Network Infrastructure

When Dependability Counts, UL 1008 Transfer Switches Deliver

By TMCnet Special Guest
Bhavesh Patel, vice president of global marketing for ASCO Power Technologies
  |  June 08, 2015

Uninterrupted power at telecom facilities is a foremost operations goal that preserves the business interests of your customers, short term, and your company reputation, long term.

Switchover to emergency backup power when utility power fails typically relies upon an automatic transfer switch that performs without fail when called for as soon as normal power goes offline. The reliability of that switch, therefore, is of paramount importance – and, unfortunately, not always a certainty.

Transfer switches have not always been as dependable as they can be now. In fact, in a recent survey of facility executives sponsored by ASCO Power Technologies, at least one in five had experience with a failed transfer switch within the previous five years. In a third of the instances, the switch failed completely. But today, given the evolving technology and standards that are becoming more widely adopted, reliable transfer switches can become the norm.

Manufacturers offer a range of transfer switch options, including those that are tested and conform to UL 1008 standards and those that have not been tested to UL 1008. (The UL stands for Underwriters Laboratory, a widely accepted third-party safety and certification company with global name recognition.)  

UL 1008 is a standard for transfer switch equipment first issued in 1972, approved by ANSI in 1976, and evolving ever since. A performance standard as well as a design and construction standard that sets criteria for short circuit testing, UL 1008 is intended to guard against transfer switch failures and potential resulting fires. It helps ensure the safety of the installation and the safety of the operation and maintenance personnel who interface with transfer switch equipment.

Since it was first introduced, there have been several revisions to ensure that transfer switches are adequately tested to safely withstand and close on the short-circuit ratings shown on their labels. Currently in its seventh edition, it is, today, the standard to which switch manufacturers build and test their switches to help ensure they perform as expected.

Achieving the label UL 1008 requires conforming to a series of rigorous requirements that cover automatic, non-automatic (manual) and bypass/isolation transfer switches intended for use in ordinary locations for power and lighting.

The standard puts forth a series of exacting requirements including: withstand and closing ratings (known as WCR), which cover severe fault currents, bolted faults, and short circuiting within the electrical distribution system; a test to ensure the device can carry rated currents; and endurance tests that specify the number of cycles the transfer switch must achieve and continue to perform its intended function. (For example, a transfer switch that has earned UL 1008 certification has the ability to transfer 6,000 times, with one-sixth of those operations at full rated load. (That leaves plenty of leeway many times over for a switch that is tested monthly and annually and also might be subjected to eight or 10 outages of normal power every year, over a period of 50 years.)

The seventh edition of UL 1008, effective as of Nov. 1, 2014, (impacting product shipped on or after that date), resulted in significant changes to the short-circuit ratings shown on all transfer switch products in the industry.

The most important change is in the method of qualifying additional circuit breakers to be shown on the label markings. Until the seventh edition, the listing of specific breakers was based on comparison of published maximum instantaneous clearing time of the breaker tested to non-tested circuit breakers’ maximum instantaneous clearing times. The published times were often conservative catchalls, based on similar published trip curve values that actually were two or even three times the actual clearing times; that ignored the fact that the circuit breaker being tested may have actually cleared the circuit a lot faster. By using the higher rating, the same WCR could be used on many breakers. Precision in WCR product labeling was not required and therefore, often, not done.   

Now, as of the seventh edition, the process of qualifying additional breakers calls for comparing circuit breaker maximum instantaneous clearing times to the actual time durations of short circuit tests conducted on the transfer switch. The change is to ensure all manufacturers use the same criteria described in the seventh edition to qualify circuit breakers to be used with transfer switches during short-circuit testing of their products. It requires transfer switch manufacturers to either test every specific breaker listed on the WCR label or conduct longer time duration tests that encompass the published clearing times of all the breakers listed.

Time-based short circuit and short time ratings are both optional for product qualification under UL 1008. Not every transfer switch manufacturer offers these ratings in addition to the specific breaker rating. However, if a specifying engineer has this added information, that person has more flexibility when selecting an appropriate circuit breaker to coordinate with the desired transfer switch rating. And, after installation, in applications where ratings are relevant, an inspector would look for the additional information.

Bhavesh S. Patel is vice president of global marketing for ASCO Power Technologies, a business of Emerson Network Power (News - Alert) (     

Edited by Dominick Sorrentino