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Addressing the EPO switch and data center downtime

An EPO switch can be a major liability, potentially leading to accidental downtime, but electrical code doesn’t require all data centers to have a big red button.

This is the first article in a two-part series on the EPO switch in data centers. Read the second article about...

the limitations of the EPO switch and the danger of downtime.

The most dangerous thing in a data center is you, and if you have an emergency power off (EPO) button next to each exit door, you could become an even bigger hazard. Reports have listed human error as the major cause of data center downtime. Of all the ways people can bring a data center down, the EPO is the most common cause and the biggest concern for data center managers. It is hit accidently, mistaken for a door exit button and pushed by disgruntled employees their last time out the door. Careless or unknowledgeable electricians breach EPO wiring, also resulting in downtime. Some EPOs are even inadvertently wired around so they are no longer functional. Many are installed in ways that violate the very code that required them in the first place. This tip explains when electrical Code may require an EPO switch, and when the "big red button" puts a data center at risk of unnecessary downtime.

Power and the EPO
If the EPO switch is so potentially dangerous in our data centers, why do we have it, and do we even need it? That’s a matter of history morphed into legality, and the legality today is often misinterpreted.

The EPO originated with big mainframe computers that used a lot of power and were so expensive it was better to “crash” them than to let them burn up. So, electricians installed big red emergency shutdown buttons that operators could push if necessary. The next logical step was to shut down power to the whole room. The modern EPO does just that, and it’s pretty dramatic. It instantly shuts down all equipment and air conditioning in the data center, including uninterruptable power supplies and batteries. This will result in a “hard crash” of the computers.

There’s no bypass (at least, there is not supposed to be), and there’s only a tiny exception that might allow for a gradual or orderly shutdown of the most critical computing equipment. If an EPO is actually needed (there are few circumstances in which it is actually necessary), it has to be readily accessible next to every exit door–that is, unless your state or municipality has adopted the 2011 code, which offers a possible alternative explained later in this tip, but local committees determine when new codes are implemented in their communities, and most state and local code committees probably won’t adopt the new code for at least several years.

National Electrical Code and the EPO
The EPO switch is a requirement of Article 645 of the National Electrical Code (NEC). Article 645 first appeared in the NEC in 1968. The title of the article, Information technology equipment, has changed over the years to recognize that equipment, such as telephone systems, are now installed in computer rooms along with traditional data processing equipment.

The current and former titles, including Data processing systems and Computer/data processing equipment could lead someone to infer that the standards in Article 645 apply to anywhere a computer is located. Quite the contrary! Chapter 6 of the NEC, where Article 645 appears, allows modifications of the general rules, which are listed in Chapters one through four. These general rules cover special spaces, certain types of equipment or unusual conditions. So, Article 645 is “permissive.” It permits you to do certain things that would otherwise be contrary to the normal “prescriptive” requirements of Chapters one through four, but only if you are willing to follow some other rules. One of those other rules is to install an “approved disconnecting means” at each main exit door, which is commonly known as an EPO, although that term is not used or recognized by the NEC. If you don’t want to do that, or don’t find the “advantages” of Article 645 beneficial, you are perfectly free to just abide by Chapters one through four, and avoid installing an EPO switch.

In fact, terms such as data center and computer room are not even recognized by the NEC. But that still does not require you to declare every room with IT equipment as an information technology equipment room, or to comply with the requirements of Article 645. You only need to do that under two conditions:

  • If you are running air and power and/or cables under a raised access floor. If there’s no raised access floor, or it’s not used for cooling, Article 645 is not applied in the first place. That is one of the reasons a number of people are advocating the elimination of the raised floor. A subsequent article will discuss the raised access floor in more detail
  • And if you want to take advantage of the deviations from requirements in Chapters one through four that Article 645 allows.

Primarily, these “deviations” are:

  • Flexible power whips (receptacles that are not secured in place under the floor).
  • Use of non-plenum communications cable in the air plenum floor cavity.
  • Use of multiple, small UPS units (750 volts-amperes (VA) or less) to enable an orderly shutdown of critical equipment instead of a “hard crash."
  • Certain grounding options.

About the author: Robert McFarlane is a principal in charge of data center design for the international consulting firm Shen Milsom &Wilke LLC. McFarlane has spent more than 35 years in communications consulting, has experience in every segment of the data center industry and was a pioneer in developing the field of building cable design. McFarlane also teaches the data center facilities course in the Marist College Institute for Data Center Professionals  program, is a data center power and cooling expert, is widely published, speaks at many industry seminars and is a corresponding member of ASHRAE TC9.9 which publishes a wide range of industry guidelines.

This was last published in July 2011

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