control systems.
How it is spelled
- Pattern
- Fail-safe keeps a hyphen when used as a compound modifier before state.
- Pattern
- State remains a separate word in the phrase.
- Breaks the pattern
- The phrase is often capitalized as Fail-Safe State in manuals and configuration software, although ordinary prose uses fail-safe state.
Spelled like
- fail-secure
- fault-tolerant
- error-resistant
Broken into chunks
-
fail
- failure
- failing
-
safe
- safety
- safeguard
-
state
- status
- static
What it sits beside
Same subject
- safe torque off
- de-energize-to-trip
- watchdog timer
- output fallback
Same shape
- fail-safe design
- fail-safe output
- fail-safe mode
failure-response terms
- fail-safe
- fail-secure
- fail-operational
control-system states
- run state
- stop state
- fault state
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.
