Hello HN39,
Originally Posted by HazelNuts39
takata;
as shown in the diagram produced by mm43 #946 (my thanks to mm43 for that) the negative relief valve is not controlled by the CPC. Intended as an 'emergency' backup, it is often no more than a simple mechanical device that requires no more than a negative pressure differential to open. As explained by Chris Scott, that happens when the aircraft overtakes (catches up with) the cabin, which is unlikely to occur above 7350 ft aircraft altitude.
Edit: Ok about the mechanical device, but this alert is unrelated to it.