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PLC programming challenges
Challenge 09 · Find the fault · Pro

The alarm you can’t clear

After the storm, Reset does nothing. During it, Reset worked.

HighAlarm latches when the water reaches the high-high float. It stays on after the water drops, until someone presses Reset. Reset must clear it only once the water is below the high-high float. Find why it does the opposite and fix it. Leave the pump logic working.

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The 3D duty standby sump pumping station this challenge runs on Pro challenge
Test replay

Operator report: Reset after the storm

Test 1 of 3
Set by the testChecked by the testStep 5 of 5 · 620 ms
Level
Medium · 20 points
Kind
Find the fault
Time
About 12 min
Pass rate
New
Machine
3D duty standby sump pumping station
Siemens
LAD and SCL
Rockwell
Ladder and Structured Text
Beckhoff
Structured Text
The operator’s report · duty standby sump pumping station
During Tuesday’s storm the high-high alarm came on, as it should. The sump has been back to normal for hours, but Reset won’t clear the beacon. Odd thing is, during the storm, holding Reset did put it out.
Automatic checks

What the checks test

  1. HighAlarm turns ON when FloatHighHigh turns ON.
  2. HighAlarm stays ON after the water drops.
  3. Reset clears it once FloatHighHigh is OFF.
  4. Reset does nothing while FloatHighHigh is still ON.
  5. Pump1 still pumps down between the floats.

The checks run 3 tests on the machine, scan by scan: Operator report: Reset after the storm; Reset during the storm does nothing; The duty pump still works.

On site

Where you’d find this

A storm-water sump has a high-high float and a beacon that the operator resets after a storm.

An alarm that cannot be reset gets ignored, or the PLC gets restarted to clear it. An alarm that resets while the cause is still there gives false comfort.

The machine

How it should behave

High-high
FloatHighHigh latches HighAlarm.
Reset
Clears it once the water is below the float.
Pump1
Starts at the high float, stops below the low float.

Pause, then in the Inputs tab switch FloatHighHigh ON for a scan and OFF again. HighAlarm stays on. Press Reset: it must clear.

Keep going

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