I've studied the NEC 2008 version and I can't find any requirement for an EPO to be mounted on each rack. For some reason the place I work requires them at the top of the rack and it's connected to the Pulizzi power distribution units at the bottom of the rack. They also have an EPO at the entrance to the server room which cuts power to all the circuits and CRACs. Is this a legacy thing?
Are these EPOs fired by the alarm panel or are they a Big Red Button Of Don't Touch? Or both?– sysadmin1138 ♦Nov 18, 2010 at 2:38
The EPOs are a manual big red button and they only shut down power to the rack.– murisoncNov 18, 2010 at 3:30
I find it odd that an EPO would be at the top of a rack, as that's over 2 meters tall, and anyone except my wife would have trouble reaching it in an emergency (which is exactly what it's for... an emergency power off). And I'm not exactly short, so if you were a shorter person then the button would be worse than useless (as it provides a sense of security but if you can't reach it then you're stuffed).
I've also never seen each rack with its own EPO, only one for the entire room. Much like a petrol station only has one emergency cutoff button for the whole station, not one for each pump.
Having an EPO on the CRAC at the exit could make sense, as if there's a fire in the server room, then air conditioning is a very easy way for it to spread to the rest of the building, so as you're bolting out of the room you hit the button on your way out.
I've seen EPOs in the rack (middle though, not top) and the reason was because each rack had independent power and UPS systems installed at the bottom of the rack. It seemed ridiculously expensive but it did seem to ensure that the racks were not overloaded (powerwise)