Safety stops must have the highest priority
Article#: 00118
Date: 2026-03-08
Author: Radim
In the previous article , I introduced different types of machine stops and their intended purpose.
This article takes a closer look at how these stop types interact in practice and how their priorities are implemented in machine control systems.
A machine is usually controlled by two types of logical controllers.
A standard PLC (Programmable Logic Controller) manages the machine control and executes the production program, while in parallel a safety PLC monitors and enforces the safety-related functions of the machine.
Both controllers run their own logic programs.
They receive signals from operator controls and sensors and generate commands for actuators, drives, and contactors that ultimately fulfill the machine functionality.
The standard PLC executes the machine program and generates commands for the actuators, drives, and other components that perform the machine’s intended functions.
However, the system architecture - both in terms of wiring and control logic - must ensure that the safety PLC can always enforce its safety functions whenever required.
If a hazardous situation is detected or a safety function is triggered, the safety system can override or interrupt the commands issued by the standard PLC.
In practice, this is achieved by designing the control architecture so that safety-related signals directly influence critical power paths or control signals, ensuring that safety actions take precedence over normal machine operation.
This architectural principle ensures that safety-related stops always have priority over any other commands, in practice mainly over commands controlling normal machine operation or executing functions.
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Related previous articles:
- Different types of machine stops
- Buttons: Start, Stop and one more
- Redundant PLC – Yes or No?
- How long will a failure keep our production down?
- Resilient machine control design
- When a touch panel isn't enough
- Hidden dangers during commissioning
- Dream of a platform-independent PLC program
- PLC network diagram
- It doesn't work without sensors
- Talk to operators
- Clear signals
- Safety relays versus safety PLC
- Machine modularization
- Modular software architecture
- Hardware concept
- A good concept = Complete set of suitable solutions
- Are there also any disadvantages?
- Think critically!
- Design machines with a focus on user's abilities and needs
- Why do automation projects fail?
- Collect all the requirements and sort them!
- HMI and PLC applications should grow together
- More than just start and stop
- Safety first!
- Do the right things and do them right!
- Define test scenarios and test cases continuously!
- Don't make it worse!
- What does the customer need?
- Don't disappoint your customer!
- Focus on target!
- Check the result!
- Success is not a matter of coincidence
- Development is evolution
- Transparency and reliability
- From an idea to a consistent system