Now I'm curious... in all of the DCs and COs I've worked in - to the best of my knowledge, I haven't personally tested this! - the EPO button does not switch to emergency power. It turns off ALL equipment power in the space - no lights, no klaxons, nothing. In simpler setups, the EPO is connected to the UPS so anything plugged in to the UPS does dark instantly. In one DC I'm familiar with, the EPO switch kills all the UPS output and uses several relays to kill commercial power at the same time.
In some, the room lights were not covered by the EPO switch, in some they were. Emergency exit lamps will continue to be lit, as they have internal batteries, and are required by building/fire code.