Substantial modification under the Machinery Regulation: when a PLC retrofit makes you the manufacturer

A substantial modification under the EU Machinery Regulation is an unplanned change to a machine already in use, made by physical or digital means, that creates a new hazard or increases an existing risk to the point where extra guards or protective devices needing changes to the safety control system, or extra measures for stability or strength, are required. Whoever carries out that modification is treated as the manufacturer from 20 January 2027, with the full duties that go with it (Regulation (EU) 2023/1230, Articles 3 and 18).

For plants planning S7-300 replacements and for integrators quoting retrofits, this is the question that decides whether a project ends with a test record or with a new CE marking. We are controls engineers, not lawyers. Use this article to structure the technical assessment, then settle the legal conclusion with your CE partner or notified body. The Regulation replaces the Machinery Directive 2006/42/EC on that date, as the IHK Region Stuttgart guide to the new rules sets out.

What the regulation actually says about substantial modification

Article 3(16) sets four conditions. The modification happens after the machine was placed on the market or put into service. It was not foreseen or planned by the original manufacturer. It affects safety by creating a new hazard or increasing an existing risk. And dealing with that requires either additional guards or protective devices whose processing needs a change to the existing safety control system, or additional protective measures for stability or mechanical strength.

Two details matter for controls engineers. First, "physical or digital means" brings software changes explicitly into scope. Second, the safety control system is named in the definition itself, so changes that touch safety logic deserve the closest look.

Article 18 then says that a natural or legal person who carries out a substantial modification is considered a manufacturer and takes on the obligations of Article 10 for that machine, or only for the affected machine within an assembly where the risk assessment shows the impact is limited to it. That includes declaring conformity and applying the relevant conformity assessment procedure. Non-professional users modifying their own machines for their own use are excluded.

How Germany has handled the question so far

Germany is not starting from zero. Under the Machinery Directive, a 2015 interpretation paper from the Federal Ministry of Labour and Social Affairs (BMAS) set out when a change to a machine counts as a wesentliche Veränderung, with a decision flowchart, replacing an earlier paper from 2000 (IHK Pfalz). The same chamber notes that operators of older machines cannot rely on grandfathering: the Ordinance on Industrial Safety and Health (BetrSichV) requires machines to be safe to use according to the state of the art, and the question of substantial modification should be considered when orders and offers are written.

Typical retrofit scenarios

The table is a guide, not a verdict. The same physical change can fall either way depending on the machine and its existing protective measures, which is why the risk assessment is the evidence that counts.

A decision process for controls retrofits

1. Document the original state before you touch anything: program versions, safety program checksum, safety functions and their Performance Levels, drawings and the existing risk assessment if one exists.
2. Describe the change precisely, including software. "Replace CPU and migrate code" is not enough; list functional changes, new modes and changed parameters.
3. Run a risk assessment of the change using the ISO 12100 method. Ask whether it creates a new hazard or increases an existing risk.
4. Check whether existing measures still cover it. If they do, the change is not substantial under the definition. Record why.
5. If new guards or protective devices are needed and they require changes to the safety control system, or stability measures are needed, treat it as substantial and plan the manufacturer's obligations: technical documentation, conformity assessment, declaration and marking.
6. Agree in the contract who carries the manufacturer role. The operator commissioning the work, the integrator doing it, or a general contractor. Leaving it open is how projects end up with nobody responsible.
7. Validate safety functions after the change either way. See our guide to safety software validation under EN ISO 13849.

Why simulation helps with the evidence

The strongest argument that a controller migration is not a substantial modification is evidence that the machine behaves exactly as before. Testing the migrated program against a simulated machine, with the same test cases as the original, produces that evidence before the shutdown. Our guides to S7-300 end of life and STEP 7 to TIA Portal migration describe how. The broader software duties under the regulation, including protection against corruption and the safety software update log, are covered in our article on the Machinery Regulation and control software.

Where Wartens fits

Wartens supports German machine builders and plants with S7-300/400 migration, safety-software documentation and the engineering evidence behind a Machinery Regulation assessment, working alongside your CE partner. We do not issue CE markings. See our automation engineering services and the Germany overview of 2027 rules.

Next step: send us a description of the planned retrofit and the existing safety documentation, and we will tell you what technical evidence the assessment will need. Contact us.