Uuuummm, STCA is not a safety tool, just a nice to have (although it saved my bacon a couple of times when I was an ATC!). Hence it can be off for Bypass mode at LATCC, DD&C etc. at LACC and so on.
Had a bit of a ruckus when I discovered this from the engineers! Seems that if it was safety then it would be a cat 2 tool with multi redundancy etc etc. Cost a fortune. As it is its a cat 5 (might have the numbers wrong, Engineers correct me please!) and costs a LOT less!!