GA800 Fault STo — Safe Torque Off

Overview

The STo (Safe Torque Off) code on a Yaskawa GA800 variable frequency drive indicates that the drive's built-in dual-channel hardware safety function has been activated or interrupted. When this state is active, the drive immediately cuts off power to the output IGBT gate transistors, preventing the motor from producing torque. While STo is often an expected indication when safety gates are opened or an E-stop is pressed, an unresetable, unexpected, or flashing STo code indicates a wiring fault, timing mismatch, or control board circuit failure.

Symptoms

When the STo condition occurs on a Yaskawa GA800 drive, look for the following system behaviors:

• Keypad Indication: The LCD digital operator flashes "STo" or displays it as an active alarm condition.
• Motor Actions: The connected motor immediately coasts to a stop, ignoring any configured deceleration ramps.
• LED Status Indicators: The drive’s run indicator goes dark, and any safety-related status LEDs on the system control panel or safety relay change from green to red.
• Interlock Refusal: The drive refuses to accept any external run commands or auto-tuning requests until both safety channels are restored simultaneously.

Possible Causes

Identifying the root cause of an unexpected STo condition requires looking at both external safety circuits and internal drive hardware. The most frequent causes include:

1. Open Safety Circuit: An E-stop button, safety light curtain, limit switch, or gate interlock in the safety loop has been tripped or damaged.
2. Missing or Loose Jumpers: If the drive does not use external safety elements and relies on physical wire jumpers (between terminals HC, H1, and H2), these jumpers may be loose, poorly crimped, or completely omitted.
3. Dual-Channel Timing Skew: The time delay between safe inputs H1 and H2 falling or rising exceeds the drive's internal tolerance window (commonly occurring when external mechanical relay contacts wear out unevenly).
4. Cabling Degradation: Corroded, broken, or high-resistance wiring between the physical safety relay/PLC outputs and the GA800 control terminal board.
5. External Power Supply Drops: Insufficient voltage from the external 24 VDC power supply feeding the safety signals, causing the optocoupler circuits to drop out intermittently.
6. Damaged Control Board Optocouplers: Internal component failure within the GA800’s control board safety isolation circuit, causing the drive to read one or both channels as "open" even when correct voltage is present.

Step-by-Step Troubleshooting

Follow these sequential diagnostic steps to isolate and resolve the STo fault safely and systematically.

Step 1: Verify the Safety Loop State

First, determine if the drive is reacting to a legitimate safety command or a fault condition. Walk the physical line of your machine. Verify that all E-stope buttons are pulled out, safety doors are fully closed, and safety grates are locked. If any safety relay module upstream of the VFD has a "Fault" or "Reset" error light on, troubleshoot that primary device first.

Step 2: Inspect Safety Jumper and Terminal Connections

De-energize the drive and wait at least 5 to 10 minutes for the internal bus capacitors to discharge fully (verify with a multimeter across the + and - terminals). Remove the drive cover to expose the control terminal panel. Locate terminals HC, H1, and H2:

• If utilizing an external safety controller, verify that each wire tip is cleanly stripped, inserted correctly into the terminal block, and tightened. Tug gently on each wire to ensure solid contact.
• If no external safety controller is used in this application, verify that physical jumper wires are installed securely bridging terminal HC to H1 and HC to H2.

Step 3: Perform DC Voltage Diagnostics

With the drive powered back up and the upstream safety circuit in a reset (ready) state, set your digital multimeter to DC voltage mode:

• Measure the voltage between terminal HC (common) and input H1.
• Measure the voltage between terminal HC (common) and input H2.
• Expected Value: You should see a stable +24 VDC (acceptable operating window is typically 20.4 VDC to 28.8 VDC) on both channels. If one or both channels read below this threshold, trace the wiring back to the safety relay or power supply to find where the voltage drop is occurring.

Step 4: Check for Output Timing Mismatch (Skew)

If you are using mechanical contacts (like safety relay auxiliary terminals) to switch H1 and H2, they can wear over millions of cycles. If one contact closes even 100 milliseconds slower than the other, the GA800 will flag a safety channel mismatch.

• Temporarily disconnect the external wiring from H1 and H2.
• Use a pair of brief, solid wire jumpers to connect HC to H1 and HC to H2 simultaneously.
• If the STo error immediately clears when jumpered directly at the terminal board, your external switching device (safety relay/PLC card) is failing to switch both contacts with precise synchronization.

Step 5: Isolate Internal Drive Card Failure

If you have applied direct wire jumpers from HC to H1 and HC to H2 and measured exactly 24 VDC across those jumpered terminals, yet the drive still displays the STo fault or flashes state continually, the problem lies within the drive itself. The optocouplers or logic diodes on the GA800 control card have degraded. In this state, the control card must be replaced.

Recommended Actions

To prevent recurrence of the STo fault, implement the following actions:

• Upgrade to Electronic Safety Switches: If contact bounce or mechanical delay on safety relays is causing timing mismatches, replace mechanical safety boundary switches with reliable solid-state, dual-channel electronic safety sensors.
• Maintain Ferruled Connections: Always use crimp-on insulated wire ferrules for all control board connections on terminals HC, H1, and H2. This prevents stray strands from shorting or creating intermittent high-resistance paths.
• Verify External Power Budgets: If utilizing an external 24V source, ensure it has enough amperage headroom to stay stable during large relay coil state-changes or contactor pick-up events.

Recommended Replacement Parts

If the troubleshooting steps point to component failure, you will need to source replacement components to restore machine operation:

• Yaskawa GA800 Control Board: The logic module that houses the safe input processing circuitry (ensure you match your exact GA800 drive frame and part spec number).
• GA800 Control Terminal Block Kit: If the spring clamp or screw terminals for the safety signals have stripped threads, cracked housings, or poor internal clamping force.
• Omron G9SA or G9SE Safety Relay: The standard, highly reliable dual-channel safety monitoring relay recommended for interface with the GA800 safety inputs when executing local hardware E-stop loops.

Related Articles

• How to Replace Yaskawa GA800 Control Boards
• Understanding Safety Integrity Levels (SIL3) in Modern VFDs
• Yaskawa GA800 Compatibility Guide with Omron Safety Relays

FAQ

Q: Can I run my machine permanently with the HC-H1-H2 terminals jumped out?

A: Absolutely not. Bypassing safety terminals with permanent jumpers violates OSHA rules, European machinery directives, and basic functional safety guidelines. Jumpers should only be used temporarily by trained electrical automation technicians to isolate problems during diagnostic procedures.

Q: What is the differences between 'STo' and 'SToF' faults on the GA800?

A: "STo" is generally an informational status or alarm indicating that the safety circuit is open (an expected condition). "SToF" represents a Safe Torque Off Hardware Detection fault (or feedback mismatch). SToF signifies that the drive detected an inconsistent state between input H1 and input H2, indicating a component or wiring failure that requires manual reset.

Q: Why does the STo fault only occur when another motor on the machine starts up?

A: This is typical of electrical noise (EMI) or power sag. High-current inrush from other inductive loads can cause a temporary dip in the 24V DC safety loop control voltage, or induce noise onto unshielded safety cables, causing the high-speed optocouplers on the GA800 to drop out for several milliseconds.

Q: Does the GA800 support safe stop categories direct from a networked PLC?

A: Yes, the GA800 supports Safe Torque Off over industrial networks (such as EtherNet/IP or PROFINET with CIP Safety or Failsafe over EtherCAT options) when equipped with the appropriate Yaskawa functional safety communication option cards.