Consider a simple example:
- there's a vessel with water and a continuos level sensor (4-20 mW signal)
- there's a water line with a magnetic valve, feeding the vessel
- When the water level is below a certain treshold, the valve opens - this is realized via a PLC unit somewhere
Now, consider the same scenario, the water level is critical for human safety so the whole refilling function needs to be SIL II. Now I - not an electrical or control systems engineer - need to understand how much more complications, cost, etc. will arise due to the SIL II requirement.
p.s. To give you an idea how much or little I know: My understanding is that I either need a safety PLC or a SIL II certified measurement amplifier with an additional potential free contact set to the correct level, hardwired to the relais controlling the valve.
Since SIL are describesd in an international IEC norm, I assume the answer will not be highly dependent on local codes.