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