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Safety-related equipment

Must an operating mode selector switch be safely evaluated?

Based on the Machinery Directive (MD) 2006/42/EC and the relevant standards, an operating mode selector switch [OMSS] must always be present if the machine can have several operating modes and/or control sequences, requiring a different level of safety.

The following minimum requirements apply for the OMSS:

  • Each switch position may exclusively enable one operating mode
  • Operation of the OMSS alone may not initiate the plant/machine operation
  • The selected operating mode must override all other control functions on the plant/machine, exception: Emergency stop / emergency off

Safe evaluation of the OMSS is not clearly regulated by the MD and relevant standards. Some product-related standards (C standards) are an exception here. Nevertheless, the requirement that "each switch position may only enable one operating mode" can only be met by safe evaluation of the OMSS. As a standard controller cannot be assigned a category in terms of safety, neither can it safely evaluate an OMSS.


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