No Code book this morn, but to add to Bob's post, the emergency unit can't be cord connected if it is a REQUIRED emergency light for means of egress. If it's just for your convenience, you can connect them with 3' cords until the cows come home.
Bob and Scott, Coming from a place like I do, we are required to wire any sort of Emergency lighting into a permanent supply. That means hard wired, with no cords. The wires would enter through the back of the fitting. It also prevents someone from in advertantly disconnecting your Em lighting system.
Oh and by the way guys, is it a requirement to have a disconnect "within sight" while working on that Cylinder?. Over here you must have an adjacent isolator at the Hot water cylinder. BTW, that cord used on that cylinder looks far too DIY for my liking.
the emergency unit can't be cord connected if it is a REQUIRED emergency light for means of egress.
Not that I have ever cord and plug connected an emergency light but it is allowed by the NEC to do that with even required emergency lights. The article I quoted is 700 which covers the required emergency lighting.
That aside I would have no trouble believing another code prohibits it. It seems like a poor practice.
Here in MA they have deleted the sentence from 700.12(F) that allows cord and plug connection.
Bob Badger Construction & Maintenance Electrician Massachusetts