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PLC programming challenges
Challenge 19 · Make it safe · Pro

Never let the pump run dry

Two seconds without water trips it. A splash does not.

Start and Stop already work. Protect the pump from running dry. If SourceLevel stays below 10% for 2 seconds, stop the pump and DryRunAlarm. A dip shorter than 2 seconds, from sloshing, must change nothing. While the alarm is on, Start does nothing. Reset clears the alarm only once SourceLevel is back to 10% or more, and the pump then waits for Start.

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The 3D pump skid this challenge runs on Pro challenge
Test replay

Two seconds low trips the pump

Test 1 of 4
Set by the testChecked by the testStep 6 of 6 · 2.5 s
Level
Medium · 20 points
Kind
Make it safe
Time
About 15 min
Pass rate
New
Machine
3D pump skid
Siemens
LAD and SCL
Rockwell
Ladder and Structured Text
Beckhoff
Structured Text
Automatic checks

What the checks test

  1. SourceLevel below 10% for 2 s stops the pump and latches DryRunAlarm.
  2. A dip below 10% shorter than 2 s changes nothing.
  3. Start does nothing while DryRunAlarm is ON.
  4. Reset clears DryRunAlarm only once SourceLevel is 10% or more.
  5. After a reset the pump stays off until Start.

The checks run 4 tests on the machine, scan by scan: Two seconds low trips the pump; A short dip changes nothing; Reset needs the level back; Start does nothing while the alarm is on.

These challenges teach the logic. On a real machine, safety functions run on a safety-rated PLC, such as a Siemens F-CPU or a Rockwell GuardLogix, and are designed to ISO 13849.

On site

Where you’d find this

A transfer pump draws from a day tank that can run low when a delivery is late.

A centrifugal pump running dry can wreck its mechanical seal in under a minute. The delay stops sloshing from tripping it for nothing.

The machine

How it should behave

Low level
Below 10% for 2 s: trip and latch.
Reset
Only once the level is back to 10% or more.
Restart
Only a fresh Start after the reset.

Press Run and Start, then let the source run down, or set SourceLevel to 5 in the Inputs tab. After 2 s the pump trips.

Keep going

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