Glossary · Stopping functions and safe states
Safe Torque Off (STO)
Also known as: STO
German: Sicher abgeschaltetes Drehmoment (STO)
In drive-based safety, Safe Torque Off (STO) is a safety function that prevents torque-producing energy from being applied to a motor. The motor cannot generate torque or force, but the drive is not electrically isolated.
- Machine safety
- Maintenance
In one sentence
Safe Torque Off (STO) blocks torque-producing energy to a motor. It is the basic drive safety function, and it is not electrical isolation.
Example
On a conveyor, opening an interlocked access door triggers STO in the belt drives; the belts coast to a stop, while the drive DC link stays charged.
How it applies
- Machine safety: STO used alone corresponds to a stop category 0 stop: the motor coasts. Where coasting is too long, SS1 is used first. Vertical axes and suspended loads need a brake or mechanical holding, because STO does not hold.
- Maintenance: STO is not a substitute for lockout/tagout. Dangerous voltages remain at motor terminals and in the drive; electrical work needs energy isolation.
- Technical documentation: Say plainly in the operating and maintenance manuals that STO stops torque, not voltage. The wiring and the achieved PL or SIL come from the drive manufacturer’s safety manual.
- Evidence: Validation checks both STO channels and the fault reaction when one channel fails.
Safe Torque Off vs. emergency switching off
STO removes torque from one motor while the drive stays energized. Emergency switching off removes electrical energy to address electrical hazards.