If systems are life critical, you go so far as to use leased lines or other physical layer segregations. Treat it like a refinery, transformer substation, or natural gas pipeline terminal SCADA system.
EDIT: Easy on the downvotes folks. If you disagree, engage me in discourse. As an infrastructure/network/devops/it generalist role, I have seen terrible things happen when you don't properly segregate critical systems from public networks.
http://scadastrangelove.blogspot.com/