I’ve always found it hard to explain why a SIL 3 fire and gas system is typically a waste of resources, especially in general area coverage situations, but have now come upon a perfect analogy. Building a very robust system that will only work if used, but is easily avoid has been done before…
At the beginning of the second world war, the French invested very heavily in a set of fortifications against a Nazi attack. These fortifications were very robust and resource intensive, a military equivalent of SIL 3.
Unfortunately, the expensive fortifications are only effective if directly attacked. Which they weren’t. Instead, the Nazis essentially went around them.
What’s the lesson for fire and gas systems? Coverage is King, and SIL only plays a modest effect in comparison. Building a fortress fire and gas system that does not have enough detectors placed to allow the fire and gas events to be detected is a waste of resources. Don’t let your gas releases do the equivalent of sneaking around your fire detectors through the Ardenne Forest… Make sure you have adequate coverage before worrying about SIL targets of your electronics.