UL 1008 Transfer Switches for Standby Generators

UL 1008 Transfer Switches Deliver Dependability.

UL 1008 transfer switches deliver dependability.Extended power outages are a real threat to Treasure Coast homeowners during severe weather events. When power is lost it completely changes the way we live our lives. When a natural disaster such as a hurricane occurs backup power is an absolute necessity. For that reason, a permanent standby generator is often viewed as the best option for homeowners because it can provide automatic, whole-home power during extended outages. Switchover to emergency backup power when utility power fails typically relies upon a UL 1008 transfer switch. That switch activates the standby generator as soon as normal power goes off-line. The reliability of that switch is of paramount importance. Unfortunately, non-UL 1008 transfer switches have not always been as dependable as they should be. In fact, in a recent survey nearly a third of the non-UL 1008 transfer switched tested failed completely. But, with evolving technology and more widely adopted standards, reliable transfer switches can become the norm. One of those evolving standards is Underwriters Laboratory’s UL 1008.

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

Labeling UL 1008 Transfer Switches.

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

The standard sets 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.
  • 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 for a switch that is tested monthly and also might be subjected to a dozen outages of normal power every year.

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

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.

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