What causes fault code AB-PF525-SAFE (PowerFlex 525 Safe Torque Off (STO) Fault)?

Safety relay or safety PLC de-energized the STO input — intentional safety stop. E-stop activated — STO circuit is in the E-stop chain. Guard door opened — STO wired through guard switch circuit. STO input wiring disconnected — terminal 01 and/or 02 not receiving 24VDC. If STO not used, jumper wires on terminals 01-02 may have fallen off or were never installed

How do I fix fault code AB-PF525-SAFE?

Check 24VDC at STO input terminals 01 (STO1) and 02 (STO2) — both must have 24VDC to enable drive output. If STO is wired through safety relay, check safety relay output status. Verify all E-stops are released and all guard doors are closed. If STO is not being used for safety, install jumper wires: terminal 01 to +24VDC and terminal 02 to +24VDC. STO cannot be reset from the keypad — the hardware input must be satisfied. Check wiring diagram for the STO circuit to identify all devices in the safety chain

PowerFlex 525 Safe Torque Off (STO) Fault

AB-PF525-SAFE

PowerFlex 525 STO safety input is de-energized. Drive output is hardware-disabled by the built-in STO function. This is a designed safety feature, not a drive malfunction.

medium allen-bradley vfd

allen-bradley powerflex 525 sto safe torque off safety

Probable Causes (ranked)

Safety relay or safety PLC de-energized the STO input — intentional safety stop
E-stop activated — STO circuit is in the E-stop chain
Guard door opened — STO wired through guard switch circuit
STO input wiring disconnected — terminal 01 and/or 02 not receiving 24VDC
If STO not used, jumper wires on terminals 01-02 may have fallen off or were never installed

Fix Steps

1 Check 24VDC at STO input terminals 01 (STO1) and 02 (STO2) — both must have 24VDC to enable drive output
2 If STO is wired through safety relay, check safety relay output status
3 Verify all E-stops are released and all guard doors are closed
4 If STO is not being used for safety, install jumper wires: terminal 01 to +24VDC and terminal 02 to +24VDC
5 STO cannot be reset from the keypad — the hardware input must be satisfied
6 Check wiring diagram for the STO circuit to identify all devices in the safety chain

Field Trick

New PowerFlex 525 drives out of the box will NOT run until you either connect the STO inputs to 24VDC or install the jumper wires. This catches a lot of people during commissioning. Check terminals 01 and 02 — they need 24VDC from a safety-rated source or jumpered.

Suggested Replacement Parts

Parts matched to fault category: Safety Relay

Allen-Bradley Guardmaster 440R-N23126 P/N: 440R-N23126 Dual-channel safety relay, 24VDC, 3 N.O. + 1 N.C., Cat 4 / PLe

AutomationDirect $125.00 Amazon $95.00 Grainger $155.00

Pilz PNOZ X3 Safety Relay P/N: 774310 24VDC/AC, 3 N.O. + 1 N.C., E-stop and safety gate monitoring

AutomationDirect $145.00 Amazon $135.00 McMaster-Carr $165.00

Prices are estimates. Verify part numbers match your equipment before ordering.

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