Chapter 8.10 – Why Selmo is a valuable tool even when conformity is lacking
Introduction
In practice we often encounter existing machines and systems that do not fully meet the requirements of the Selmo standard. There can be many reasons for this: Missing sensors, incomplete feedback, older control systems or simply economic limits in a retrofit project.
The central question then is:
"If a system is not fully Selmo-compliant – is it worth using the process model at all?"
The answer is: Yes – absolutely. Because even under these conditions Selmo offers a decisive added value: It makes the behavior of the system visible, explainable and documented – and that is an essential component of the state of the art and the legal responsibility in mechanical engineering.
1. The formal framework remains valid
Selmo is based on the principle that every technical behavior of a system can be described in a formal model This model does not replace hardware – but it orders it logically. Even if feedback is missing or only partially available, the model remains a structured mirror of the process.
Therefore:
Every state, every zone, every action is defined, even if their monitoring is incomplete is.
The model clearly shows: "An action takes place here – a reaction is missing here."
This gap becomes visible, assessable and documented.
That alone already represents progress compared to conventional programming, in which such gaps are mostly implicitly hidden in the code remain.
2. Documentation creates responsibility and legal certainty
The Machinery Regulation (EU) 2023/1230 explicitly requires that safety according to the state of the art is demonstrated by appropriate technical and documented procedures .
A Selmo model meets this requirement – even if it does not fully monitor all signals. Because:
It documents objectively, which functions are fully monitored and which are not.
It makes every deviation conscious and comprehensible.
It enables the manufacturer or operator to assess and justify these deviations in the risk assessment (according to ISO 12100) .
This turns uncertainty not into a hazard, but into a conscious, documented decision. And that is the decisive difference legally and in terms of safety.
3. The process model as a cognitive framework
Even in incomplete systems Selmo provides a formal thinking aid: It forces you to think in cause-effect relationships rather than in signals and addresses.
This means:
Every action must be assigned to a condition (cause).
Every effect must – if present – be secured by feedback.
Where this is not possible, a fallback mechanism (timer, plausibility, acknowledgement) is defined.
This creates a complete representation of the behavior, even if the physical monitoring is incomplete.
In teaching we call this a reduced but complete model – it does not perfectly map reality, but it is complete within the scope of the known information.
4. Integration of special functions
In industrial practice there are repeatedly situations in which certain functions do not fit into the standardized process model. This can be, for example, a very specific control, a communication module or a safety block.
Selmo offers three graded options for this:
Function blocks within the model → special logic, but formally embedded in states and zones.
Extended logic outside the model, but documented → manual programming with clear labelling "outside Selmo structure".
System extension at the engineering level → integration in E-Plan, CAD or PLCopen environment – with reference back to the Selmo model.
As a result the system remains traceable and verifiable, even if not every function is directly formulated in the Selmo syntax.
5. Comparison with established design documents
In engineering it is self-evident that a P&ID, a Circuit diagram or a CAD drawing documents the technical structure. None of these plans directly creates safety – but they create traceability and responsibility.
Selmo takes on this role for the behavior of the machine .
One can say:
Selmo is to behavior what the circuit diagram is to the electrical system.
Even if not every valve or every sensor is perfectly integrated, the model remains the central document, which shows how the machine should behave and what actually is present . is.
Thus Selmo forms a bridge between engineering and reality:
formal thinking,
structured representation,
and a clear documentation of deviations.
6. Conclusion: Awareness before completeness
In engineering science a simple principle applies:
"What is described can be checked. What is not described remains chance."
Selmo ensures that everything is described – including what is (still) not perfect. And that is precisely state of the art in terms of responsibility:
Technical uncertainties become visible.
Decisions are made consciously and documented .
The entire process remains formally comprehensible – regardless of whether every feedback signal exists.
Selmo is thus not only a tool for ideal systems, but a tool for reality: for existing plants, for retrofit projects, for step-by-step digitalization and safe further development.
In short: Even if Selmo conformity is (still) not fully achieved, Selmo provides formal clarity, technical responsibility and legally secure documentation – and that is the basis of every modern, safe machine.
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