Machine Directive & CE Marking After PLC Migration

You are replacing an S5 PLC with an S7-1500. The machine has a CE mark from 1998. Does the PLC migration invalidate the CE mark? Do you need a new conformity assessment? The answer depends on one critical concept: substantial modification.

This is one of the most misunderstood topics in industrial automation. Getting it wrong can mean operating a non-compliant machine — or spending tens of thousands of euros on unnecessary re-certification. This guide explains the rules clearly.

The Regulatory Framework#

Current: Machinery Directive 2006/42/EC#

The Machinery Directive 2006/42/EC has been the law since December 2009. It applies to machinery placed on the EU market and requires the CE marking to demonstrate conformity with Essential Health and Safety Requirements (EHSRs).

Coming: Machinery Regulation (EU) 2023/1230#

The new Machinery Regulation replaces the directive and applies from 20 January 2027. Approximately 90% of the requirements are identical. Key changes relevant to PLC migration:

• Cybersecurity requirements (Annex III, §1.1.9): Connected machinery must be protected against third-party attacks that could compromise safety functions. This is new and directly affects modern PLCs with Ethernet/OPC UA.
• "Substantial modification" clarified: The new regulation provides clearer definitions of what constitutes a substantial modification.
• Digital documentation permitted: Instruction manuals can be provided digitally (print only on request).
• AI in safety functions: If machine-learning algorithms influence safety-related decisions, additional conformity assessment may be required.

The Central Question: Is Your PLC Migration a "Substantial Modification"?#

Under EU law, the person who makes a substantial modification to a machine takes on the legal role of the manufacturer — with all obligations including CE marking, risk assessment, and declaration of conformity.

A modification is considered substantial if it meets ALL of the following:

1. It was not foreseen by the original manufacturer in the original risk assessment
2. It creates a new hazard or increases an existing risk
3. Existing protective measures are no longer adequate to address the changed risk

When a PLC Migration Is NOT a Substantial Modification#

Like-for-like replacement: Replacing a failed S7-300 CPU with the same model (or an identical functional equivalent) is maintenance, not modification. The CE mark remains valid.

Equivalent replacement with identical safety function: Replacing an S7-300 with an S7-1500 where the control program is functionally identical — same I/O, same safety logic, same interlocks, same behavior — is typically NOT a substantial modification if:

• The safety functions work identically
• No new hazards are introduced
• The risk assessment from the original machine still applies
• All safety-relevant hardware (E-stop, guards, sensors) remains unchanged

Key principle: If the machine behaves exactly the same way after the PLC swap as before — same inputs, same outputs, same safety behavior — the modification is not substantial.

When a PLC Migration IS a Substantial Modification#

Changed safety functions: If you use the migration to modify safety interlocks, change guard monitoring logic, or alter the E-stop chain, you have changed the safety concept. This is a substantial modification.

Added functionality: If the new PLC adds features the original machine did not have — new operating modes, higher speeds, new motion paths, remote access to safety-relevant functions — this may create new hazards.

Changed I/O configuration: If the migration includes rewiring sensors, adding outputs, or changing the electrical layout, this can change the hazard profile.

Network connectivity where none existed before: An S7-1500 connected to PROFINET/OPC UA introduces cybersecurity considerations that the original machine design did not address. Under the new Machinery Regulation (from January 2027), this is explicitly a safety consideration.

What You Must Do (Regardless of Substantial Modification)#

Even if your PLC migration is NOT a substantial modification, you should:

1. Document the modification — Record what was changed, why, and by whom. Keep this with the machine's technical file.

2. Verify safety functions — After every PLC swap, test all safety functions: E-stop, guard interlocks, safety sensors. Document the test results.

3. Update the risk assessment — Review the existing risk assessment. If the PLC swap does not change any hazards, note this explicitly. If it does, update the assessment.

4. Keep the original CE documentation — The original declaration of conformity, technical file, and instruction manual must remain available.

What You Must Do If It IS a Substantial Modification#

If the PLC migration qualifies as a substantial modification, you effectively become the manufacturer of the modified machine:

1. New risk assessment according to EN ISO 12100 (risk assessment and risk reduction for machinery)

2. New conformity assessment — Apply the appropriate conformity assessment procedure. For most machines, this is self-certification (internal production control). For high-risk machines listed in Annex I of the new regulation, a notified body may be required.

3. New CE marking — The modified machine receives a new CE mark with your name/company as the manufacturer.

4. New declaration of conformity — Referencing the applicable directive (2006/42/EC before Jan 2027) or regulation (EU 2023/1230 from Jan 2027).

5. Updated technical documentation — Including the new risk assessment, safety concept, circuit diagrams, and PLC program documentation.

The 2027 Transition: What Changes for Migrations#

If your PLC migration qualifies as a substantial modification AND is completed after 20 January 2027, you must comply with the new Machinery Regulation (EU) 2023/1230 — not the old directive.

Key new requirements:

• Cybersecurity assessment: If the new PLC is network-connected, demonstrate that remote access or network connections cannot compromise safety functions (Annex III, §1.1.9)
• Digital documentation: You may provide the instruction manual digitally
• If AI/ML is used in safety functions: Additional conformity assessment through a notified body may be required

How PLCcheck Pro Helps with Compliance#

PLCcheck Pro directly supports the documentation requirements of CE compliance:

• Program documentation: Generates the technical documentation of the PLC program that the technical file requires
• Safety function identification: Identifies safety-relevant code sections (E-stop, interlocks, guard monitoring)
• Before/after comparison: Documents the functional behavior before and after migration — evidence that safety functions are unchanged
• Risk assessment input: Provides the code-level understanding needed for an accurate risk assessment

Document your PLC program for CE compliance →

Frequently Asked Questions#

Do I need to involve a notified body for every PLC migration?

No. Most PLC migrations do not require a notified body. Only if the migration is a substantial modification AND the machine falls into a high-risk category (Annex I of the new regulation) is notified body involvement potentially required. For the vast majority of industrial machines, self-certification is sufficient.

My machine was CE-marked in 1998 under 98/37/EC. Is it still valid?

Yes. A CE mark is valid indefinitely as long as the machine is not substantially modified. Older machines CE-marked under 98/37/EC or 2006/42/EC do not need to be updated to the new regulation unless substantially modified after January 2027.

Who is responsible for CE after a PLC migration — the machine owner or the integrator?

If the migration is a substantial modification, the party who made the modification takes on manufacturer responsibility. This is typically the system integrator or engineering firm that performed the work. This should be clarified contractually before the project starts.

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Maintained by PLCcheck.ai. Last update: March 2026. Not affiliated with Siemens AG. This article is general guidance, not legal advice.