Relay Redundant System
Let’s see a simple example of how we can increase the safety of our design using conventional electromechanical relays.
Suppose we have a high pressure steam line with three safety pressure switches wired to an intermediate relay. We also make a wired 2 out of 3 logic, so that the circuit is open when at least two switches are detecting high steam pressure.
The second contact of each relay is wired to a digital input of PLC to program a 2oo3 logic like the other one.
The output to the valve is considered in our design as a “critical output”, so we doubled the output, each wired to an external relay, and the contacts are wired in H-shaped. It is a typical high availability wiring, tolerant to digital output failure or relay failure.
The H-shaped output is a good solution in many cases but sometimes has certain risks. If it’s an output normally energized it can short-circuited without notice. If there is a short-circuit in the second digital output our system would remain in a high risk situation.
If we do not use a safety PLC, we can at least increase safety by wiring in series the “Redundant Relay System” that is completely independent of the PLC.