Hi,

I am in the process of designing a Hardwired Safety System (HWSS), which requires a significant number of safety relays. A decision has been made to use safety relays through out the system even if they dont for part of a SIF.

In my system, apart from Safety Instrumented Functions (SIFs), there are logic conditions that determine specific system states. For example, a combination of limit switches can indicate when the system reaches a particular state.

I initially planned to use Siemens 3SK series safety relays throughout the design—for both checking logic states and switching the final element.

However, while exploring options, I found that safety coupling relays like 3RQ1000-1HB00 are significantly cheaper compared to the 3SK safety relays. This has led me to consider using 3SK relays only for the final element, while using 3RQ relays to determine logic states.

According to the datasheet, the 3RQ1000-1HB00 safety coupling relay is primarily intended as an interface or expansion module, with safety PLC or safety relays. However, after reviewing the block diagram and description, I don't see a technical reason why it couldn't be used to determine a logic state.

My question:
Can the 3RQ1000-1HB00 safety coupling relay be used for logic state determination? For example, if a set of valve limit switches are connected in series and fed into the relay input, would the relay output reliably indicate that the valves are in position?

Any insights or experience with this approach would be greatly appreciated!