History of the EPO (Emergency Power Off)

Thu Jul 26 03:22:49 UTC 2007

At 08:10 PM 7/25/2007, Sean Donelan wrote:
>Sometimes you need to revisit the rules.  For example, for folks
>thought having automatic water sprinklers in data centers was a bad 
>thing. Slowly folks have started to rethink it, and now automatic 
>sprinklers are
>found in more data centers.  I don't have hard data, but my experience
>is there have been fewer outages from accidental sprinkler discharges
>than from accidental EPO activations.

There was an interesting study conducted by the US Air Force about 
fires and other failure modes in computing facilities protected with 
Halon/FM200/FE227 vs. dry pipe preaction. I know I saved the PDF, but 
I can't seem to find it at the moment. If my memory is correct, it 
boiled down to the fact that there had only been two fire incidents 
at all US Air Force installations and both were due to (surprise, 
surprise) human factors. One was a stray incendiary munition which 
breached the datacenter and other was due to a Jet A fuel spill and 
fire - which is odd because it is hard to ignite kero, diesel, jet A 
without atomization. The point of the study was that there was zero 
damage over a 30 year period from water based fire protection systems 
and I suspect it was pretty handy to have sprinklers when both 
datacenter fires happened. The munition breach of the physical 
structure would have rendered any gas based fire suppression system 
ineffective.

In theory, I'm not a big fan of EPOs due to the "Is this the button 
to exit/open the door?" problem. One of our redundant 150KVA UPS 
units caught fire a couple years ago, the input breaker became the 
EPO since the on-board front panel EPO was completely ineffective 
(and it still would have been ineffective had it been connected to an 
external EPO button.) That incident prompted a design change in all 
of our new datacenter power systems since and all existing systems 
were also updated. Now all UPS units have separate input and bypass 
breakers and feeds. Previously we used a single feed, but you can't 
isolate a burning UPS without dropping your attached load when they 
share a single breaker and are tied together inside the unit where 
the fire is happening. Having discrete A & B power systems is also a 
very good thing!

Many years ago when we were much, much smaller, the EPO was wired to 
a special EPO circuit breaker on the main panel which fed the 
subpanel for the datacenter room. A short on that breaker was like 
pressing the "test" switch on a GFCI breaker. Do most people who do 
have functional (as opposed to decorative) EPO buttons have them 
connected to the building/suite mains disconnect? or to the output of 
your UPS units? to a special EPO panel which trips the EPO cutoffs on 
other units?

-Robert

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