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PLC programming challenges
Challenge 17 · Make it safe · Free

A broken wire must stop it

Normally closed stop and overload contacts, the safe way round.

The Stop button and the motor overload are wired with normally closed contacts. StopOK and OverloadOK are ON while all is well, and turn OFF when Stop is pressed, the overload trips, or a wire breaks. Write the start and stop logic: Start latches the pump on, and either signal going OFF stops it at once. When the signal comes back, the pump stays off until Start.

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The 3D pump skid this challenge runs on
Test replay

Start, then Stop

Test 1 of 3
Set by the testChecked by the testStep 5 of 5 · 400 ms
Level
Easy · 10 points
Kind
Make it safe
Time
About 8 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. A press of Start latches PumpOn ON while StopOK and OverloadOK are ON.
  2. StopOK OFF, from a press or a broken wire, turns PumpOn OFF on the same .
  3. OverloadOK OFF turns PumpOn OFF too.
  4. When the signal comes back, the pump stays off until Start.
  5. Start does nothing while either signal is OFF.

The checks run 3 tests on the machine, scan by scan: Start, then Stop; A broken wire stops it and keeps it stopped; An overload trip stops it.

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

Stop buttons, guard switches and overload contacts are wired normally closed on almost every machine.

A normally open stop button with a broken wire looks healthy, and nobody can stop the machine until someone finds the fault.

The machine

How it should behave

StopOK
ON while Stop is not pressed and the wire is whole.
OverloadOK
ON while the motor is healthy.
Start
Latches the pump on while both are ON.

Press Run and Start. Switch StopOK OFF, as a broken wire would: the pump stops, and switching it back ON does not restart it.

Keep going

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