The Pluses of Specifying UL-Listed Transfer Switches for Critical Systems

When specifying transfer switches for critical systems, reliability and durability are of paramount importance. Electrical manufacturers offer a range of transfer switches to meet the needs of various types of facilities and customer goals.

Options include UL-certified products that meet requirements based primarily on UL’s published and nationally known standards for safety. For best assurance of operation as expected, select a switch that, specifically, is UL-1008 listed.

To help ensure transfer switches operate dependably, UL developed a standard, UL 1008, to which transfer switches for installation in emergency backup power systems can be certified. The standard was established to guard against transfer switch failures and potential resulting fires and is both a performance standard and design and construction standard.

UL-1008 listed transfer switches have undergone rigorous third-party testing and been found to comply with strict requirements that can sustain and meet needs of data centers, healthcare facilities, telecom facilities, and other mission critical businesses and establishments where uninterrupted power is critical. A UL 1008 listed transfer switch can transfer at least 3,000 times, with 1,000 of those times under at least 100% of rated load.

In addition, if rated at lower amperes, a UL 1008 listed transfer switch must be capable of completing 6,000 transfers under 100% load or greater. Given that a transfer switch might be tested monthly and annually, those requirements help ensure durability over the long run of potentially many decades of operation.

With a UL 1008 listed switch, the range of tests show that the switch can operate after withstanding and closing-on a severe fault current, bolted fault, or short circuit within the electrical distribution system. This testing meets the NEC Articles 517, 700, 701, and 708.

When specifying a UL 1008 switch, look at the literature accompanying the transfer switch for clear documentation of that testing. If the literature merely says “as per UL standards” or “meets UL standards” the switch might not actually have been UL tested.

Look for more precise language, such as “tested and certified by UL 1008” or “UL 1008 listed” and – of utmost importance – the switch should carry a UL inside a circle logo label and identification as cited in the standard as to the type of switch it is. For example, for an automatic transfer switch (ATS) it should say just that – “automatic transfer switch.”

Along the same lines, a UL 1008 listed non-automatic switch would carry the UL logo and be labeled ‘non-automatic transfer switch.’ Switches that come with all this documentation and labeling have been third party tested to meet those requirements.

Some manufacturers certify their equipment at 480VAC only. At least one manufacturer goes further, certifying its ATSs for 480 and 600VAC equipment.        

Selecting a UL 1008 ATS may also make sense for other reasons, as well, including heightened efficiency. Specifications for automatic transfer switches at some installations at critical facilities may call for meeting the National Fire Protection Association’s (NFPA) Life Safety Code and other applicable NFPA standards, such as NFPA 99 and 110. Selecting a transfer switch manufactured by a company that offers only transfer switches that are UL 1008 listed makes it easier for inspectors at installations where a UL 1008 listed transfer switch is required.