According to Article 5 of the Machinery Directive (MD), before a machine is placed on the market or put into service, the manufacturer must ensure that it satisfies the relevant essential health and safety requirements. The application of particular standards is not specified in law.
"Presumption of conformity" – User is acting in compliance with the law / directives
However, application of harmonised standards does trigger the "presumption of conformity", i.e. presumption that the machine has been designed and constructed in accordance with the relevant essential health and safety requirements. The burden of proof is therefore reversed in the event of a claim. If harmonised standards have not been applied, the burden of proof is on the manufacturer in the event of a claim.
Harmonised standards can also be called on to define the state of the art, which is the minimum requirement for the manufacturer (according to Annex 1 of the MD). A standard is deemed to be harmonised when it has been listed in the Official Journal of the European Union.
As a design engineer, if you should establish that the state of the art has not been applied in the machine design or that the machine does not satisfy machinery safety requirements, then you should notify your superiors of these defects in writing. Legally there is no obligation between the purchaser and the design engineer, but between the two firms. So normally the design engineer will not be sued in court. However, written documentation is advisable.