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Practice missions
Mission 66 · Pro PLC programming exercise

Prove the contactors let go

Check the contactors' mirror contacts, and refuse a reset while one is welded shut.

Accept a Reset only when both contactors are proven open, and a fault when either one fails to follow its command within 0.3 s.

Training only. This is not a certified safety system. On a real machine these rules run in a safety relay or a safety PLC, designed and validated to ISO 13849-1 or IEC 62061.

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The 3D robot safety cell this mission runs on Pro mission
Test replay

Start and stop with healthy contactors

Test 1 of 4
Set by the testChecked by the testStep 12 of 12 · 1.6 s
Level
Advanced
Time
About 40 min
Reward
+450 XP
Rig
3D robot safety cell
Siemens editors
Ladder (LAD), SCL
Rockwell editors
Ladder, Structured Text
Beckhoff editors
Structured Text
Course stage
33. Safety lab 2: trust nothing single
Automatic tests

What the tests check

  1. Reset is refused unless K1Open and K2Open are both ON.
  2. A contactor that stays closed after its command goes OFF latches a fault within 0.3 s.
  3. A contactor that does not close when commanded latches a fault too.
  4. The fault switches K1, K2 and DriveRun OFF and lights FaultLamp.
  5. Only a Reset with both contactors proven open clears it.
  1. Try it Use the buttons below the model and watch the rig respond.
  2. Test solution Every check runs on your code in one press.
  3. Replay If one fails, step through it one scan at a time to see why.
Why it matters

Where you’d see this on site

Safety relays have a feedback loop for the contactors' mirror contacts. Checking it before every reset is called EDM, external device monitoring.

Two contactors in a row mean one welded contactor cannot keep the power on. But the second fault, the other contactor welding, would. EDM finds the first weld before the second can happen.

Where you'd find thisKeep people out while the arm moves, and prove it every time

A robot stacks cartons inside a fence. A gate lets a fitter in, a light curtain lets the cartons in, and an emergency stop sits outside. Two contactors in a row cut the robot's power, so one that sticks cannot keep it running.

  1. InfeedCartons through the curtain
  2. PickRobot takes the carton
  3. GuardStop, reset, restartYou are here
  4. PalletSix cartons, then change
On a real site: places, parts and safety
  • Robot palletising and packing cells
  • Machine tending: robots loading presses and machine tools
  • Any guarded machine with a door, a curtain or an E-stop

Training only: this rig is not a certified safety system. Real safety functions run in safety relays or a safety PLC, designed and validated to ISO 13849-1 or IEC 62061.

IEC 60204-1 sets out stop categories 0, 1 and 2 and how an emergency stop must behave. Which category a machine uses comes from its risk assessment.

The sizes, timings and cutaways are teaching choices, not a drawing of any one real installation.

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