Why cannot Programmable Controller (PLC) or Programmable Terminal (PT) be used for safety circuits?

Industrial standards IEC60204-1(JIS B 9960-1) prescribes that the safety circuit should not be configured only with the Programmable Controller.

This comes from the general requirements of the IEC60204-1(JIS B 9960-1) that safety-related functions should be configured with the verified circuit technologies and components. 

However, since circuits depending on the movement of programmable electronic devices may sometimes not fulfill these requirements, for this reason, Programmable Controller or Programmable Terminal cannot be used for safety circuits.

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