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Limitations of the EPO switch and the danger of downtime

An EPO switch is not required in all data centers. Data center owners should consider whether the risk of unplanned downtime is worth the advantages that installing an EPO can offer.

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

addressing the EPO switch and data center downtime.

In many cases, data center owners will see few tangible benefits to applying Article 645, but will put IT operations at risk of downtime caused by accidental or malicious use of the emergency power off (EPO) switch. This tip will explain the limitations and drawbacks of an EPO switch, and detail what precautions data center managers should take if one is needed.

The EPO switch made sense years ago, and there are many data center owners and operators that still tout the value of Article 645 and the use of EPO switches. However, there are some strong arguments to be made against electing to apply Article 645 in today’s data centers:

  • The amount of power required by today’s high-density racks requires so many dedicated circuits that the advantages data center managers once found with flexible power whips under the floor have virtually disappeared. Further, it is becoming more common to run power overhead, with products such as the Power Bus Way, which provides flexibility similar to the whips without adopting Article 645.
  • The large amount of copper cable once run in data centers is being reduced, as well as replaced by optical fiber, due to the newer network and storage topologies and increased speed requirements. Fiber carries many connections in a single relatively small cable, so the cost difference between plenum and non-plenum cables is greatly reduced. This can make the cost and benefit of Article 645 compliance, and the potential for accidental downtime, hard to justify.
  • Very few critical computing devices today will operate within the power limits of a 750 VA UPS (which is only 525 Watts with most small UPSes). To single out the most critical devices and power them individually with such small UPS units is probably not practical in most operations. Hundreds could be required to power all the interrelated critical devices, and maintenance would be an operational nightmare. It is very rare that the advantage of an orderly shutdown is worth maintaining so many UPSes, or the downtime that could occur if these non-enterprise grade units fail.
  • The use of isolated grounds in data centers is of no value, because everything is bolted into grounded metal cabinets anyway. At today’s processor clock frequencies, full signal reference ground grids are of debatable value as well. No matter how you try to isolate your grounding system, it still must ultimately be tied to the building ground, both by NEC requirements and for life safety. Better to follow the standards set forth by ANSI-J-STD-607-A (Commercial Building Grounding (Earthing) and Bonding Requirements for Telecommunication – 2002), than to succumb to the requirements of Article 645 for what is almost certainly of no benefit in any normal data center.

Cautions when using EPOs
There are cases where Article 645 can benefit the data center. In other cases, local authorities, such as the electrical inspector, fire inspector or local code board, may insist that an EPO be installed, even if Article 645 doesn’t actually apply. It’s important to observe some of the nuances involved, which are often overlooked or misinterpreted:

  • The EPO switch: This requirement is often misinterpreted in an attempt to make this dangerous switch “safe” by using a “break glass” unit--the type sometimes seen for fire alarms, where hitting the glass with an attached hammer allows the button inside to be released and popped out. This is not code compliant! Article 645 clearly states that pushing the button in shall disconnect the power.

Until the 2011 update to the NEC, Article 645 also required these buttons to be “readily accessible at the principal exit doors." The 2011 NEC update modifies this rule to allow the EPO to be in a nearby room, but only with prior agreement of the authority having jurisdiction (“AHJ”), and only if it is clearly identified and the local fire marshal is made aware of the location.

  • The IT equipment room must be fully contained within fire-rated walls, and all penetrations must be sealed with UL Listed fire-stopping material suitable for the size of each penetration and the types of construction through which pipes or ducts are passing.. All ducts passing through the walls must have automatic fire dampers.
  • A separate HVAC system must be provided and dedicated to the IT equipment room, or the system serving the room must be able to isolate the IT equipment  room with automatic fire dampers in the event of smoke detection or operation of the EPO. Fire dampers are often overlooked on ducts simply passing through the room.
  • The room must be occupied only by the personnel that are needed for the maintenance and functional operation of the IT equipment.
  • The EPO switch must instantly disconnect all power to the IT equipment and air conditioning, which includes disconnecting the UPS batteries (with the small exception cited above).

The installation of an EPO simply because a gaseous fire protection system (e.g., based on FM-200, Inergen or Novec) is used is unnecessary and can lead to accidental downtime. There are two reasons some people do this: First, all equipment power and air conditioning has to be shut down before releasing the fire-suppressing gas to keep the many fans in these systems from dispersing the gas and rendering it ineffective. The room must also be sealed with fire dampers that close before gas activation. Since this requires all the same circuitry that’s behind an EPO, many people have thought it logical to go ahead and install the “red button." Second, there is the mistaken idea that the EPO is simply required with a gas-based system. This is both untrue and illogical. The purpose of the EPO has always been for the fire department to have a foolproof way of shutting down power before entering a room that might have an electrical fire under the raised floor. Since the controls for the gas-based system already do that, there is no need for another means. Power will already be down by the time the fire department arrives.

Boxing in the EPO
If you’re going to have an EPO switch, it should be clearly marked and located inside a transparent, protective, lift-cover box that sounds an alarm if the cover is raised. This will usually prevent accidental downtime, and startle anyone who gets curious enough to look at what’s inside.

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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