Drive Shows STO Active—That Does Not Mean the Motor Is Electrically Isolated
Safe Torque Off prevents torque-producing drive energy according to the implemented safety function, but it does not provide electrical isolation. Treating an STO indication as permission for electrical work confuses two different requirements. Follow the machine's hazardous-energy control procedure and exact equipment documentation.
Safety-function distinction; qualified personnel must apply the actual machine's procedures.
What is happening?
Siemens explicitly states that its documented STO function does not electrically isolate the power unit from the motor. Motion can also require separate consideration when external forces or gravity act on the mechanism. The complete safety design determines the required stopping, holding, and isolation measures.
Check these five things
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Identify the exact drive, safety function, and diagnostic indication involved.
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Read the manufacturer's function limitations and the machine's validated safety documentation.
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Distinguish an operational or safety-related stop from the isolation required for maintenance work.
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Have qualified personnel apply the approved hazardous-energy procedure, including stored and mechanically driven energy.
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After work, complete the required restoration and validation before operation.
Worked example
A vertical mechanism has torque production disabled, but gravity still acts on the load. Whether it remains supported depends on the designed mechanical and braking arrangements. The STO label alone does not establish that holding requirement.
Quick diagnostic reference
| Requirement | Separate question |
|---|---|
| Prevent drive torque | What does the validated STO function do? |
| Prevent load movement | What handles external forces? |
| Electrical maintenance | What provides verified isolation? |
The mistake to avoid
Do not use a PLC status bit, drive display, or ordinary stop command as a substitute for the required verification of isolation.
How to verify the fix
Use the documented safety-validation and maintenance-release procedures; this educational distinction is not a substitute for either.
Field-work boundary: These are educational diagnostic guides. Use the exact equipment manuals and approved site procedures. Electrical testing and machinery changes belong to qualified, authorized personnel; control hazardous energy and validate affected functions before release.
Technical reference
Siemens: Safe Torque Off function and limitations. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.