Insight and analysis on the data center space from industry thought leaders.

Data Centers Should Trust Emergency Power to UL 1008 Listed Automatic Transfer Switches

When utility power at a data center fails, automatic switchover to an emergency backup power system that enables “business-as-usual” is the ideal scenario, writes Bhavesh Patel of ASCO. But sometimes that doesn’t happen because the transfer switch itself fails.While automatic transfer switches (ATSs) are the norm in data centers, they are not all the same.

Industry Perspectives

February 6, 2014

6 Min Read
Data Center Knowledge logo

Bhavesh Patel is director of marketing and customer support, ASCO Power Technologies, Florham Park, NJ, a business of Emerson Network Power.

ASCO-Bhavesh Patel




When utility power at a data center fails, automatic switchover to an emergency backup power system that enables “business-as-usual” is the ideal scenario. But sometimes that doesn’t happen because the transfer switch itself fails.

While automatic transfer switches (ATSs) are the norm in data centers, they are not all the same. There are various standards to which automatic transfer switches can be certified.
For reliability and the greatest likelihood that a switchover to backup power will occur flawlessly, many experts in business-critical continuity recommend selecting an automatic transfer switch certified to UL 1008. UL 1008 Certification requires conforming to extremely rigorous industry-recognized requirements.

What Does Underwriters Lab Certification Mean?

Established by UL (Underwriters Laboratories) in 1970 to guard against transfer switch failures and resulting potential fires, UL 1008 is both a performance standard and a design and construction standard.

In a recent survey commissioned by ASCO, a business of Emerson Network Power, of executives at facilities with transfer switches, 20percent of respondents reported at least one failure of a switch in the previous five years. And 42 percent of those failures left a facility without power. One third of those respondents stated that the transfer switch failed completely and became totally non-operable. Age of the switch was not a determinant – the number of failed units less than five years old was the same as the number of failed units 15 or more years old.

Aimed at ensuring reliability and durability of operation, UL 1008 requires rigorous testing of the transfer switches by an independent testing and certification agency. The stringent requirements include: withstand and close on ratings (WCR) covering severe fault currents, bolted faults and short circuits; a test to ensure the device can carry rated currents; and endurance tests which use tables specifying the number of cycles the transfer switches at each ampere level must achieve and still perform the intended function. A typical transfer switch that has earned UL 1008 certification can transfer 6,000 times with a minimum of 1,000 operations at 100percent of the rated load.

To put those numbers in perspective, a UL listed transfer switch that is tested monthly might be operated 15 or so times a year. Normal power outages may happen 7-10 times a year in a facility, when the switch also transfers power over to an on-site source. So the standard certainly has lots of leeway built in – over a 50 year period a particular transfer switch might be operated about 1,200 times, give or take.

Two more advantages of installing a UL 1008 listed transfer switch are simplification of the inspection on code-required emergency power systems and the spelling out of construction standards for mechanical operation, essentially contributing to ensuring safe operation of the switch.

ATC Switch. On the upper left hand corner of the door, the UL label is applied.


ATC Switch. On the upper left hand corner of the door, the UL label is applied.


If a manufacturer’s transfer switches have successfully passed the battery of tests, that manufacturer is permitted to label those switches with the UL mark. UL 1008 transfer switches are labeled as "non-automatic transfer switch," "automatic transfer switch," or "transfer and bypass isolation switch." Literature that is associated with the transfer switch should state "UL tested" or "tested and certified by UL 1008." The UL 1008 switch itself must have the UL inside a circle logo and the word “listed” along with the exact transfer switch words cited in the standard. In addition, each switch carries a code that identifies the manufacturer.

Certification Is Not Commonly Understood

Despite the importance of UL 1008 certification, many data center executives are unaware of any certifications of a transfer switch or why they should look for it. In fact, when asked to which UL standard their transfer switch(es) were certified given the choices UL 67, UL 98, UL 891, UL 1008, “others,” or “not sure,” 92 percent responded not sure.

When asked who ensured the transfer switch(es) were certified to the appropriate standard, 15percent were not sure and 10percent said themselves. Almost half the responses indicated it was the design engineer’s responsibility and 47percent felt it was the installing contractor’s responsibility. About one quarter of the responses chose “authorities who approved occupancy certificate.” (Responses added up to more than 100percent because multiple answers were allowed.) Whoever has the responsibility for the transfer switch selection, it is important that individual has extensive experience in the process. To avoid any misunderstanding, the UL 1008 requirement should be written into any master construction specifications.

In addition to installation of a UL 1008 certified transfer switch, it is also essential to ensure that proper periodic maintenance and testing are in place. Typically, a qualified service company should be contracted to regularly perform periodic inspection and maintenance on the transfer switch and associated equipment and make any repairs as needed. Indeed, transfer switches have moving parts that, if left in the same position for months or years, can seize up.

The service provider may be the original equipment manufacturer’s service organization or, more commonly, a third-party service provider. If possible, it is also a good idea to have in-house personnel with expertise on the switches and their maintenance.

Don’t take the quality of potential service providers for granted. The lowest bidder is not necessarily the best choice. One of the first questions is whether the service provider’s techs are trained by the manufacturer of the automatic transfer switch to work on that switch. Expertise on and familiarity with protocol for one manufacturer’s switch does not guarantee “easy-fit” comfort on another manufacturer’s switch. Beyond that, does the provider offer round-the-clock service? Will the tech arrive onsite with spare parts from the manufacturer or does the tech first have to survey the damage or problem and then call for parts?

Data center managers should know the type of transfer switch the data center’s emergency backup power is relying on. If an upgrade is warranted, make sure the design and specifying engineers select from an established manufacturer of UL 1008 listed switches suitable for the given application. The aim is for the transfer switch to operate flawlessly when called upon over the span of many years.

Industry Perspectives is a content channel at Data Center Knowledge highlighting thought leadership in the data center arena. See our guidelines and submission process for information on participating. View previously published Industry Perspectives in our Knowledge Library.

Subscribe to the Data Center Knowledge Newsletter
Get analysis and expert insight on the latest in data center business and technology delivered to your inbox daily.

You May Also Like