5091 Safe Torque Off fault in ABB ACS880 Drive

5091 Safe Torque Off in the ABB ACS880 Drive indicates that the Safe Torque Off (STO) safety function has become active because one or both safety circuit signals connected to the XSTO connector have been interrupted during motor start or while the drive is running. When the STO function is activated, the drive immediately disables the inverter output, preventing the motor from producing torque while the control electronics remain powered.

Unlike a hardware failure (Fault 5090), Fault 5091 usually indicates that the STO safety circuit has been intentionally or unintentionally opened by an external safety device, wiring fault, or safety circuit interruption.

This fault is programmable using:

Parameter 31.22 – STO Indication Run/Stop

which determines how the drive responds when the STO function becomes active during operation.

Possible Causes

• Safe Torque Off (STO) safety circuit activated.
• STO1 or STO2 safety signal interrupted.
• Open circuit in the XSTO safety connector.
• Loose or disconnected STO wiring.
• Damaged STO cable.
• Emergency Stop (E-Stop) activated.
• Safety relay opened the STO circuit.
• Safety PLC commanded STO activation.
• Faulty safety relay contacts.
• Incorrect STO wiring after installation or maintenance.
• Poor electrical connection at the XSTO connector.
• Defective external safety switch.
• Intermittent wiring due to vibration.
• Incorrect configuration of the external safety circuit.
• Parameter 31.22 STO Indication Run/Stop configured for fault indication during operation.

Solutions

• Verify whether the STO function has been intentionally activated.
• Inspect the STO circuit connected to connector XSTO.
• Check STO1 and STO2 wiring for continuity.
• Ensure all XSTO connector terminals are securely connected.
• Inspect the safety relay for proper operation.
• Verify the Emergency Stop circuit is reset.
• Check the safety PLC outputs if used.
• Inspect all STO wiring for loose, broken, or damaged conductors.
• Measure continuity through the STO safety circuit.
• Verify correct wiring according to the ABB hardware manual.
• Check the setting of Parameter 31.22 – STO Indication Run/Stop.
• Reset the fault after restoring the STO circuit.
• Replace faulty safety components if necessary.

Additional Information

The Safe Torque Off (STO) function is a certified functional safety feature that safely removes torque-producing power from the motor without disconnecting the AC supply to the drive.

When either STO channel (STO1 or STO2) opens, the inverter gate signals are disabled immediately, preventing motor torque generation.

Fault 5091 indicates that the STO function has become active because the safety circuit connected to the XSTO connector has been interrupted during motor start or while the drive is running.

Unlike Fault 5090 (STO Hardware Failure), this fault generally indicates a valid or external activation of the STO safety circuit rather than an internal hardware malfunction.

The drive behavior when STO becomes active is influenced by:

Parameter 31.22 – STO Indication Run/Stop

Refer to the ABB ACS880 hardware manual for the wiring requirements and operating description of the STO safety circuit and Parameter 31.22.

Common Symptoms

• Drive trips with Fault 5091.
• Motor stops immediately during operation.
• Motor cannot be started.
• STO status remains active on the control panel.
• Emergency Stop has been activated.
• Safety relay output is open.
• Drive enables normally after restoring the STO circuit.
• Fault occurs intermittently due to loose STO wiring.
• No inverter output despite a valid run command.

Recommended Troubleshooting Sequence

1. Verify whether the Emergency Stop or safety system has been activated.
2. Inspect the STO circuit connected to the XSTO connector.
3. Check STO1 and STO2 input signals.
4. Verify all XSTO connector terminals are secure.
5. Inspect STO wiring for damage or loose connections.
6. Check the safety relay for proper operation.
7. Verify the safety PLC outputs if installed.
8. Measure continuity of the STO safety circuit.
9. Review the setting of Parameter 31.22 – STO Indication Run/Stop.
10. Reset the STO circuit and clear the fault.
11. Restart the drive and verify normal operation.
12. Replace faulty safety devices or wiring if the fault persists.

STO Safety Circuit Check Table

Check Point	Action
XSTO connector	Verify secure connection
STO1 input	Check continuity and signal
STO2 input	Check continuity and signal
STO wiring	Inspect for loose or damaged wiring
Emergency Stop	Verify reset and proper operation
Safety relay	Confirm correct operation
Safety PLC	Verify STO output signals
Safety switches	Inspect for proper operation
Parameter 31.22	Verify STO Indication Run/Stop configuration
Drive restart	Reset fault after restoring STO circuit
Hardware manual	Verify STO wiring complies with ABB requirements

Preventive Maintenance Tips

• Inspect STO wiring during routine maintenance.
• Periodically test the Emergency Stop and STO safety circuit.
• Ensure XSTO connector terminals remain clean and securely tightened.
• Verify proper operation of safety relays and safety PLC outputs.
• Protect STO wiring from vibration and mechanical damage.
• Replace damaged safety cables immediately.
• Follow ABB wiring recommendations for all STO connections.
• Periodically verify the setting of Parameter 31.22 – STO Indication Run/Stop.
• Record any STO activations to help identify intermittent wiring faults.
• Perform functional safety tests after maintenance or modifications to the safety system.

Proper inspection of the Safe Torque Off safety circuit, secure XSTO wiring connections, regular testing of emergency stop devices and safety relays, correct configuration of Parameter 31.22 – STO Indication Run/Stop, and timely repair of faulty safety components will help prevent recurring 5091 Safe Torque Off faults in ABB ACS880 drives while ensuring safe, reliable, and compliant machine operation.