The phrase "closed circuit" means that something is happening within a self-contained system or loop, where it does not connect or interact with anything outside of that system. Full definition
Dogs are monitored on closed circuit television while they sleep. (tworockdogranch.com)
Children well enough to come into a public area attended in person and about 150 other children, unable to move about, watched the demonstrations on closed circuit TV in their rooms. (amcny.org)
You might even want to install a security system or closed circuit cameras around your home's perimeter. (trustedchoice.com)