What it protects against
STO acts on the drive’s ability to supply the motor so that it generates torque. It is one part of Machinery safety functions and performance level, alongside the device that detects the need to stop and the logic processing that demand. An STO-equipped drive does not establish the safety of the entire installation.
Removing torque can leave a shaft coasting. The time until the hazard disappears must therefore be compared with a person’s possible access time. An illuminated status indicator cannot establish that the motion is already safe.
Limits that matter most
A vertical load can descend under gravity, and another machine can transmit movement into a mechanism. STO is not a mechanical brake and does not hold these loads by itself. The designer must determine how hazardous movement is prevented and verify the complete solution.
It does not necessarily remove voltage from terminals or discharge energy stored inside the drive. Electrical intervention requires the applicable isolation rules and Lockout/Tagout (LOTO) procedure. An active STO indication is not permission to touch conductors or start maintenance.
Stopping and restarting are separate decisions
Some applications need controlled deceleration before torque is removed. Selection depends on the hazard and the engineered stopping function; changing a parameter without reviewing the machine is insufficient.
Closing a guard or restoring power must retain the intended restart behaviour. Review the interaction with Interlocking devices, pending commands and anyone who might remain inside the enclosure. These circumstances belong in the application’s assessment rather than being left to default drive settings.
Evidence to request
Ask for the function description, the hazards covered, the response to faults and the completed validation records. Identify installed equipment and versions, change control and the personnel authorised to alter safety settings.
For example, removing torque from a conveyor with substantial inertia may be a valid part of its stop function. It does not show that immediate access is safe. That conclusion needs an assessment of residual motion and validation of the complete arrangement under representative operating conditions. Retain the result with the machine’s technical information.
