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EPO/NEC (was Re: Why do we use facilities with EPO's?)

  • From: Alex Pilosov
  • Date: Wed Jul 25 19:14:55 2007

On Wed, 25 Jul 2007, Leo Bicknell wrote:

> What I found interesting is that a single EPO is not a hard and fast
> rule.  They walked me through a twisty maze of the national electric
> code, the national fire code, and local regulations. Through that
> journey, they left me with a rather interesting tidbit.
> The more "urban" an area the more likely it is to have strict fire
> codes.  Typically these codes require a single EPO for the entire
> structure, there's no way to compartmentalize to rooms or subsystems.
> However in more rural areas this is often not so, and they had in fact
> built data centers to code WITHOUT a single building EPO in several
> locations.  That's to say there was no EPO, but that it may only affect
> a single room, or even a single device.
> If they can be avoided, why do we put up with them?  Do we really
> want our colo in downtown San Francisco bad enough to take the risk
> of having a single point of failure?  How can we, as engineers, ask
> questions about how many generators, how much fuel, and yet take
> for granted that there is one button on the wall that makes it all
> turn off?  Is it simply that having colo in the middle of the city
> is so convenient that it overrides the increased cost and the reduced
> redundancy that are necessitated by that location?
This is an interesting question.

National Electric Code (NEC) requires EPO. Sort of. Articles 645 and 685
deal with it.

While NEC is not binding on every jurisdiction, almost every US
jurisdiction bases its code on NEC with additions/subtractions. I don't 
know offhand if the local changes deal with EPO much, however, here's some 
food for thought regarding EPO and NEC.

With regard to "putting up with them" - EPOs are designed to protect life,
not property or uptime. If there's a short causing electrical fire because
breaker did not open, firefighter better be sure he can cut the power
*before* stepping next to it.

Here's how NEC works:

1) If a room is designed to comply with Article 645, it must have EPO, 
*except* if it qualifies under Article 685.

Being under Article 645 gives couple of things that are generally not 
permitted otherwise, as follows:

645.4 D) permits underfloor wiring for power, receptacles and 

645.4 E) "Power cables;  comunications cables; connecting cables;
interconnecting cables; and associated boxes, connectors plugs and
receptacles that are listed as part of, or for, information technology
equipment shall not be required to be secured in place".

In other words, you can have crossconnects that are laying on the floor
(or under raised floor but not otherwise secured), and that is OK, 
normally they'd need to be secured every X feet.

645.17) (too lazy to retype NEC language) You can have PDUs with "multiple 
panelboards within a single cabinet" - not all that clear what exactly 
does it permit (PDUs with multiple breaker panels essentially).

My understanding is that if you are willing to forego things that 
Article 645 permits, you do not have to install EPO. Frankly, I don't see 
all that much logic in 645 requirements and linking it to EPO (except, 
possibly, to make operation of datacenters not in compliance with 645 to 
be annoying enough that everyone would opt to comply with EPO).

The Article 685 exception from EPO applies if "An orderly shutdown is
required to minimize personnel hazard and equipment damage". It is really
intented for industrial (like chemical plants control) systems where EPO
shutoff can cause damage to life/property. I doubt this applies to 

Above is an armchair engineer's understanding. To be sure, you should 
consult a real engineer who can stamp and seal your plans!