PLC Simulator
← Back to all lessons
Fault 12
Pro

Fault 12 — Welded Safety Output Contact

What you'll learn

**Reported symptom:** The light curtain is interrupted, yet the contactor coil path remains continuous.

Lab time: ~14 minutes.

Lesson briefing

Welded Safety Output Contact

Reported symptom: The light curtain is interrupted, yet the contactor coil path remains continuous.

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

Isolate the panel and prove whether KSR1 output 13–14 actually opens when the safety relay drops.

Choose the fault category only when the measurements support it.

Hints

Hint 1

First confirm both OSSD inputs change state so the fault is not at the curtain.

Hint 2

Sign up to unlock the full hint sequence.

Hint 3

Sign up to unlock the full hint sequence.

This lesson uses 7 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.

Sign up to preview

Create a free account to preview this Pro lab; upgrade to open the interactive circuit.

Sign up to preview →

You might also like

Wiring 1A — Power the PLC
Wiring 1 — E-Stop Control Circuit
Wiring 2 — NPN / PNP Sensors (Sinking & Sourcing)

Competency and practice field guide

Welded safety output fault lesson: practice plan

Direct answer

The learner can separate logic command from final-element state, interpret monitoring feedback and explain why restart remains inhibited after a detected discrepancy.

Written for safety and maintenance learners studying why a command-off state may not remove energy when an output contact or final element is welded.

a supervised industrial electrical bench used to trace 24 VDC control power, terminals, relay contacts and PLC I/O with a correctly selected meter while studying welded safety output-device detection and restart inhibition
System map / 02

NODE 01observable

Scope

Hazard, safe state, safety output, contactor or final element, force-guided or mirror feedback, discrepancy monitoring, diagnostic coverage, reset and restart interlock.

NODE 02observable

Signal path

Safety demand through logic output, final switching element, hazardous energy path, monitored feedback and restart decision.

NODE 03observable

Baseline practice

A demand removes the commanded outputs and independent feedback confirms every final element reached its expected safe state.

NODE 04observable

Edge cases

Welded pole, welded auxiliary, open feedback, cross fault, delayed dropout, reset held, bypass, second channel and power return.

NODE 05observable

Fault practice

A demand, logic, output, final-element, feedback, timing, reset or restart mismatch.

NODE 06observable

Transfer to the job

The complete function inspected and validated on target machinery under the approved safety plan.

Answer surface / 07

What is a welded safety output?

It is a switching element that remains electrically or mechanically closed when the safety system commands it open.

Why monitor contactor feedback in a safety function?

Independent feedback can reveal that a final element failed to reach its expected state and prevent an unsafe restart.

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.