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PLC programming challenges
Challenge 20 · Make it safe · Pro

Trip before it overflows

An independent high-high switch that works when the transmitter doesn’t.

Normal level control already fills to 80%. Add an overfill trip that does not rely on the level transmitter alone. LevelHH, a separate high-high switch, or Level at 95% or more must close FillValve at once and OverfillTrip. While tripped, Start does nothing. Reset clears the trip only when Level is below 95% and LevelHH is OFF, and filling waits for Start.

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The 3D process tank this challenge runs on Pro challenge
Test replay

The high-high switch trips on its own

Test 1 of 5
Set by the testChecked by the testStep 4 of 4 · 240 ms
Level
Medium · 20 points
Kind
Make it safe
Time
About 15 min
Pass rate
New
Machine
3D process tank
Siemens
LAD and SCL
Rockwell
Ladder and Structured Text
Beckhoff
Structured Text
Automatic checks

What the checks test

  1. Normal filling still stops at 80%.
  2. LevelHH, or Level at 95% or more, closes FillValve on the same and latches OverfillTrip.
  3. LevelHH trips on its own, even when Level reads normal.
  4. Start does nothing while OverfillTrip is ON.
  5. Reset clears the trip only once Level is below 95% and LevelHH is OFF.

The checks run 5 tests on the machine, scan by scan: The high-high switch trips on its own; 95% on the transmitter trips too; Latched until reset, then Start; Reset does nothing while the switch is still made; Normal filling still stops at 80%.

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 storage tank has a level transmitter for control and a separate high-high switch for protection.

A stuck transmitter can read a normal level while the tank overflows. A separate switch catches what the transmitter misses.

The machine

How it should behave

Normal
Start fills; 80% or Stop ends the fill.
Trip
LevelHH, or 95%, closes the valve and latches.
Reset
Only once both have cleared. Then Start.

Press Run and Start. While it fills, switch LevelHH ON in the Inputs tab: the valve closes and OverfillTrip lights.

Keep going

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