Safe Torque Off (STO) and drive safety functions
What STO does and does not do, stop categories 0 and 1, SS1 and other drive-integrated safety functions per IEC 61800-5-2, and wiring and validation notes.
What STO is
Safe Torque Off (IEC 61800-5-2) safely prevents the drive from generating torque: the pulses to the power semiconductors are blocked by a certified path, independent of the normal control. The motor coasts to standstill (unless a brake acts).
STO does not disconnect the motor from voltage — it is not an electrical isolation for maintenance work on the motor or cable. Use a lockable isolator for that.
Stop categories (IEC 60204-1)
| Category | Behaviour | Drive implementation |
|---|---|---|
| 0 | Immediate removal of power, uncontrolled stop | STO |
| 1 | Controlled braking, then power removed | SS1: ramp down, then STO (time- or speed-monitored) |
| 2 | Controlled stop, power maintained | SS2 / SOS: safe operating stop at standstill |
Emergency stop usually uses category 0 or 1 — the risk assessment decides (a large flywheel coasting for minutes may be the bigger hazard).
Other integrated safety functions
SLS (safely limited speed — e.g. setup mode), SDI (safe direction), SBC (safe brake control), SLP (safely limited position). They may be controlled by safety-rated digital inputs or by a safety protocol (PROFIsafe, CIP Safety, FSoE).
Wiring and validation notes
- Two-channel STO inputs; follow the drive manual for test pulses (OSSD) and discrepancy times.
- The achieved safety level (PL/SIL) depends on the whole chain: sensor, safety relay/PLC, wiring, drive — not only on the drive's certificate.
- Validate the function after commissioning and after every change; drives often require an acceptance test record for their safety settings.
- The normal PLC program and normal drive enable are not safety functions.
Connections
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Sources
Updated 2026-10-02