I don't run an emergency drain. No need if you know nothing will completely stop flow. If link doesn't work. Just type gmac herbie which is a total explanation of it.. I've been using this type of drain since this Herbie guy came up with it. I got into reefing about that time. Only type I've ever used and will.
http://gmacreef.com/herbie-overflow-reef-tank-plumbing-method-basics/
http://gmacreef.com/herbie-overflow-reef-tank-plumbing-method-basics/