How to Fix ABB ACS880 Fault 5091 Safe Torque Off (STO)?
Getting Fault 5091 on your ABB ACS880 drive? Learn how to troubleshoot the Safe Torque Off (STO) circuit, check XSTO connections, and configure parameter 31.22 to clear the error.
Fault 5091 indicates that the Safe Torque Off (STO) function is active. This means the drive has lost the required 24V DC signal on its safety input terminals, and it has disabled the power stage to prevent the motor from generating any torque. Here is a guide on how to find the break in the circuit and get your process back online.
Why is Fault 5091 Triggering?
The ACS880 monitors the XSTO connector (typically a specialized orange or gray terminal block on the control unit). The fault occurs in two primary scenarios:
- During Start or Run: If the safety circuit is opened while the motor is spinning or receiving a start command, the drive trips immediately to ensure a safe stop.
- While Stopped: If the safety circuit is open even while the drive is idle, and Parameter 31.22 is configured to treat this as a "Fault," the drive will display 5091.
Common Causes
- Emergency Stop (E-Stop): A physical E-stop button has been pressed or has a loose contact.
- Safety Relay Trip: An upstream safety relay or light curtain has detected a violation and cut the 24V supply to the drive.
- Loose Wiring: Vibration in the cabinet has caused one of the small wires in the XSTO terminal to wiggle loose.
- Blown Fuse: The 24V DC supply used for the safety loop has a blown fuse or has failed.
Step-by-Step Troubleshooting Guide
1. Inspect the XSTO Connector
The XSTO connector on the ACS880 control unit has two separate channels (IN1 and IN2). Both must receive 24V DC for the drive to run.
- Check the physical connector. Ensure it is snapped firmly into the board.
- Gently tug on the wires leading into the XSTO terminals. If a wire pulls out, you’ve found your problem.
- If you are not using an external safety system, ensure the factory-installed jumper wires are tightly secured across the terminals.
2. Trace the Safety Loop
Use a multimeter to check the voltage at the XSTO terminals:
- Measure between OUT1 and IN1, and OUT1 and IN2 (or reference your specific wiring diagram).
- You should see 24V DC. If you see 0V, follow the wire back to your E-stop buttons, safety gate switches, or safety relays to find where the signal is being interrupted.
3. Check Parameter 31.22 (STO Diagnostics)
The drive allows you to choose how it reacts when the safety circuit is opened. This is often the cause of "nuisance" faults while the drive is stopped.
- Navigate to Parameter 31.22 (STO indication run/stop).
- Fault/Fault: The drive will show Fault 5091 whether it is running or stopped. This requires a manual reset every time a safety gate is opened.
- Warning/Warning: The drive shows a warning instead of a fault. This allows the drive to restart automatically (if commanded) once the safety circuit is closed.
- Fault/Warning: This is a common setting where it faults if running, but only warns if stopped.
4. Verify Simultaneous Switching
The ACS880 expects both safety channels (IN1 and IN2) to switch within 500ms of each other. If your safety relay is old or one contact is "lazy," the timing difference can trigger a fault. Ensure your safety hardware is switching both channels simultaneously.
How to Reset Fault 5091
- Clear the Safety Event: Reset the E-stop button or close the safety gate.
- Verify 24V: Ensure the "STO" or "Safe" LED on the drive (if applicable) indicates a healthy state.
- Acknowledge: Press the [Reset] button on the control panel or toggle the digital reset input.
Summary Checklist
| Component | What to Look For |
|---|---|
| XSTO Terminals | Loose wires or disconnected plug. |
| Safety Loop | E-stops, interlocks, or safety relays in the "Open" state. |
| Parameter 31.22 | Setting matches your required safety logic (Fault vs. Warning). |
| 24V Supply | Verify the external or internal 24V source is active. |
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