We are working in a modification project on a chemical plant where we have to implement a logic for a protection system where we have valves limit switches involved. In the Hazop arised a question related to the protocol used to wire them. For the alarms we use negative logic, that is to say no signal => problem, and for the limit switches in other parts of the plant that are not involved in the protection system we use the closed switch of the SPDT to indicate the position of the valve. There are some discrepancies between different engineers in the best protocol to follow.
Someone of you knows about a methodological recommended practice to follow for the wiring of those limit switches involved in a plant protection system?
Thanks in advance.