5091 Safe Torque Off (STO) Fault in ABB ACS580 Drive

5091 Safe Torque Off (STO) in the ABB ACS580 Drive indicates that the Safe Torque Off (STO) safety function is active. This occurs when one or both safety signals connected to the STO connector are interrupted during start or while the drive is running. When STO is activated, the drive immediately disables torque production to the motor, allowing the motor to coast to a stop without removing power from the drive.

This fault generally indicates a problem with the safety circuit, STO wiring, or safety device, rather than the motor or inverter hardware.

Possible Causes

• Safe Torque Off (STO) function activated.
• Broken or open STO safety circuit.
• Emergency stop (E-Stop) activated.
• Safety relay opened.
• Loose or damaged STO connector.
• Broken STO wiring.
• Faulty safety switch or interlock.
• Defective safety relay.
• Incorrect STO wiring during installation.
• Incorrect setting of Parameter 31.22 – STO indication run/stop.

Solutions

1. Check the safety circuit connections.
2. Verify that the STO connector is securely connected.
3. Inspect the STO wiring for loose or broken connections.
4. Check the operation of the emergency stop and other safety devices.
5. Verify that the safety relay is functioning correctly.
6. Check Parameter 31.22 – STO indication run/stop, as this is a programmable fault.
7. Restore the safety circuit and reset the drive if required.
8. Restart the drive after confirming that all safety conditions have been satisfied.

Additional Information

The Safe Torque Off (STO) function is a built-in safety feature that prevents the drive from generating torque without disconnecting the main power supply.

Fault 5091 is generated whenever the STO safety inputs become inactive during startup or normal operation.

According to ABB, the recommended corrective actions are to:

• Check the safety circuit connections.
• Verify Parameter 31.22 – STO indication run/stop.

Unlike an Overcurrent or Hardware Fault, 5091 STO is typically a safety-related fault intended to protect personnel and machinery.

Common Symptoms

• Drive displays 5091 Safe Torque Off.
• Motor cannot start.
• Motor coasts to a stop if STO activates during operation.
• Drive remains powered but produces no motor torque.
• Run command is ignored while STO is active.
• Fault appears after an emergency stop is pressed.
• Safety relay activation triggers the fault.
• Fault clears after the safety circuit is restored and the drive is reset.
• No inverter output despite normal control power.
• Machine remains safely stopped until STO conditions are cleared.

Recommended Troubleshooting Sequence

1. Verify that the fault is 5091 Safe Torque Off (STO).
2. Disconnect power if maintenance is required and follow lockout/tagout procedures.
3. Check the STO connector and wiring.
4. Verify continuity of the safety circuit.
5. Inspect the emergency stop push button and safety switches.
6. Check the safety relay operation.
7. Verify Parameter 31.22 – STO indication run/stop.
8. Restore any open or broken safety circuit.
9. Reset the drive if required.
10. Confirm normal drive operation after the STO circuit is restored.

5091 Safe Torque Off Check Table

Check Point	Action
STO connector	Check for secure connection
STO wiring	Inspect for open or damaged wires
Safety circuit	Verify continuity
Emergency stop	Check for proper operation
Safety relay	Verify correct operation
Safety switches	Inspect interlocks and guard switches
Parameter 31.22	Verify STO indication run/stop setting
Drive operation	Confirm normal operation after restoring STO

Preventive Maintenance Tips

• Periodically inspect STO wiring and connectors.
• Test the emergency stop and safety circuit regularly.
• Verify safety relay operation during preventive maintenance.
• Keep STO terminals clean and securely tightened.
• Protect safety wiring from mechanical damage.
• Confirm STO operation after commissioning or maintenance work.
• Record recurring 5091 STO faults to identify intermittent safety circuit issues.
• Follow ABB safety and maintenance recommendations for the ACS580.

Proper inspection of the STO connector, safety circuit wiring, emergency stop, safety relay, and Parameter 31.22 – STO indication run/stop will help prevent recurring 5091 Safe Torque Off faults in ABB ACS580 drives, ensuring reliable safety operation and compliance with machine safety requirements.