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Fault 17
Pro

Fault 17 — VFD STO Channel 1 Open

What you'll learn

**Reported symptom:** The guard is closed and the safety relay resets, but the VFD remains in STO active / inhibit.

Lab time: ~14 minutes.

Lesson briefing

VFD STO Channel 1 Open

Reported symptom: The guard is closed and the safety relay resets, but the VFD remains in STO active / inhibit.

The circuit was commissioned from a known-good drawing. Treat every terminal as suspect, but test from source to load instead of guessing. Use voltage mode on an energised control circuit and continuity only after the simulated circuit is isolated.

Your task

Compare both safety output channels at the relay and at the VFD STO inputs.

Choose the fault category only when the measurements support it.

Hints

Hint 1

Confirm the normal run command separately; it cannot override active STO.

Hint 2

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Hint 3

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This lesson uses 9 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.

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Competency and practice field guide

VFD STO channel-open fault lesson: practice plan

Direct answer

The learner can distinguish ordinary run logic from the safety path, preserve diagnostics and identify the open channel without bypassing the protective function.

Written for drive and maintenance learners diagnosing a VFD that remains inhibited because one STO channel is open or discrepant.

a guarded motor-control training cell used to commission contactors, protection, a variable-frequency drive and independent speed feedback while studying safe-torque-off channel-open diagnostics and validation boundaries
System map / 02

NODE 01observable

Scope

Hazard-based function, dual channels, supply, source device, wiring, discrepancy, drive safety input status, reset, diagnostic code and safe state.

NODE 02observable

Signal path

Protective-device state through both channels, drive STO inputs, certified internal path, torque prevention, feedback and deliberate restart conditions.

NODE 03observable

Baseline practice

Both healthy channels permit the separately controlled drive-enable path and a demand removes torque according to the declared function.

NODE 04observable

Edge cases

One open channel, cross fault, timing discrepancy, supply loss, reset held, communication loss, maintenance bypass and power return.

NODE 05observable

Fault practice

A source-device, channel, supply, wiring, input, diagnostic, reset, final-element or feedback mismatch.

NODE 06observable

Transfer to the job

The complete safety function inspected and validated on target equipment by competent people.

Answer surface / 07

What happens when one VFD STO channel is open?

The drive should remain in its defined safe-torque-off state and report channel or discrepancy diagnostics according to the exact product design.

Can STO be bypassed to test a motor?

Do not defeat a safety function. Diagnose and validate it under an approved procedure using the manufacturer instructions and qualified personnel.

Motor, wiring and VFD path

Connect the control command to physical motor behavior

Move from safe control wiring and contactors into VFD parameters, measurements, faults and PLC command paths.