An (old) plant is fitted with a general emergency off (PNOZ5 with PZE5, single-channel) and some area emergency stops (PNOZ5 with PZE5, single channel). Part of the plant is now to be modernised, as part of which the area emergency stop will be supplied as a dual-channel device. The installer is now claiming that you cannot mix single and dual-channel circuits and that everything will be need to be converted to dual-channel!?
Details of how the emergency stop or area emergency stop must be designed are set out in the plant's risk assessment. However, an emergency stop or area emergency stop should always be designed in such a way that it stops the worst hazard on which the pushbutton acts, with sufficient safety.
If the area emergency stops are dual-channel due to the hazards from the plant sections, then the higher level emergency stop must also be dual-channel to ensure that safety is not diminished.