Implementing an override on safety PLC

A non-safety PLC cannot be part of a safety solution. Also, non-safety inputs cannot be part of the essential safety solution, even if they are in a safety PLC.
edit. In the typical safety PLC program, you simply cannot connect safety functions to non-safety inputs. However, you can still create something that is principally wrong, such as connecting non-safety devices to safety inputs. In that case the safety PLC program cannot know that you are using non-safety devices. However, the next step is to validate the safety, and it will fail.
Sounds like what I was thinking, but wanted to confirm.

I am thinking that the switch in question must be a key-switch that traps the key when it enables the override. And the key must be a specially ordered key with a unique encoding. Only 1 (one) key with the encoding must exist, and when not used in the switch the key must be in the hands of a specially trained person.

The key switch will only allow the key to be removed in the disabled position of the override. It's also the be kept in the safe of the site manager... and to be released under a permit to work.

There should be a way to also prevent startup of the potentially dangerous process if the key is in wrong position.

The valve that feeds the process is already a SIS function that gets activated on high temperature. When switching over the key, that valve will be shut thus not allowing any more material in.

One way could be to have two key-switches for the same key. Each enabling one of the two modes. It is called trapped key with key interlock (*). But that would cause a state of none of the modes being enabled for the time that you transport the key from one switch to the other, so you cannot have a change-over without stopping. I think that is not acceptable for cardosocea.
I've used them in the past (made by CASTELL) and for this application wouldn't really add that much. I could obviously default the none state to the lowest temperature level, but then it defeats the purpose of the key position in my view.
 

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